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THE ECOLOGY OF THE MESQUITE TWIG GIRDLER,
ONCIDERES RHODOSTRICTA BATES, AND ITS
EVALUATION AS A BIOLOGICAL
CONTROL AGENT
by
KENITH LEE POLK, B.S .
A THESIS
IN
ENTOMOLOGY
Submitted to the Graduate Faculty of Texas Tech Univers i ty in
P a r t i a l Fulfil lment of the Requi rements for
the Degree of
MASTER OF SCIENCE
Approved
May, 1971
f07( ^JQ Y P ACKNOWLEDGEMENTS
I am deeply indebted to Dr. D. N. Ueckert for his guidance,
encouragement, and help in collecting data for my thesis, I would
also like to express my appreciation to the other members of my
committee, Dr. C. R, Ward, Dr. D. Ashdown, and Dr. J. E.
George, for their helpful criticisra. I also thank the many land
owners whose cooperation made this project possible,
I am thankful to Drs. B. D, Burks and P. M. Marsh of the
U.S.D.A, Insect Identification and Parasite Introduction Research
Branch for specific deterrainations of parasites of the mesquite
twig girdler.
I would also like to acknowledge the State of Texas and Texas
Tech University for their financial assistance and support in the
completion of this project.
II
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
LIST OF TABLES vi
LIST OF FIGURES viii
I. INTRODUCTION AND LITERATURE REVIEW. . . 1
L i t e r a tu r e Review 2
Descr ip t ion 4
II. , METHODS AND PROCEDURES 6
Study A r e a s 6
Mesquite Twig Girdler 7
Life His tory and Behavior 7
Number of Eggs /Branch , Pe rcen t Egg Hatch, and Diameter of Girdled b r a n c h 8
E ly t ra , Egg and Head Capsule Measu re ments 9
Volume of Wood Consumed by Larvae . . . . 10
Population Densi t ies , . 10
Damage to Mesquite by Gi rd l e r s . . . . . . 11
Field Cage Studies , 12
P a r a s i t i s m , Preda t ion , and Competit ion . . . 14
Labora to ry Rearing 15
iii
Bio log i ca l Check Method . . . . . . . . .
E f f e c t s of T e m p e r a t u r e on Twig G i r d l e r L a r v a e ,
B r a n c h Method of G i r d l e r Co lon iza t ion . .
Adu l t Method of G i r d l e r Co lon iza t ion . . .
IIL RESULTS AND DISCUSSION
Life H i s t o r y
Adul t
L a r v a e
P u p a e .
D e s c r i p t i o n
Adul t
• Egg
L a r v a e
P u p a e
P o p u l a t i o n D e n s i t i e s
D a m a g e to M e s q u i t e by G i r d l e r s
F i e l d Cage S tud ies . . . . . . . . . . .
P a r a s i t i s m , P r e d a t i o n , and C o m p e t i t i o n . .
B io log ica l Check Method .
Ef fec t s of T e r Q p e r a t u r e on Twig G i r d l e r L a r v a e 52
iv
Page
16
17
19
20
23
23
23
27
28
28
28
30
30
31
34
37
38
41
51
Page
Branch Method of Gi rd le r Colonization . . . 54
Adult Method of Gi rd le r Colonizat ion. . . . 57
IV. SUMMARY ^^
LITERATURE CITED ^^
LIST O F T A B L E S
T a b l e P a g e
1. A c o m p a r i s o n of s e x r a t i o s of O. r h o d o s t i c t a Ba t e s c o l l e c t e d by hand and in l ight t r a p s . T e x a s . A u t u m n , 1970 24
2. P o p u l a t i o n d e n s i t i e s of adu l t s of ^ ^ r h o d o s t i c t a Ba t e s on two 5 0 - t r e e t r a n s e c t s in W i n k l e r -W a r d and G a r z a C o u n t i e s , T e x a s . Au tumn , 1970 35
3. Mean n u m b e r s of m e s q u i t e b r a n c h e s g i r d l e d p r i o r to 1969, in 1969, in 19V0, and the m e a n n u m b e r s of adu l t s of O. r h o d o s t i c t a B a t e s , on two 5 0 - t r e e t r a n s e c t s in both G a r z a and W i n k l e r - W a r d C o u n t i e s , T e x a s . Au tumn , 1970 36
4. Ef fec t s of popu la t ion dens i ty of jO^ r h o d o s t i c t a Ba t e s on d a m a g e to m e s q u i t e and egg laying hab i t s in cage s t u d i e s . D ickens Co. , T e x a s . A u t u m n , 1970 39
5. P a r a s i t e s and p r e d a t o r s of ^ ^ r h o d o s t i c t a B a t e s l a r v a e d i s s e c t e d f r o m m e s q u i t e b r a n c h e s co l l e c t e d in G a r z a and Ward C o u n t i e s , T e x a s , b e t w e e n M a r c h and S e p t e m b e r , 1970 45
6. N u m b e r s of l a r v a e of O^ r h o d o s t i c t a B a t e s and t h e i r p a r a s i t e s and p r e d a t o r s d i s s e c t e d f r o m m e s q u i t e b r a n c h e s co l l ec t ed in 1970 and 1971 in Ward and G a r z a C o u n t i e s , T e x a s 47
7. Mean no . of l a r v a e oi 0_^ r h o d o s t i c t a Ba t e s p e r b r a n c h u n d e r five t e m p e r a t u r e r e g i m e s in Ward and G a r z a C o u n t i e s , T e x a s . June 26 to A u g u s t 28, 1970 53
8. R e s u l t s of co lon iza t ion a t t e m p t s u s ing b r a n c h e s infe s t ed wi th l a r v a e of O, r h o d o s t i c t a Ba t e s in 6 un in fes t ed T e x a s s i t e s u s ing two m e t h o d s of shad ing b r a n c h e s . A u t u m n , 1970 55
vi
T a b l e P a g e
9. R e s u l t s and eva lua t i on of adu l t r e l e a s e s of O. r h o d o s t i c t a Ba t e s co l l ec t ed in W i n k l e r Co. , T e x a s , and i n t r o d u c e d into 3 un in fes ted T e x a s s i t e s in e a r l y O c t o b e r , 1970 58
10. A c o m p a r i s o n of adul t and l a r v a l m e t h o d s of co lon iza t ion of O. r h o d o s t i c t a B a t e s in T e x a s . - 1970 ' 59
V L l
LIST OF FIGURES
F igure Page
1. Adult mesqui te twig g i rd le r ^i^ rhodost ic ta Bates . Ward Co. , Texas . Autumn, 1970 29
2. Egg of mesqui te twig girdlerjO_. rhodost ic ta Ba tes . Ward Co. , Texas . Autumn, 1970 29
3. Las t l a rva l ins ta r of mesqui te twig g i rd le r , O. rhodost ic ta Bates . Ward Co . , Texas . Autumn, 1970 32
4. Frequency graph of head capsule m e a s u r e m e n t s of 250 la rvae of ^^ rhodosticta Bates . Measu remen t s "were made per iodical ly during 1970. Ward Co. , Texas 32
5. Pupae of mesqui te twig g i rd le r ^_^ rhodosticta Bates . Ward C o . , Texas . Autumn, 1970. . . 33
6. Effect of O. rhodost icta Bates on raesquite at a population density of 50 / t r ee : A, T ree before caging; B, Tree after al l girdling had stopped; C, Tree with girdled b ranches r e moved. Dickens Co. , Texas . Autumn, 1970 40
7. Trend in density fluctuations in interact ing populations of l a rvae of ^^ rhodost ic ta Bates and the i r p a r a s i t e s and p reda to r s d issected from ga l l e r i e s in mesqui te b ranches collected in 1970, 12 mi les nor theas t of Post , Garza County, Texas 43
8. Trend in density fluctuations in interact ing populations of l a rvae of O. rhodost icta Bates and the i r p a r a s i t e s and p r e d a t o r s d issec ted from ga l le r ies in mesqui te b ranches collected in 1970, 12 mi les south of Wink, Ward County, Texas 44
vi i i
F i g u r e P a g e
9. T o t a l n u m b e r s of i n s e c t s r e a r e d from, four l a b o r a t o r y c a g e s , e a c h conta in ing 200 r n e s q u i t e b r a n c h e s g i rd l ed by O. r hodo s t i c t a B a t e s . B r a n c h e s w e r e col icc te t l on A p r i l 8, May 28 , Ju ly 2, and Augus t 13, 1970, 12 m i l e s n o r t h e a s t of P o s t , G a r z a County, T e x a s , . 49
10. T o t a l n u m b e r s of i n s e c t s r e a r e d f rom four l a b o r a t o r y c a g e s , e a c h conta in ing 200 m e s q u i t e b r a n c h e s g i r d l e d by _0^ rhodo s t i c t a B a t e s . B r a n c h e s w e r e co l l ec t ed on M a r c h 2 5, May 27, July 29, and Augus t 17, 1970, 12 m i l e s south of Wink, Ward County, T e x a s 50
I X
CI-L^PTER I
INTRODUCTION AND LITERATURE REVIEW
About 56 ])iilIion acres of Texas rangeland are infested with
mesquite, Prosopis glandulosa Torr. var. glandulosa, one-fourth
of which has greater than 20% canopy cover (Smith and Rechenthin,
1964). Mesquite is a noxious plant on rangeland because it reduces
herbage production! by competition for sunlight, moisture, and nutri
ents. Thick stands of mesquite make it almost impossible for
ranchers to gather- their livestock. Many present day chemical
and mechanical control methods are often infeasible due to cost
or incffcctivcnesc.
The inesquite twig girdler, Oncideres rhodosticta Bates,
attacks the crowns of mesquite trees. Preliminary observations
on the life history and description of this insect have been presented
by Ueckert, Polk, and Ward (1971). This insect chews through the
bark and living xylem layers of limbs 0. 5 to 2. 0 cm in diameter,
killing the branch above the girdle. In several areas of Texas,
mesquite tv/ig girdlers have reduced the canopy cover of mesquite
considerably, and the need for more detailed studies concerning its
ecology, life history, and parasite and predator complex and its evalu
ation as a biological control agent has been suggested. (Ueckert
et al. , 1971).
1
This study was begun in 1969 to study the mesquite twig girdler
as a possible biological control agent for raesquite. The purpose of
this study was to determine the biology and ecology of the mesquite
twig girdler and its effect on mesquite in Texas.
The use of insects in biological control has proven highly
successful with other noxious perennial plants, including Klamath
weed, Hypericum perforatum, and pricklypear, Opuntia spp. (Hollo-
way, 1964),
Literature Review
Bates (1885) first described the mesquite twig girdler, 0>.
rhodosticta Bates, in 1885 from Lerdo, Mexico, however, there
is much confusion in the literature due to misidentification of this
insect, Linsley (1940) reported the taxonomy of this insect and
distinguished it by the presence of three callsoities in a transverse
row on the pronotum, the pale anti-median fascia of the elytra, and
the yellowish or tav/ny spots throughout the elytral pubescences.
High (1915) reported the habits of the huisache girdler,
Oncideres putator Thomson, but most of the published records to
which he refers pertain to O. rhodosticta Bates according to Linsley
(1940). Swenson (1969) reported that the mesquite twig girdler
inflicted an average damage of about 10% to mesquite trees during
the autumn of 1968 on the Spining Ranch in Garza County, Texas.
Uecker t et a l . (1971) es t imated that about 90% of the mesqui te t r e e s in
infested a r e a s in the T r a n s - P e c o s region of Trxas had been attacked
by this insect , and that about 40% of al l l imbs fxcm 0, 5 to 2, 0 cm in
d i ame te r had been girdled.
Closely related species and the i r distril .ution and host records
were repor ted by Linsley (1940). They include: (1) 0_^ pus tula ta
Lee . , which is abundant in the lower Rio Grande Valley of Texas
where it g i rd les b ranches of Acacia farnesi.j.r,a, Pi thecolobium
f lexicaul is , P rosop i s glandulosa. Acacia be i l and ie r i , Parkinsonia
aculeata and Mimosa l indheimer i ; [2) 0_^ ci:'i;:',ulata (Say), which
g i rd les hickory, pecan, pe r s immon , elm, poplar , sour -gum,
bass -wood , honey locust , dogwood, apple, pear , peach, plum,
c i t ru s , eucalyptus and probably other t r e e s ; (3) O, texana Horn,
which occurs in southern Texas where it g i rdles the twigs of Acacia
and probably a lso P rosop i s and Pithecolobium; and (4) O^ que reus
Skinner, which is found in southern Arizona where it a t tacks oaks,
Quercus spp.
Linsley (1940) repor ted that ^L rhodosticta Bates occurs in
the southwestern United States and nor thern Mexico where it occurs
on mesqui te (Prosop is ) . Howard (1900) was the f irs t to repor t this
bee t l e ' s damage to inesqui te ; however, he misidentified the beetle
as O. putator Horn according to Linsley (1940). Howard 's repor t
was documented from specimens and information sent to him by
Mr. M. R. Wise, who speculated that the mesquite in Calabasas,
Arizona, would ultimately be exterminated due to excessive pruning
which eliininated seed production. According to Linsley (1940),
Craighead (1926) reported this species frora Sarcobatus. Essig
(I926) illustrated the adult, egg, larvae, and its injury to mesquite
under the name O^. pustulatus Lee. (Linsley, 1940).
Description
The raesquite twig girdlers (Figure 1) belong to the family
Cerarabycidae, subfaraily Lamiinae, and tribe Onciderini. One of
the raaior characteristics of the tribe is that the front coxal cavities
are angulated on.the outer side and closed behind. The antennae
of the raales are rauch longer than the body, while those of the females
are as long as the body.
The genus Oncideres was first described by Serville in 1885.
Linsley (1940) described the beetles of this genus as stout, cylindri
cal insects of raedium to large size, dark gray or grayish brown
in color, and often colorfully marked. The antennae are as long as
or a little longer than the body and the thorax often has a spine on
each side. The legless larvae are cylindrical, shining borers, with
heads longer than they are wide and the anterior raargin of the front
beset with transverse rows of short carinae or ridges. The pronotum
is raised, very shining, and finely striate; and the arapullae bear two,
or rarely three, regular rows of tubercles. The genus Oncideres can
be distinguished from Taricanus by the eyes not being so finely granu
lated, the lower lobe of the eye is raore elongate, and the antennae
are slender in both sexes. The vertex of the head is flat instead of
concave as in Taricanus. ' Linsley, 1940).
CHAPTER II
METHODS AND PROCEDURES
Study Areas
Field studies were conducted in three areas which were infested
by high population densities of twig girdlers: (1) 6 and 12 miles
south of Wink in Winkler and Ward Counties, Texas; (2) 12 miles
northeast of Post in Garza County, Texas; and (3) 6 miles south
west of Quannah in Hardeman County, Texas.
The Winkler-Ward County study area is in the Trans-Pecos
region of Texas and is located principally on the Ron Slaughter
Ranch. Vegetation in the area is typical of arid raesquite-creo
sote bush associations. Soils in this area are Springer loamy fine
sand or sandy loara, and the topography is nearly level. The average
annual precipitation is about 30 era. The mesquite is shrub-type
and has a high percentage of the small limbs typically preferred
as oviposition sites by the mesquite twig girdler.
The Garza County study area is in the Rolling Plains of Texas
and is located on the Post-Montgomery Estate where the average
annual precipitation is about 47 cm. Vegetation consists of buffalo
grass, tobosa grass, sideoats grama, and scrubby mesquite and
soils are Vernon clay loam with a 3-5% slope. Mesquite in this area
is generally scrubby, but may be quite large in low lying areas.
6
The Harderaan County study area is in the Rolling Plains of
Texas and is located on the Joe Ed Steep Ranch. Vegetation con
sists of tobosa grass, juniper, and mesquite. Tellman-Vernon
soils are predorainant and the topography is rolling. The average
annual precipitation is about 63 era and mesquite in this area is
mostly tree-type with basal steras frora 7 to 13 cm in diaaieter.
The twig girdlers at the Hardenaan County study area were
deterrained to be Oncideres cingulata (Say) instead of 0_^ rhodosticta
Bates, thus research at this site was curtailed.
Mesquite Twig Girdler
Life History and Behavior
Mesquite twig girdlers were observed under field conditions
to determine their feeding and mating habits and their response to
light traps. Field cages were used to observe longevity, fecundity,
and ovipositing habits.
Large numbers of raesquite twig girdlers were needed for
various experiraents in this study, thus the effectiveness of light
traps for collecting specimens was determined. The girdler's
response to artificial light at night was determined using light
traps in Garza and Winkler Counties on October 3 and 8, respec
tively, when the air temperature was above 70°F. Light used for
these traps was supplied by a 100-watt bulb suspended from a
8
5-ft. tripod, and powered by a portable generator. Beetles were
collected by placing a sheet under the tripod for background con
trast. They were sexed by the differential length of the antennae.
Large numbers of mesquite twig girdlers were hand collected
from raesquite branches. Their sex was determined and the numbers
of males and fennales collected with light traps were compared with
the nurabers collected by hand. The chi-square test (Snedecor and
Cochran, 1967) was used to deterraine if the sex ratios were equal
using these two collecting methods. The mean antennal length was
also determined on both raale and female beetles.
Nuraber of Eggs/Branch, Percent Egg Hatch, and Diameter of Girdler Branches
On October 14 and 27, 1970, 60 recently girdled branches
were collected at both the Garza and Ward County study areas to
determine the raean number of eggs/branch, the time required for
the eggs to hatch, and the percent egg hatch. The incubation period
was determined by selecting branches in which beetles were oviposit
ing and dissecting these oviposition sites daily in the lab until the
eggs hatched. Sixty branches were dissected about three weeks
after they had been girdled to determine egg hatch. It was assumed
that this was an adequate incubation period. Dial calipers were used
to determine twig diameters immediately above the girdles.
Elytra, Egg, and Head Capsule Measurements
Elytra, eggs, and head capsules of mesquite twig girdlers were
measured with a microscope equipped with an ocular scale at 6, 12,
and 25 raagnifications for these three measurements, respectively.
Scale values were converted to metric units with a micrometer.
Elytral and overall length measureraents were made on raes
quite twig girdlers hand collected in Winkler County in late Septem
ber. Forty-five elytral raeasurements were taken from both male
and female specimens. These measurements v/ere used to compare
maleifemale size ratio instead of the overall length to avoid errors
due to flexing of the head and prothorax. The "t" test was used to
determine if males and feraales were the same size (Steel and Tor-
rie, I960).
The length and width of 60 eggs v/ere measured frora branches
collected in Garza and Winkler County study areas on October 14
and 19, respectively. Branches containing unhatched eggs were
selected from representative trees at each study area. The bark
was reraoved frora the oviposition site, leaving the egg exposed,
and measurements were made.
Head capsule widths of 250 mesquite twig girdler larvae,
ranging in size from first instars to pre-pupal instars, were deter
mined, Dyar's Law was used to attempt to determine the nuinber
of instars in the larval development of this insect (1890),
10
Volurae of Wood Consumed by Larvae
The volurae of wood consuraed by a raesquite twig g i rd le r l a r
vae was deterra ined frona 35 ga l l e r i e s . Branches , f rom which gird
l e r s had eraerged , were collected in Carza County in mid-Noveraber .
At each emergence hole a smal l opening about 2 ram wide v/as cut
the en t i re length of the gal lery . Larva l or pupal exuvae and f rass
were removed and individual ga l l e r ies were filled with fine sand
which was weighed and converted to a volume by applying a weight:
volume ra t io .
Population Densi t ies
Population densi t ies of raesquite twig g i rd le r s v/ere determined
at the Winkler -Ward and the Garza County study a r e a s during the
fall of 1970. Fifty t r e e s on each of two t r a n s e c t s were permanent ly
marked with wooden s takes at each of the two s i t e s . Population
density counts at the Winkler-Ward County site were made on Septera-
b e r 24, October 7, 19, and 27, 1970. In Garza County, counts were
made October 7, 14, 18, and November 5, 1970. Turnover in the
twig g i rd l e r population was determined by raarking the e lyt ra of
each beet le with a smal l dot of model a i rp lane paint, using different
co lors of paint on different da tes .
On each t r an sec t the following were recorded: (I) mean
number of b ranches girdled p r i o r to 1969; (2) mean nuraber of
11
branches girdled in 1969;. (3) mean number of branches girdled in
1970; and (4) mean nuraber of aiesquite tv ig girdlers present in
1970. These variables were used to determine if future population
densities and daraage could be predicted froin raeasurable statistics.
The statistical method outlined in Snedecor and Cochran (1967)
was used to test for significant correlation coefficients between the
following: Xj , mean number of branches girdled prior to 1969, with
Y]^,mean nuraber of branches girdled in 1969; X2, mean nuraber of
branches girdled in I969, with Y2,naean number of branches girdled
in I97O; X3,mean number of branches girdled in I969, with Y:,,mean
number of twig girdlers present in 1970; and X4, mean number of
branches girdled in 1970, with Y , mean nuraber of beetles in 1970.
Damage to Mesquite by Girdlers
Estimates of the extent of injury to inesquite by the mesquite
t"wig girdler, O, rhodosticta Bates, and the twig girdler, O, cingulata
(Say) were made in late May of 1970 at the three study sites. Damage
by O. rhodosticta Bates was estimated in Ward and Garza Counties,
while damage by O. cingulata (Say) was estimated in Hardeman County.
The extent of girdling damage on 200 mesquite trees in 10 line tran
sects was estimated at each site. Injury for each tree was the aver
age of two observers' estimates of the percent of the total volume of
the photosynthesizing area that had been killed by twig girdlers.
12
These estimates were made on May 27, 28, and 29 for Ward, Garza,
and Hardeaian Counties, respectively. Damage estimates included
girdling activity in 1969 and all previous damage.
Field Cage Studies
Field cages were used to confine beetles in an attempt to deter
raine damage to mesquite froai known population densities of adult
twig girdlers. These cages were also used to determine fecundity,
longevity and the number of branches girdled/female. This phase
of the study was conducted 5 miles east-northeast of Dickens on the
7-Bar Ranch in Dickens County, Texas. The mesquite in this area
had been controlled by chaining about 10 years prior to this study
and the resprouts had a high percentage of stems of the size pre
ferred by twig girdlers for oviposition sites. Three 6X6 ft. cages,
constructed of wood and saran screen, were placed over three trees
of approximately equal size.
The first cage trial was conducted in June, 1970 using twig
girdlers, O. cingulata (Say), which were collected on June 11, 12
miles east of Guthrie on the 6666 Ranch in King County, Texas.
Population densities of 26, 50, and 100/tree were used with a 1:1
sex ratio. The beetles were placed in the cages on June 12 and the
cages were removed on July 24 and damage estimates were made.
13
A siaiilar cage experiment was conducted in late September,
1970, using mesquite twig girdlers, O. rhodosticta Bates, which
were collected at the \7ard County study area on Septeraber 24,
Population densities of 10, 26 and 50 adults per tree were used,
also at a 1:1 sex ratio. The beetles were collected as early after
emergence as possible in order to obtain females which had not yet
laid eggs. The cages were removed and damage estimates were
made on November 4,
A third cage experiment was conducted at the Garza County
study site. A similar cage was placed over a small mesquite
tree that had sustained heavy girdling for several years prior to this
study in order to determine if girdlers would girdle branches of
greater diameter at extremely high population densities. On June
11, 100 twig girdlers, O. cingulata (Say), collected on the 6666
Ranch in King County, Texas, with a 1:1 sex ratio were caged on
the tree. The cage was removed on July 24, 1970, and damage
estimates were made. Twenty-six adult mesquite twig girdlers,
O. rhodosticta Bates, with a 1:1 sex ratio were caged in the same
tree on September 11, 1970. These beetles were reared in the
laboratory from mesquite branches collected at the Garza County
study area. The cage was removed and damage estimates were
made on November 5, 1970.
14
Permanent photo-points were established for each caged tree.
Pictures were taken: (1) before each tree was caged; (2) after
girdling had ceased; and (3) after girdled branches had been
removed. After girdling had ceased and the beetles were dead
the following data were recorded for each tree: (l) percent of
crown volume removed; (2) number of branches girdled; (3)
mean diameter of girdled branches; and (4) mean number of
eggs laid per girdled branch. The "t" test (Steel and Torrie,
i960) was used to determine if the mean numbers of eggs/branch
were equal under the three population densities and if the mean
diameters of the girdled branches were equal under these treat
ments.
Parasitism, Predation, and Competition
The numerical relationship between twig girdler larvae and
their parasites and predators was determined by periodically dis
secting girdled branches from Garza and Ward Counties. Each
oviposition site or gallery was examined to determine if the lar
vae had been attacked by a parasite or predator. The number of
twig girdler larvae and the number of parasites and predators
was recorded for each branch.
Larval parasitization or predation was determined by the
presence of a parasite or predator in the gallery or the presence
15
of material produced by parasites or predators. This material,
v/hich attached the parasite pupae to the gallery wall was usually
deposited in the upper end of the gallery, above the oviposition
site. Predators deposited several small black spots, probably
feces, on the floor of the gallery.
Parasites or predators in the late larval instar or pupal
stage were reared to the adult stage in the lab. The adults of
these parasites and predators were sent to the Entomology Research
Division, A. R, S. , U. S. D. A. in Beltsville, Maryland, for
identification.
Laboratory Rearing
Laboratory cages were used to study relationships between
twig girdlers and other insects inhabiting girdled branches. Some
of these insects were parasites or predators, while others compete
v/ith the twdg girdler larvae for food and space. On each of four
sampling dates, 200 girdled branches were collected from the Ward,
Garza, and Hardeman County study areas. The sampling dates for
each of these three counties were as follow : for Ward County - -
March 25, A ay 27, July 29 and Aug. 17; for Garza County - - Apr. 8,
May 28, July 2, and Aug, 13; for Hardeman County - - March 31,
May 29, July 9, and Aug, 18. The smaller twigs of these girdled
branches were removed and the main steras were placed in 2 ft.
16
X 3 ft. rearing cages, made from aluminum window screens and
covered with paper to exclude light, with ends made of fiber board.
A pint jar was attached to the bottom corner of each cage to collect
positively phototactic insects as they emerged from the mesquite
branches.
Insects reared from the branches were periodically collected
from the jars and preserved in 70 percent ethanol. The total number
of twig girdlers reared frora each cage was compared with the
numbers of parasites, predators, and competitors to determine
their numerical and chronological relationship to the twig girdler
population.
Biological Check Method
DeBach and Bartlett (1964) stated that the evaluation of the
effectiveness of natural enemies of all pests in a ecosystem is a
fundamental prerequisite to intelligently manipulate insect popu
lations ecologically. This concept seemed applicable tn the evalu
ation of the effects of parasites on a potential biological control
agent of mesquite. A biological check method was therefore used
to evaluate the effects of parasites on natural populations of mes
quite twig girdlers. This study utilized the natural aggressiveness
of ants against parasites of honeydew-producing insects (Bartlett,
1961; DeBach, Fleschner, and Dietrick, 1951; Ilanders, 1951),
17
In this biological check study an attempt was made to increase
ant activity on girdled branches. It was assumed that the ants would
be aggressive against all parasites, regardless of their hosts. The
treatments applied in the field and in two replications, each consisted
of 50 girdled branches and included: (1) applying honey to girdled
branches to encourage ant activity, and (2) a non-treated control.
Four-dram plastic vials taped to the girdled branches above the
highest oviposition site were filled with honey, and vial caps were
held open about 3-4 mm with insect pins to exclude larger insects.
The honey was replaced periodically as it was removed from the
vials by ants.
Honey was applied to 50 girdled branches in Ward and Garza
Counties on July 2, and 6, 1970, respectively. On August 28, 1970,
the treated branches and the non-treated branches were collected at
both sites. All oviposition sites and galleries were examined to
determine the percent parasitization and the mean nurabers of larvae/
branch. The "t" test (Steel and Torrie, I960) was used to determine
if the mean percent parasitization levels and the mean numbers of
larvae surviving was equal under the two treatments.
Effect of Temperature on Twig Girdler Larvae
High (1915), working with Oncideres putator Thomson, observed
that when girdled huisache branches remain high in the top of trees
18
the larvae would die frora lack of moisture, while severed branches
or those only a few feet above the ground apparently were under
higher huaiidity conditions and therefore produced raore adult beetles.
Based on High's observations, a study was designed to evaluate
four methods of augmentation of native mesquite twig girdlers by
larval habitat manipulations. Larval survival was used to determine
the favorability of micro-climatic conditions in plots established
in Garza and Ward Counties, Texas, on June 26 and July 6, 1970,
respectively. The treatments, each applied to 50 girdled branches,
included: (I) hanging girdled branches in the canopy of mesquite
trees; (2) placing girdled branches on the ground in the shade of
mesquite trees; (3) placing girdled branches on mid-grass vegeta
tion (tobosa and black grama); (4) placing girdled branches on bare
ground or short-grass vegetation (buffalo grass and false-buffalo
grass), and (5) a non-treated control. The mean number of live
larvae/branch at the beginning of the experiment was determined by
dissecting 50 branches from each study site.
Temperatures were recorded at positions corresponding to the
5 treatments to determine if teraperature affected larval survival.
Two temperature measurements were taken during the hottest part
of the day (between 3:00 and 4:00 PM) in early and late July. On
August 28, 1970, these girdled branches were collected and dissected
19
in the lal; to de te rmine the mean numbers of la rvae surviving as
well as p . j rcentages of l a rvae , pupae, and adul ts .
The analys is of var iance and Duncan's new raultiple range
t e s t (Steel end T o r r i e , I960) was used to de termine if the mean
n u m b e r s of l a rvae surviving under these t r ea tmen t s were equal.
A co r re l a t ion coefficient (Snedecor and Cochran, 1967) was cal
culated between the mean t empe ra tu r e and the mean number of larvae
surviv^ing under each t rea t raent to de termine the effect of t empera
tu re upon l^-rval survival .
Branch Method of Gird ler Colonization
Six field introductions v.'-ere made involving from 500 to 6, 000
girdled mesqui te b ranches infested with mesqui te twig g i rd ler larvae
These introduction plots were 30 m X 30 m and were in a r e a s with
no twig g i rd le r activity in 1969. Two methods of shading girdled
b ranches were used: (1) placing from 10 to 15 branches on tall
g r a s s under a mesqui te t r e e (light shading): and [2) placing about
500 b ranches in a pile under a raesquite t r e e and covering them
with ca rdboard , green aiesqui te b ranches , and leaves (heavy
shading). The mean number of l a r v a e / b r a n c h was determined for
each introduction by dissect ing oviposition s i tes on 25-50 r ep re sen
tat ive b r a n c h e s .
20
Locat ions and dates of field introductions were as follow:
(1) 7 -Bar Ranch, 5 mi les e a s t - n o r t h e a s t of Dickens, Dickens County,
Texas (May 20, 1970); (2 and 3) Sclrarbauer Land and Cattle Company,
16 m i l e s e a s t - n o r t h e a s l of Andrev/s, Andrews County, Texas (June 8
and 18, 1970); (4) L-7 Ranch, 7 mi les south of Crosbyton, Crosby
County, Texas (August 5, 1970); (5) Stephen F a r m s , 12 mi les south
of Draw, Borden County, Texas (August 5, 1970); and (6) White
F a r m s , 3 mi les eas t of Grass land , Lynn County, Texas (August 5,
1970). At each s i te , 13 mesqui te t r ee s were permanent ly marked
in each of the four card ina l d i rec t ions; the last t r ee marked in each
d i rec t ion was about 35 m from the center of the introduction plot.
Populat ion es t in ia tes of adult twig g i rd le r s and the number of branches
g i r d l e d / t r e e were determined on these 52 t r ee s in late October.
The re la t ive success of these introductions were determined
by dividing the mean nurabers of b ranches g i rd l ed / t r ee by the total
number of b ranches introduced. This number was then multiplied by
a constant , 10^ , to adjust success indices to a value between 0. 1
and 1, 000.
Adult Method of Gi rd le r Colonization
Th ree field introduct ions, involving from 2, 000 to 4, 100 live
adult mesqui te twig g i r d l e r s , were made in locations with no twig
g i rd l e r act ivi ty in 1969. The locations and dates of these introductions
21
were: (1) Caswell Farms, 3 miles north of Draw, Lynn County, Texas
(September 28 and October 8, 1970); (2) Dennis Ranch, 16 miles
southwest of Post, Borden County, Texas (September 25 and 28,
1970); and (3) Koosman Ranch, 1 mile south of Dickens, Dickens
County, Texas (September 28, 1970).
The beetles used in these introductions were hand collected
by dislodging thera from mesquite branches. The beetles were trans
ported to introduction sites in 3-gallon ice-cream cartons. Small
mesquite branches were placed in these containers to provide food
and resting areas, and to reduce the tendency of the beetles to dis
member each other before they could be released into mesquite trees
at the introduction sites. Population estimates and the raean number
of branches girdled/tree were made on each of 52 permanently marked
trees, 13 in each of the four cardinal directions from the center of 30
m X 30 m introduction plots.
The relative success of these adult introductions was determined
by dividing the mean numbers of branches girdled/tree by the total
number of beetles released. This value was then multiplied by a
constant, 105 , to give a success index between 0, 1 and 1,000.
Adult and branch colonization methods were compared by adjust
ing numbers of branches introduced to represent the numbers of twig
girdlers that would have emerged from these branches had they been
left intact on mesquite trees. The mean number of adult beetles that
22
e m e r g e d f rom branches left intact on mesqui te t r e e s was determined
by counting the number of emergence holes p r e sen t on 50 branches
in Garza County on November 6, 1970, after emergence of adults
had ceased . At tempts were a lso made to collect s imi la r data at
the Ward-Winkle r County study a r ea in late November; however,
e m e r g e n c e had caused severe breakage of l imbs and counts were
not poss ib le .
CHAPTER III
RESULTS AND DISCUSSION
Life History
Adult
Adult mesqui te twig g i rd l e r s began emerging from gal le r ies
in late August and ea r ly September and continued until late Novem
be r . Branches d issected August 28, 1970, at the Garza County study
si te contained 2% adul ts , 33% pupae, and 65% la rvae . Branches d is
sected at the Ward County study site on August 28, 1970, contained
14% adul t s , 47%) pupae, and 37% la rvae . Thus adults probably emerge
in Ward County one to two weeks e a r l i e r than in Garza County, which
is located about 180 mi les to the north.
Longevity-In field cages mos t adults had died in 20 to 30
days; however , a fe^v adult beet les lived more than 45 days.
Behavior-Only female mesqui te twig g i rd l e r s were observed
girdling mesqui te b r a n c h e s . Males were a t t rac ted to light t r aps
significantly m o r e than females (0.05 level) . About 87% of the adult
twig g i r d l e r s a t t r ac ted to lights were ma les (Table 1), Both sexes
behaved in a s imi l a r manner when dis turbed. Instead of flying, the
bee t les dropped from the mesqui te t r e e to the ground and remained
mo t ion l e s s . They made a noise by rubbing the pro thorax against
23
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25
the a n t e r i o r marg in of the e ly t ra when captured. Both of these
c h a r a c t e r i s t i c s were p r e sumed to be protect ive mechan i sms .
Feeding habi ts-Adul t beet les fed for severa l days p r i o r to
mat ing and girdling on tender ba rk around the buds, thorns , and
s m a l l l imbs of mesqu i te . The adults did l i t t le obvious damage to
mesqu i t e by feeding, and only at population densi t ies of 100/ t ree
in the G a r z a County cage exper iment was there any noticeable
damage .
Mating- Mating was observed both at day and at night, even
while females were involved in girdling or egg laying ac t iv i t ies .
Males and females were usually found on the same branch. Observ
ing bee t les marked with paint showed that males frequently mated
with m o r e than one female .
Fecundi ty- Cage studies on the 7-Bar Ranch in Dickens County,
Texas , in September showed that the average number of branches
girdled by mesqui te twig g i rd l e r s var ied from a low of 0, 96/female
at a population density of 50 b e e t l e s / t r e e to a high of 1. 5/female at
a population density of 26 b e e t l e s / t r e e . The average numbers of
eggs l a id /b r anch for Garza and Winkler Counties were 8, 1 jF 1,1
and 8 , 2 + 1,1 (95% confidence in terva ls ) , respect ive ly , with 97%
and 98% egg hatch, respec t ive ly .
Oviposit ion- Female beet les girdled branches before laying
eggs , by chewing through the p e r i d e r m , nontranslocat ing phloem,
26
t r ans loca t ing phloem, cambium, and far enough into the xylem to
p reven t the upward t rans loca t ion of water and nut r ien ts . Branches
died soon after they were girdled. The leaves remained on these
g i rd led b ranches because an absc i s s ion zone - lid not form. These
girdled b ranches averaged 9 m m _+ 0, 05 mrn ''range 6 to 15 mm)
and 10 m m _+ 0, 06 m m (range 6 to 20 nim) in d iameter in Winkler
and Garza Counties , respect ive ly (95% confidence in tervals ) .
Meyer et a l . (1971) stated that the smal l s tems of naesquite
t r e e s were impor tant for s torage of carbohydra tes as s t a rch gran
u l e s . When these smal l b ranches were girdled by the mesqui te
twig g i rd l e r the amount of s tored carbohydrates available to the
plant was l ikely reduced. The reduction of this s tored food by the
act ion of these insects may make it n e c e s s a r y for the plant to use
food r e s e r v e s s tored in the roots for budding and leaf development
the following spr ing, thus weakening the plant.
After the b ranch was girdled, the female made an incision in
the ba rk with her mandibles and placed the egg between the bark
and wood. The female faced downward while making the incision,
then turned around and inser ted the ovipositor so the egg was inser ted
p a r a l l e l to and beneath he r abdomen. After extract ing the oviposi tor ,
the incision was sealed with an aa iber colored secre t ion .
It took about 20 to 30 minutes for the female mesqui te twig
g i r d l e r to lay each egg, while the girdling p r o c e s s required as much
27
as two days . The distance between oviposition s i tes var ied with
the s ize of the b ranch , but they were usually about 40 m m apar t .
The eggs hatched in 10 to 14 days.
La rvae
Feeding hab i t s -The mesqui te twig g i rd le r la rvae fed toward
the oviposit ion s ca r and opened it after about three months of feed
ing. The l a rvae used this opening to expell f rass from the gal lery
and were observed feeding on only one side of the branch thus,
s eve re ly weakening the branch at that point. Many weakened branches
broke during high winds, probably killing the larvae direct ly or
indi rec t ly .
Behav io r -The la rvae exhibited severa l behaviora l cha rac t e r
i s t i c s . In the la te r s tages of development when ga l le r ies were d is
sec ted the l a rvae were usually found with thei r venter toward the
bark . The l a rvae used smal l s t r ips of wood to plug up any smal l
holes that were made in the ga l l e r i e s . When the la rvae fed within
about 3 m m of another ga l le ry they usually veered to one side.
The la rvae of the mesqui te twig g i rd le r did not cut an e m e r
gence hole in the ba rk nor did they p r e p a r e a pupal cavity as do some
other spec ies of Onc ideres . A prepupat ion stage occured in the
l a rva in which the head capsule became deflexed, the mouthpar ts
b e c a m e hypo{i,nathous, and movements c h a r a c t e r i s t i c of the pupae
28
w e r e exhibited. P e r i s t a l t i c - l i k e movements and many dorsa l spines
were used by the pupae to remove the l a rva l exuvium after it spli ts
immedia te ly caudad to the head capsule .
Pupae
Desc r ip t ion -The pupae were very raobile and moved inside
the ga l l e r i e s using the elongated las t abdominal segment which
has a row of smal l spines around its p e r i m e t e r , by p re s s ing the
spines agains t the s ides of the gal lery . The pupal stage occur red
in late August and ea r ly Septeraber and lasted for about 14 days.
Descr ipt ion
Adult
Adult raesquite twig g i rd l e r s were black in color with a
m i d - t r a n s v e r s e grey band a c r o s s the e ly t ra . Each elytron was
dotted with about 35 to 40 smal l patches of orange pubescence
(Figure 1). The mean length of the male and female elytron was
10. 1 m m jf 0. 3 m m (range 8. 0 to 12, 1 mm) and 10, 6 m m 4- 0, 3 m m
(range 8, 0 to 12. 0 mm) (95% confidence in tervals ) , respect ively ,
while the mean overa l l lengths were 16, 3 and 15. 1 mm, respect ively .
Male and feaiale e ly t ra l lengths were not significantly different
(0. 05 level) .
Males were dist inguished from females by the length of the
an tennae . The average antennal length in raales was about 30. 3 a im.
29
Figure 1. Adult ones Bates . W
quite twig g i rd le r O, 1 ^ 2 ^ 2 5 ^ a r d C o . , Texas . Autumn, 1970.
2 Egg of mesqui te twig g i rd le r O ^ r h o d c ^ F igure 2. Egg^^^ ^ ^ ^ d Co. , Texas . Autumn, 1970.
30
a s c o r a p a r e d to 18. 4 m r a in the f e a i a l e s . The distal jo in t of m a l e
a n t e n n a e w e r e about four t i m e s l o n g e r than tha t of the f e m a l e .
The l eng th of th is s e g m e n t in the m a l e v a r i e d f r o m 3, 2 to 6, 5 m a i ,
whi le in the f e m a l e , it v a r i e d f rom 0, 9 to 1.5 m m .
M a l e s can a l s o be d i s t i ngu i shed f roa i f e m a l e s by the shape
of the l a s t v e n t r a l a b d o m i n a l s e g m e n t . The p o s t e r i o r m a r g i n of
t h i s s e g m e n t in the m a l e is indented o r s l ight ly "V" shaped , whi le
in the fe raa le th is m a r g i n was not "V" shaped and was b r o a d e r .
The eggs w e r e e l l i p t i c a l l y ovate in shape , about four t i m e s
a s long a s wide ( F i g u r e 2), of c r e a r a y whi te co lo r , with a m e a n
l eng th of 4. 3 m m jf 0. 06 m m ( range 3. 8 to 4. 7 m m ) (95% confidence
i n t e r v a l ) . T h e i r m e a n width was 1. 1 m m jf 0. 02 m m ( range 0. 9 to
1. 3 m m ) (95% conf idence i n t e r v a l ) . The eggs w e r e s l igh t ly pointed
on the end away f r o m the ov ipos i t ion s i t e . J u s t be fo re the egg ha tched
the y e l l o w i s h e m b r y o n i c l a r v a e could be seen th rough the chor ion .
The head deve loped t o w a r d the ov ipos i t ion s i t e , and the d o r s u m
deve loped ad j acen t to the b a r k .
L a r v a e
The r aean length of newly ha t ched l a r v a e , which w e r e ye l lowish
wh i t e in c o l o r , was 3. 2 rara Grange 2. 6 to 3. 6 m m ) . The p r o t h o r a x
w a s abou t 1 m m wide and w a s w h i t e r than the r e s t of the body.
31
The head was light brown, while the mandibles were black.
(Figure 3).
Just before pupation, the larvae were som what scrabei-
form and about 26 mm long (Figure 3). Each abdominal seg-
raent possessed two callous-like ridges on its dorsum and ven
ter; these ridges were probably used to stabilize the larvae while
feeding.
Out of 250 head capsule measurements there were no distinct
size groups, therefore it was impossible to determine the number
of larval instars by use of a size-frequency gra-|.h (Figure 4). Ap
parently Dyar's Law does not apply to this species.
Volume of wood consumed-A mesquite twig girdler larva ate
1. 44 cc of wood (range 0. 91 to 2. 1 cc). The small volume of wood
eaten indicated that these larvae were very efficient in extracting
essential nutrients from aiesquite wood.
Pupae
The pupae were about 20 mm long (Figure 5), and were white
when they first eaierged froaa the last larval skin; however, they
gradually became darker until transformation into the adult stage.
The eyes darkened first, then the mandibles, vertex of the head,
and prothorax in rapid succession.
32
Figure 3, Las t l a rva l ins ta r of mesqui te twig g i rd le r . Or rhodosticta Bates . Ward Co. , Texas . Autumn, 1970.
F igure 4.
0 .31 1.9 3.4 Head capsule d iameter (mm)
Frequency graph of head capsule m e a s u r e m e n t s of 250 la rvae of ^^ rhodosticta Bates . Measure raents were made periodical ly during 1970, Ward Co. , Texas .
33
Figure 5, Pupae of mesqui te twig g i rd ler 0_^ rhodosticta Bates , Ward Co, , Texas . Autumn, 1970,
34
Popu la t i on D e n s i t i e s
The h i g h e s t popu la t ion dens i ty of m e s q u i t e twig g i r d l e r s o c c u r
r ed on S e p t e m b e r 24, 1970, the f i r s t s a m p l i n g date (Table 2). S a m
pl ing m a y have begun too la te to d e t e r m i n e peak popula t ion d e n s i t i e s ,
and it w a s o b s e r v e d tha t the n u m b e r of twig g i r d l e r s / t r e e d e c r e a s e d
a s the s e a s o n p r o g r e s s e d . Sampl ing p r o b a b l y should have begun in
e a r l y S e p t e m b e r and cont inued t h rough e a r l y N o v e m b e r . A t u r n
o v e r in the popula t ion o c c u r r e d wi th in a 10 day p e r i o d , thus the
s t and ing c r o p of b e e t l e s cannot be u s e d a s a r e l i ab l e e s t i m a t e of the
t o t a l ef fect ive popu la t ion .
P r e v i o u s and c u r r e n t g i rd l ing d a m a g e and c u r r e n t popula t ion
d e n s i t i e s on t r a n s e c t s in G a r z a and Ward Count ies a r e show^n in
Tab le 3. T h e r e was no s ign i f ican t c o r r e l a t i o n be tween : (1) the
r aean n u m b e r of b r a n c h e s g i r d l e d / t r e e p r i o r to 1969 and the m e a n
n u m b e r of b r a n c h e s g i r d l e d / t r e e in 1969; (2) the m e a n n u m b e r of
b r a n c h e s g i r d l e d in 1969 and the m e a n n u m b e r of b r a n c h e s g i rd l ed
in 1970; (3) the m e a n n u m b e r of b r a n c h e s g i rd led in 1969 and the
m e a n n u o i b e r of b e e t l e s / t r e e in 1970; and (4) the m e a n n u m b e r of
b r a n c h e s g i r d l e d in 1970 and the m e a n n u m b e r of b e e t l e s / t r e e in
1970 (0. 05 leve l ) (Table 3). T h u s , p r e v i o u s d a m a g e r e c o r d s o r
popu la t ion d e n s i t i e s m a y not be r e l i a b l e v a r i a b l e s for p r e d i c t i n g
fu ture t r e n d s . The twig g i r d l e r popula t ion a p p e a r e d to be d e c r e a s i n g
in G a r z a County and i n c r e a s i n g in Wink le r and W a r d C o u n t i e s .
35
T a b l e 2. P o p u l a t i o n d e n s i t i e s of adu l t s of ^ r h o d o s t i c t a Ba tes on two 5 0 - t r e e t r a n s e c t s in W i n k l e r - W a r d and G a r z a C o u n t i e s , T e x a s , Au tumn , 1970,
Mean no. g i r d l e r s / t r e e
G a r z a Co, W i n k l e r - W a r d Co,
Sep t . 24 - 1 /
T r a n s e c t T r a n s e c t S a m p l i n g no . no.
d a t e s 1 2 1
4. 7
Oct. 7 0. 8 - 3. 5 2. 5
Oct , 14 0. 5 0, 5 2, 8 1. 1
Oct . 27 0. 2 0. 2 0. 6 0. 1
0, 5
0. 2
0. 1
44
2 , 8
0 ,6
—
583
Nov, 5 -
T o t a l no. 74 44 583 185
1/ D a s h e s ind ica te tha t no data w e r e co l lec ted .
36
Table 3. Mean numbers of raesquite b ranches girdled p r i o r to 1969, in 1969, in 1970, and the mean numbers of adults of ^L rhodost ic ta Bates , on two 50- t ree t r a n sec t s in both Garza and Winkler-Ward Counties, Texas . Autumn, 1970.
Var iables Mean no. Mean no. Mean no.
Location b ranches b ranches Mean no. b ranches and girdled girdled beet les girdled
t r a n s e c t p r i o r I969 in I969 in 1970 in 1970
Garza A 7. 3 5. 9 1. 5 0. 9
Garza B 7. 7 7, 1 0, 9 1. 4
W i n k l e r - W a r d A 2 . 8 1.8 11.6 13.7
Winkler -Ward B 3. 0 7.0 3.7 9.0
37
I n a d e q u a t e m e t h o d s for d e t e r m i n i n g popula t ion d e n s i t i e s in 1970
p r o b a b l y a c c o u n t e d for ins ign i f i can t c o r r e l a t i o n be tween th i s and
the m e a n n u m b e r of b r a n c h e s g i r d l e d / t r e e in 1970,
Daraage to Mesqu i t e by G i r d l e r s
D a m a g e e s t i m a t e s showed that 34% + 3% and 36% j - 3% (95%
conf idence i n t e r v a l s ) of the c r o w n s of m e s q u i t e t r e e s had been
k i l l ed by n a t u r a l popula t ions of m e s q u i t e twig g i r d l e r s , _0 , r h o d o
s t i c t a B a t e s , in Ward and G a r z a Coun t i e s , r e s p e c t i v e l y . In H a r d e
raan County, 17% jf 2% (95% confidence in te rva l ) of the c rowns of
m e s q u i t e t r e e s had been ki l led by the twig g i r d l e r , 0_^ c ingula ta
(Say),
V a r i a t i o n s .in the d e g r e e of d a m a g e w e r e a t t r i b u t e d to s i ze
o r g rowth hab i t of the m e s q u i t e t r e e s , p e r s i s t e n c e of the in fes ta
t ion , and popu la t ion d e n s i t i e s of g i r d l e r s . In H a r d e m a n County a l l
t h r e e f a c t o r s a p p e a r e d to a ccoun t for l e s s i n s e c t d a m a g e . T r e e s
w e r e l a r g e r , the in fes ta t ion was fa i r ly r e c e n t and the popula t ion
dens i t y a p p e a r e d to be low.
M e s q u i t e t r e e s in G a r z a County w e r e l a r g e r than those in
Ward County and m a i n l y s i n g l e - s t e r a m e d ; howeve r they v. e r e d a m a g e d
m o r e than the m u l t i p l e - s t e m a i e d m e s q u i t e in Ward County. T h e r e
f o r e , the h i g h e r d a m a g e e s t i m a t e s in G a r z a County raay have been
the r e s u l t of a l o n g e r p e r i o d of in fes ta t ion , or popula t ion d e n s i t i e s
38
in G a r z a County in I969 and p r e v i o u s y e a r s m a y have been h i g h e r
than those in W a r d County.
F ie ld Cage S tud ies
Adul t twig g i r d l e r s , O^ c ingula ta (Say), co l l ec ted in June 1970
on the 6666 Ranch in King County, k i l led 15%, 5 5%, and 75% of the
c r o w n v o l u m e of t h r e e caged m e s q u i t e t r e e s a t popula t ion d e n s i t i e s
of 26, 50, and 1 0 0 / t r e e , r e s p e c t i v e l y . The nu rabe r of b r a n c h e s
g i r d l e d / f e m a l e w a s 1. 3, 2. 6, and 1. 1 a t the t h r e e popula t ion d e n s i
t i e s , r e s p e c t i v e l y . In th i s s tudy the h ighes t popula t ion dens i ty caused
the g r e a t e s t d a m a g e , while each feraale g i rd led m o r e b r a n c h e s u n d e r
the i n t e r m e d i a t e popula t ion dens i ty .
M e s q u i t e twig g i r d l e r s , O^ r hodos t i c t a B a t e s , co l l ec ted in
l a t e S e p t e m b e r in Ward County k i l led 10%, 75% and 65% of the c rown
v o l u m e s of caged m e s q u i t e t r e e s a t popula t ion d e n s i t i e s of 10, 26,
and 5 0 / t r e e , r e s p e c t i v e l y '^Table 4), ( F i g u r e 6). The n u m b e r of
b r a n c h e s g i r d l e d / f e m a l e w a s 1. 6, 1. 15, and 0, 96 a t the t h r e e popu
l a t i on d e n s i t i e s , r e s p e c t i v e l y . The m e a n d i a m e t e r of the g i rd l ed
b r a n c h e s did not i n c r e a s e a s popula t ion d e n s i t i e s w e r e i n c r e a s e d in
t h e s e field c a g e s . The m e a n d i a a i e t e r of the b r a n c h e s g i rd l ed in
t h e s e c a g e s was 9 m m ( range 6 to 12 m m ) , which a g r e e d with the
r e s u l t s ob ta ined u n d e r n a t u r a l cond i t i ons . T h u s , m e s q u i t e twig
g i r d l e r s p r o b a b l y would not g i r d l e l a r g e r b r a n c h e s if n a t u r a l popu
l a t i o n d e n s i t i e s w e r e to be s ign i f ican t ly i n c r e a s e d .
39
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A . B .
Figure 6.
C.
Effect of ^ rhodosticta Bates on mesquite at a population density of 50/tree: A, Tree before caging; B, Tree after all girdling had stopped; C, Tree with girdled branches removed. Dickens Co. , Texas. Autumn, 1970.
41
The cage experiment in Garza County, designed to determine
if twig girdlers, O cingulata (Say), would girdle larger branches if
population densities were extremely high showed that no stems over
19 mm in diameter were girdled even at a population density of
100/tree. Sixteen smaller branches were girdled and eggs were
laid in some of the larger branches, even though they were not
girdled.
When this experiment was repeated in September, using the
mesquite twig girdler, 0_ rhodosticta Bates, at a population density
of 26/tree, one branch, 23 mm. in diameter, was girdled. These
beetles were reared in the insectary from branches collected in
Garza County; thus, the beetles had done no previous girdling.
Both experiments showed considerable feeding damage with popu
lation densities at these levels.
Parasitism, Predation, and Competition
The larva is the life stage of the mesquite twig girdler usually
attacked by both parasites and predators. No parasitized eggs were
observed, and the pupae were attacked by predators on very few
occasions. In Garza County, by August 28, 1970, about 34% of
the original 8. 1 larvae/branch had died of unknown causes, 22%
were killed by parasites or predators, and 44% reraained alive.
The percent of the larval population that died from exposure to
42
excessive heat when girdled branches broke off and fell on the ground
due to high winds was not deterrained; however observations on many
occasions implied that this factor is frequently important under West
Texas conditions. The number of larvae/branch decreased as the
year progressed and there was a corresponding increase in the number
of parasites and predators/branch (Figure 7). By early November,
20% of the larvae had emerged as adults, 8% had not pupated, and 72%
had died from attack by parasites or predators or unknown causes.
In Ward County, by August 28, 1970, 55% of the original 8, 2
larvae/branch had died of unknown causes, 15% were killed by para
sites or predators, and 30% remained alive (Figure 8). The number
of larvae/branch decreased and the number of parasites and predators
increased as the year progressed. By mid-December, 24% of the
larvae had emerged as adults, and 76% had died from attack by para
sites or predators or other causes.
A wide variety of parasitic Hymenoptera were dissected from
galleries of raesquite twig girdler larvae from Ward and Garza
Counties. These parasites were found in about equal numbers at
both areas. Five families, 8 genera, and 10 to 12 species were
identified as parasites of mesquite twig girdler larvae ^Table 5).
Apparently all of these parasites attack the larvae in about equal
nuaibers except for Tenuipetiolus n. sp. and braconids, which were
rauch less nuoierous.
43
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45
Table 5. Parasites and predators of Q rhodosticta Bates larvae dissected from mesquite branches collected in Garza and Ward Counties, Texas, between March and September, 1970.
Faaiily Species
Parasites
Chalcedectidae Euchrysia hyalinipennis Ashm.
Pte r omalidac Lampoterma sp.
Eupelmidae
Eurytomidae
Eupelmus bruchivorus (Cwfd. )
Eurytoma magdalidia Ashm.
Eurytoma dorcaschemae Ashaa.
Eurytooia sp.
Tenuipetiolous n, sp.
Braconidae Odontobracon nigriceps Cam.
Predators
Iphiaulax sp.
Glyptocolastes sp.
Cleridae Enoclerus sp.
unidentified sp.
46
The stage and condition of p a r a s i t e s or p reda to r s of the m e s -
quiie t vig g i r d l e r s at var ious dates a r e shown in Table 6. In Ward
County b ranches were d issec ted per iodical ly from March 25, 1970,
to A p r l 8, 1971. In Garza County branches were d i s sec ted per iod
ically froaa March 31, 1970, to January 15, 1971. The highest
incidence of p a r a s i t i s m occur red between mid-Apr i l and ear ly July.
Laboratc^ry r ea r ing -The date or season that twig g i rd l e r s were
at tacked by p a r a s i t e s or p reda to r s could not be determined by cage
rea r ing t r i a l s . Very few g i rd le r s emerged from branches in the
cages . More adult g i rd l e r s emerged from those branches collected
in Axigust; thus these cages were apparent ly not suitable for rear ing
mesqui te twig g i r d l e r s . The low nurabers of beet les r ea red is
probably due to low t empera tu re and the unnatural position of the
b ranches in the cage.
Many insects were r ea red from girdled mesqui te b ranches ,
soaie of which were p a r a s i t e s and p r e d a t o r s , while o thers a r e
believed to compete with twig g i rd le r larvae for food and space.
The nua ie r ica l re la t ionship between mesqui te twig g i rd le r s and
other insec ts occupying girdled b ranches is shown in Figures 9 and
10 for b ranches collected in Garza and Ward Counties, respect ively .
The species of insects r ea r ed from branches collected in Ward and
Garza Counties were the s ame , but the i r numbers differed great ly .
All Coleoplera , except Cler idae , compete with ^ rhodosticta Bates
47
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51
for food and space, with bostrichids probably the most imporant
competitor. The larvae of clerids are presumed to be predacious
on both mesquit, twig girdler larvae and all other immature insects
within girdled branches. Ichneumonids, chalcids, braconids, other
Hymenoptera and Diptera are parasites of insects within girdled
branches; however, no ichneumonids were dissected from twig girdler
galleries. From both sites, numerous bostrichids of the genera
Xylobiops and Amphicerus were reared, with Xylobiops emerging in
much greater numbers than Amphicerus. Ichneumonids were reared
in the greatest numbers in Garza County fFigure 9), while in Ward
County, chalcids v/ere the most nuoierous parasites (Figure 10).
Biological Check Method
The mean number of twig girdler larvae in mesquite branches
was not significantly increased nor was the mean number of parasites
and predators significantly decreased by the biological check method
used in this study (0. 05 level). Parasites probably attack the larvae
in the spring (Table 6), thus the biological check method which was
begun in July had little or no effect on parasites and predators of the
larvae. The biological check raethod might have been effective if
ant activity had been increased between April and June; however more
branches would probably have to be used to detect significant differ
ences between the treatraent and the control.
52
Effect of Teraperature on Twig Girdler Larvae
Survival of mesquite twig girdler larvae was not successfully
increased by the methods of habitat naanipulation used in this study.
In Ward County, girdled branches left intact, in shade on the ground,
or hanging in raesquite trees, had significantly more surviving lar
vae than branches placed on tall grass, or on bare ground (0. 05
level) (Table 7). In Garza County girdled branches left intact con
tained significantly more larvae than any of the treatments, while
those placed on the ground in the shade and those hanging in trees
contained significantly more than those on tall grass or those on
bare ground (0. 05 level) ' Table 7). Branches nearer the ground
were exposed to higher temperatures, and had higher larval mor
tality. Higher temperatures did not decrease the time required to
complete the life cycle. Thus, if mesquite twig girdler larvae infested
branches are used for colonization, they should be shaded or placed
in a situation simulating natural conditions.
There were no statistical correlations between the maximum
air temperatures corresponding to the five twig placement areas and
the mean numbers of larvae surviving under these t reatraents (0, 05
level); however, it appears obvious that excessive temperatures
reduced larval survival. The data obtained in this study did not
substantiate High's theory (1915). Our data indicated that unless
severed branches were shaded, the larvae would die frora excessive
T a b l e 7. Mean no . of l a r v a e of O^ r h o d o s t i c t a Ba tes p e r b r a n c h u n d e r five t e m p e r a t u r e r e g i m e s in Ward and G a r z a C o u n t i e s , T e x a s . June 26 to Augus t 28, 1970.
Mean N u m b e r of L a r v a e / B r a n c h 1/
L o c a t i o n (County)
In shade Hanging C o n t r o l on ground in t r e e
(1) (2) (3)
M a x i m u m te rap . 40 °C 42 °C 40°C
On ta l l On b a r e g r a s s ground
(4) (5)
46°C 61°C
W a r d 2. 32 1. 92 1, 86 0. 12 0
G a r z a 3. 52 1. 88 1, 58 0 0
1/ M e a n s u n d e r l i n e d by the s a m e l ine a r e not s igni f icant ly diff e r e n t a t the 5% l e v e l , ^Duncan 's n e w - m u l t i p l e range t e s t ) .
53
54
heat , even though this study did not produce s ta t i s t ica l ly conclusive
r e s u l t s .
T e m p e r a t u r e s inside tb.e ga l le r ies would have been a more
meaningful m e a s u r e m e n t and inight have resul ted in a negative cor
re la t ion between t empera tu re and the mean number of surviving l a r
vae .
Branch Method of Girdler Colonization
Placing irrfested b ranches under the shade of mesqui te t r e e s
resu l ted in 3. 5 to 15 t imes more effective introductions than shading
b ranches with cp.rclboard (Table 8). Covering branches with card-
boa,rd may have cooled the branches so much that the larvae did not
develop. Even those branches collected in ear ly August and covered
with cardboard did not produce many adult bee t les , although these
b ranches remained under the more favorable na tura l conditions for a
longer per iod.
This study indicated that if b ranches a r e used for introduction
purposes they should be collected in May. This would se rve two pur
poses ; f i rs t , by collecting the b ranches ear ly , only a few pa ra s i t e s
and p r e d a t o r s would be introduced; second, when the branches a r e
collected ea r ly the twig g i r d l e r ' s gal lery is not large enough to
weaken the branch, and the branches can be collected without being
broken, resul t ing in death of the l a rvae . P e r h a p s , adult emergence
55
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57
might be higher if the branches were placed vertically, and well above
the soil surface to stimulate the natural micro-climate.
Adult Method of Girdler Colonization
The introduction of adult beetles on the Dennis Ranch in Borden
County resulted in about 5 times more girdled branches than similar
releases at two other locations (Table 9). There is no obvious explan
ation for this difference nor the difference in the nurabers of girdlers/
tree. The beetles were collected in the same locality and released
on the same date at both the Koosaian and Dennis Ranches, there
fore the females in both introductions had done about the same amount
of girdling prior to being collected. The success indices for the
Caswell -Farms and Koosman Ranches were about equal even though
over twice as many beetles were released at the Caswell Farms.
The branch and adult colonization attempts are compared in
Table 10. These comparisons were made on the basis of the number
of adult girdlers the branches would have produced had they been left
intact and under natural conditions. On this basis the adult release
method was many times more successful than the infested branch
method.
Both colonization methods would have advantages and disadvan
tages. Adult beetles would need to be collected before the females
have laid their eggs, while branches can be collected over a period
58
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T a b l e 10. A c o m p a r i s o n of adu l t and l a r v a l m e t h o d s of co lon iza t ion of Q. r h o d o s t i c t a Ba tes in T e x a s . 1970.
59
L a r v a e
L o c a t i o n
L - 7 R a n c h C r o s b y Co,
W i n d e r I a r m s Lynn Co,
Eva lua t i on index 1/
0. 2
0. 6
Adul ts
Loca t ion
C a s w e l l F a r m s Lynn Co.
Dennis Ranch B o r d e n Co.
Eva lua t ion index ^ /
56. 0
280. 0
S te phens I a r m s Borden Co.
0. 8 Koosraan Ranch Dickens Co.
59. 0
7- B a r Ranch D i c k e n s Co.
0. 6
S c h a r b a i i c r Ranch 2, 5 A n d r e w s Co,
S c h a r b a u e r Ranch 3 ,0 A n d r e w s Co.
1 / Mean no. b r a n c h e s g i r d l e d / 5 2 m a r k e d t r e e s No. b r a n c h e s i n t roduced X no. adu l t s p r o d u c e d / b r a n c h
in c o n t r o l .
2 / Mean no. b r a n c h e s g i r d l e d / 5 2 m a r k e d t r e e s -^ , ^5 No. of a d u l t s i n t roduced .
X 10
60
of several months. Adult beetles must be collected during the brief
period prior to girdling and ovipositing activities, thus requiring
several collectors over a short period of time; however, once the
beetles have been collected they can easily be released. Large
numbers of branches can be collected with a small amount of man
power; however, branches are bulky and hard to handle and they
also must be protected from heat by simulating the natural micro
climate as nearly as possible.
CHAPTER IV
SUMMARY
Adult mesqui te twig g i rd le r s began emerging in late August
and ea r ly September . Peak population densi t ies occur red in mid-
September and population densi t ies ranged from 0. 1 to 4. 7 g i r d l e r s /
t r e e . These beet les lived from 20 to 30 days. Each female girdled
1 to 2 b ranches and deposited about 8 eggs /b ranch , with an eventual
brood success of 20 to 24%, the mean diameter of girdled branches
was about 9. 5 mm (range 6 to 20 mm).
Under na tura l conditions mesquite twig g i rd le r s , O. rhodo-
st ic ta Ba tes , killed up to 36% of the crown volume of mesquite t r e e s .
The removal of the b ranches from the t r ee probably reduced the
amount of s tored carbohydra tes available to the plant, thus weaken
ing it. In cage t r i a l s , at a population density of 2 6 / t r e e , these
g i r d l e r s killed 75% of the crown volume of mesquite t r e e s . Inc reas
ing the population density of bee t les , did not resul t in an increase in
the mean d iamete r of girdled b ranches . Thus, g i rd le r s probably
w^ould not a t tack l a r g e r b ranches even at high population densi t ies .
P a r a s i t e s and p r e d a t o r s killed about 20% of the potential brood,
while about 55% to 60% of the brood died from unknown causes . Ten to
61
62
t w e l v e s p e c i e s of p a r a s i t e s and two s p e c i e s of p r e d a t o r s w e r e found
a t t a c k i n g the twig g i r d l e r l a r v a e . An e x p e r i m e n t des igned to aug-
r aen t n a t u r a l popu la t ions of g i r d l e r s by us ing an t s to d i s c o u r a g e
p a r a s i t e s was p r o b a b l y u n s u c c e s s f u l b e c a u s e it was i m p l e m e n t e d
s u b s e q u e n t to the p a r a s i t e a t t ack .
L a r v a l s u r v i v a l was not i n c r e a s e d by the l a r v a l hab i t a t m a n i
pu l a t i on r ae thods u s e d in th i s s tudy. U n l e s s the b r a n c h e s w e r e p r o
t e c t e d f r o m e x c e s s i v e hea t the l a r v a e died. In th is study the m e a n
n u m b e r of l a r v a e s u r v i v i n g / b r a n c h u n d e r n a t u r a l condi t ions and in
s h a d e d b r a n c h e s w e r e s igni f icant ly h ighe r than in n o n - s h a d e d b r a n c h e s .
N a t u r a l popula t ion of m e s q u i t e twig g i r d l e r s w e r e e x t r e m e l y
l o c a l i z e d and i so l a t ed g e o g r a p h i c a l l y . Twig g i r d l e r s w e r e r e l e a s e d
into un in fe s t ed a r e a s , us ing both adu l t s and l a r v a e . In t roduc t ions
u s i n g adu l t s a p p e a r e d m u c h m o r e succe s s fu l than l a r v a l i n t roduc t i ons ;
h o w e v e r , it w i l l take s e v e r a l y e a r s to d e t e r m i n e if co loniza t ion
a t t e m p t s w e r e s u c c e s s f u l .
The a c t i v i t y of the m e s q u i t e t r i g g i r d l e r has been shown to
p a r t i a l l y c o n t r o l m e s q u i t e in s e v e r a l a r e a s in T e x a s ; h o w e v e r , s e v e r a l
f a c t o r s m u s t be c o n s i d e r e d in eva lua t ing th is i n sec t a s a b io log ica l
c o n t r o l agen t . In heav i ly infes ted a r e a s g i r d l e r s r educe the c rowns
of m e s q u i t e thus d e c r e a s i n g i ts shading e f fec ts . Th i s pruning m a y
a l s o r e d u c e the a m o u n t of s t o r e d c a r b o h y d r a t e s a v a i l a b l e , thus
63
dec reas ing the vigor and ability of mesqui te to compete with other
plants for water and nu t r i en t s . Gird ler activity has also been
observed to c rea te conditions favorable for the attack of mesqui te
by other insects such as flat-headed b o r e r s (Buprest idae). The
raesqui te twig crirdler 's inability to corapletely control mesqui te
may be explained in seve ra l ways: (1) the broad success of this
insect is fairly high compared to that of other insects ; however, the
fecundity is ve ry low; and (2) these insects tend to exist in localized
infestat ions and a r e slow to d i spe r se . While girdling activity causes
an immedia te dec rease in the crov/n volume of mesqui te , this dam
age may eventually inc rease the basa l or crown a rea due to sprout
ing of sub te r ranean and a e r i a l l a t e ra l buds.
Studies concerning the effect of sustained heavy twig girdler
act ivi ty on the total carbohydrate r e s e r v e s and vigor of mesqui te ,
and on forage production would contribute significantly to the evalua
tion of this insect as a biological control agent.
LITERATURE CITED
Bar t l e t t , B. R. 1961. The influence of ants upon p a r a s i t e s , p reda t o r s , and scale insec ts . Ann. Entomol. Soc. Amer . 54:543-51.
Ba tes , H. W. 1885. Class Insecta , Order Coleoptera, Tr ibe Longicornia . Biol. Cent, A m e r . , Coleopt. 5:367.
Cra ighead, F . C. 1923. North Amer ican cerambycid l a rvae . Can. Dep. Agr . Bull. 27 (n. s. ).
DeBach, P . , and B. R. Bart letL 1964. Methods of colonization r e c o v e r y and evaluation, p. 402-26. ^ P . DeBach, (ed. ) Biological control of insect pests and weeds, Reinhold Publishing Corp. , New York.
DeBach, P . , C. A. F leschner , and E. J . Dietr ick. 1951. A biological check method of evaluating the effectiveness of entomophagous insec ts . Jour . Econ. Entomol. 44:763-5.
Dyar, H. 1890. The number of molts of Lepidopterous l a rvae . P syche . 5;420.
Es s ig , E, O. 1926. Insects of wes tern North Amer ica . Macmlllan & Co. , New York. 460 p.
F l a n d e r s , S. E. 1951. The role of the ant in the biological control of homopterous insec ts . Can. Entomol. 83:93-8.
High, M. M. 1915. The huisache g i rd le r . U. S. Dep. Agr. Bull. 184. p. 1-9.
Holloway, J. K. 1964. P ro jec t s in biological control of weeds, p. 650-70. ln_ P . DeBach, (ed. ) Biological control of insect pes t s and weeds . Reinhold Publishing Corp. , New York.
Howard, L. O. 1900. A t roublesome twig g i rd ler of the southwest. U . S . Dep. Agr. Div. Entomol. (n. s. ) Bull. 22:94-5.
Lins ley , E. G, 1940. Notes on Oncideres twig g i rd l e r s . J. Econ. Entoraol . 33:562,
64
65
Meyer , R. E. , H. L. Morton, R. H. Haas, E. D. Robins and T. E. Riley, 1971. Morphology and anatomy of honey mesqui te . Agr . Res . Serv ice , U. S. Dep. Agr. Bull. 1423. 186 p.
Smith, N. H. , and C. A. Rechenthin. 1964. Grass land r e s to ra t i on - -The Texas b rush problem. U.S . Dep. Agr. Soil Conserv. Se rv i ce , Temple , Texas .
Snedecor , G. W. , and W. G. Cochran. 1967, Stat is t ical methods . Sixth Edition. Iowa State Univ. P r e s s . 593 p.
Steel , R. G. D. , and J. H. T o r r i e . I960. Pr incip les and procedures of s t a t i s t i c s , McGraw-Hil l Book Company, Inc, , New York. 481 p.
Swenson, W. H. 1969. Comparison of insects on mesquite in burned and unburned a r e a s . M.S. Thes is . Texas Tech. Coll. , Lubbock, Texas . 62 p.
Uecker t , D. N. , K. L. Polk, and C, R. Ward. 1971. Mesquite twig g i rd le r : a possible means of mesquite control. Jour . Range Manage. 24:116-18.
