THE JOURNAL OF EXPERIMENTAL ZOOLOGY 220:117-120 11982)

RAP I D CO M M U N I CAT I 0 N Host Location by the Japanese Beetle: Evidence for a Key Role for Olfaction in a Highly Polyphagous Insect

SAM1 AHMAD Department of Entomology and Economic Zoology, New Jersey Agricultural Experiment Station, Cook College, Rutgers. The State University, New Bruns- wick, New Jersey a89a7

ABSTRACT Olfaction plays a key role in host location and possibly also in host preference in a highly polyphagous insect, the Japanese beetle, Popillia japonica Newman. Moreover, this study also shows for the first time that impair- ment in olfaction by antennectomy affects feeding intensity. This suggests prob- able integration of olfactory and gustatory inputs in eliciting maximal/optimal feeding response.

Secondary plant substances known as allelo- chemics (Whittaker and Feeny, '71) guide herb- ivore insects in their search and location of suitable host plants (Fraenkel, '59; Dethier et al.. '60; Visser et al., '79; Cates, '80). In many specialist species, i.e., monophagous and oligo- phagous, the host location and selection have been shown to be dependent on one or more volatile plant attractants (Visser et al., '79; and other Refs. therein). However, except for work on a few acridids, which are generally known to be attracted to vertical objects as well as plant scents, host location mechanisms in most other polyphagous species are little known. The Japanese beetle, Popillia japonica New- man, also apparently is attracted by odors emanating from its host plants. This old obser- vation has been the basis for mass-trapping of beetles using geraniol or a mixture of phen- ethyl propionate and eugenol (Ladd Jr. et al., '75, '76). The natural attractants have not been identified, however, and the overall role of olfaction in host exploitation also remains, in large measure, unknown. I now provide evi- dence that olfaction plays a key role in host location and possibly also in host preference of the Japanese beetle. This report also shows for the first time that impairment in olfaction by antennectomy affects feeding intensity, sug- gesting probable integration of olfactory and gustatory inputs in eliciting maximal/optimal feeding response.

MATERIALS AND METHODS

Feeding behavior and foodplant preference were examined with the use of a leaf-disc tech- nique (Jermy et al., '68). The tests were de- signed to include five plant species known to be attacked by the beetle to varying degrees in

the field (Fleming, 72); common sassafras (Sassafras albidum), broccoli (Brassica oleraceae botrytis), weigela ( Weigela florida), perennial phlox (Phlox paniculata), and Chi- nese holly (Ilex cornuta). Leaf discs of these plants, 15 mm in diameter, were punched out and immediately weighed. Each disc was then fixed on a pin between two pieces of celluloid film (2 X 2 mm, 0.2 mm thickness) for support, and pin stuck into paraffin base of the test con- tainer so that each disc was exactly 1.5 cm from the metal screen that separated the feed- ing chamber from paraffin base (see Jermy et al., '68, for details of design of the test con- tainer). Leaf discs were arranged in a circular fashion, and one female beetle was placed at the center of the test container. The bioassays were terminated when the consumption of any of the five plants reached ca. 50% of the total disc area of that plant. By terminating the experiment at T50r enough of the most-con- sumed plant remained so that the insect was not forced to locate or feed on other plants simply because of starvation. Since the leaves were preweighed. accurate assessment of the consumption of the leaf (area vs. mass) was possible despite unequal thickness of the foli- age of the various species. The area of leaf disc consumed was determined with the use of LI-3000'" portable area meter (LI-COR, Lin- coln, Nebraska). In each bioassay, time taken to first location of plant discs, total time for continuous feeding episodes, and the number of probing events (testing of foodplant before initiation of actual feeding) were recorded.

RESULTS AND DISCUSSION

Data in Table 1 show the order of preference to be: sassafras > broccoli = phlox, with no

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118 S. AHMAD

TABLE 1. Location and consumption of leaf discs (1.77 cm') of five plant species by the female Japanese beetles. Plant sDecies were: sassafras (A): broccoli (B); phlox (C); weiaela (D); and Chinese holly (E)

Mean consumotion f S.E.'

Plant species

A B c D E

B C D E

C D E

D E

E

A B C

A B C

"

4 (4) 4 10) 4 10)

4 10) 4 (0)

4 (0)

Leaf Leaf area area

(cm? I%)

Experiment 1: A+B+C+D+E 0.941 k 0.090 a 0.076 k 0.003 b 0.075 f 0.002 b 0 0 0 0

53.2 k 5.2 a 4.3 f 0.2 b 4.2 f 0.1 b

Experiment 2: B+C+D+E 0.775 f 0.029 a 0.599 f 0.027 a 0 0 0 0

43.8 f 1.6 a 33.8 f 1.5 a

Experiment 3: C+D+E 0.655 f 0.074 0 0 0 0

33.6 f 1.1

Experiment 4: D+E4 0 0 0 0

Experiment 5: E4 0 0

Experiment 6: A+B+C+D+E5 Immediately after antennectomy 0.085 * 0.061 a 0.040 f 0.040 a 0.063 f 0.042 a

4.8 f 3.4 a 3.4 k 2.3 a 3.5 f 2.4 a

48 hr after antennectomy 0.150 f 0.140 a 0.078 k 0.064 a 0.078 k 0.039 a

8.5 f 0.8 a 4.4 f 3.6 a 4.4 f 2.2 a

9.1 k 1.0 a 1.2 f 0.1 b 1.7 +<0.1 b 0 0

12.5 f 0.6 a 13.2 f 0.8 a 0 0

14.7 f 0.7 0 0

0 0

0

0.8 f 0.5 a 1.0 f 0.9 a 1.4 f 0.7 a

1.4 k 0.1 a 1.3 k 1.5 a 1.7 f 0.6 a

'Total number of replicates, with number of replicates in parentheses on which actual location and feeding was observed and quantified. 'Means followed by the same letter are not significantly (P > 0.05) different by multiple comparisons using LSD method. 3These calculations are based on fresh weights of leaf discs (1.77 cm') as follows: sassafras 117.10 mg f 0.19 S.E.); broccoli (28.59 mg f 0.75 S.E.); phlox (39.09) mg f 0.30 S.E.); weigela (39.58 mg f 0.49 S.E.); and Chinese holly (61.20 mg f 1.60 S.E.). 4No feeding was recorded on either weigela or Chinese holly, as in Experiments 1-5. However, in Experiment 4, in two of the four replicates, the beetles did locate weigela discs and probed the discs with their mouthparts, but walked away without any feeding. Chinese holly was entirely ignored in all choice tests, including Experiment 5 where this was the only foodplant pro- vided in the test container. 'Leaf discs of weigela or Chinese holly were neither eaten nor probed by the antennectomized beetles.

feeding on weigela or Chinese holly. My data are at variance with those of Fleming ('72), who ranked the plants I tested in the following order of preference: sassafras > broccoli > weigela > phlox, with no feeding on Chinese holly. Fleming compiled a list of some 295 plant species that are attacked by the beetle to varying degrees, but this compilation is based on field observations only, which date back 30 to 50 years. Therefore, my results clearly indi-

cate the need for a thorough reinvestigation of the preferences of the Japanese beetles.

These experiments establish the importance of olfaction in host location in a highly poly- phagous insect. Supporting evidence for this conclusion is fivefold. (1) In Experiment 1 (Table l), the beetles showed a definite prefer- ence for sassafras; more beetles located discs of sassafras than other plants, and the con- sumption of sassafras was concomitantly

OLFACTION IN JAPANESE BEETLES 119

higher. (2) Where sassafras was excluded (Experiment 2, Table 11, more beetles located broccoli and phlox discs than in Experiment 1, with an increased consumption of these host plants. (3) Antennectomy (for removal of olfac- tory sensilla that are mainly associated with insect antenna; Schoonhoven, ’77) abolished the beetles’ ability to discriminate between sassafras, broccoli, and phlox, even following 48 hours of recovery. The encounter frequency with discs and consumption of all plants was drastically reduced compared to the normal beetles. (4) Records of “first arrival” (Table 2) clearly showed enormous delays for antennec- tomized beetles compared to normal ones in making their first contact with the host plant. ( 5 ) Finally, the number of probings clearly in- creased in the antennectomized beetles, sug- gesting that in the absence of olfactory cues the beeltes had to depend more on gustatory evaluation of foodplants (Table 2).

Reduced consumption by the antennec- tomized beetles apparently resulted from loss of olfactory location of leaf discs, and also from reduction in feeding duration. Whereas the normal beetles fed continuously for periods ranging from 1 to 2 hours, the antennectomized insects did not eat for more than 10 minutes at a time. Furthermore, the antennectomized beetles wandered aimlessly in between their short meals and, unlike the normal ones, sel- dom returned to the original host for resump- tion of feeding. Differences in feeding intensity on different foods are generally attributed to the taste, e.g., for larvae of Pieris brassicae (Schoonhoven, ’77). This is the first report that shows that removal of an insect’s antennae could also affect feeding intensity.

TABLE 2. Time for first arrival (location) on host plant and number ofprobings before initiation o f feeding o f

plant leaf discs by normal and antennectomized female Japanese beetles

x Number of Number of first arrival probings prior

x Min to

Plant replicates (range in min) to feeding (range) .. ~ - ~ ~~~~~~ ~ ~~ .

Normal beetles Sassafras 6 8.3 (1-29) 1.3 (1-2) Broccoli 4 45.8 11-120) 2.2 (2-3) Phlox 9 61.3 11-150) 3.7 (3-5)

Antennectomized beetles’ Sass a f r a s 7 150 1100-2001 3.3 13-61 Broccoli 4 240 1200-280) 5.6 (4-9) Phlox 5 105 ( 95-300) 6.0 (4-101

‘The data obtained from beetles immediately and 48 hours after excision of antennae were essentially the same and, therefore, were pooled to obtain the mean values.

In some insects random search behavior leads to chance contact with a preferred host where search is terminated by detection of “arrestant” chemicals (Dethier et al., ’60; Fraenkel, ’69). In my observations, Japanese beetles always oriented straight to leaf discs of the acceptable hosts. This was further con- firmed by observing beetle behavior in r e sponse to foliage odor (sassafras, broccoli, and phlox) using a Y-tube olfactometer (McIndoo, ’26). Thus an arrestant action does not appear to be sufficient to explain host location. The actual feeding process most likely depends on the detection of phagodeterrent and phage stimulant substances of the hosts by contact chemoreception via receptors on the mouth- parts (Schoonhoven, ’77). When beetles were offered weigela and Chinese holly (Experiment 1, Table l), some beetles did locate weigela discs, probed the discs with their mouthparts, but walked away without eating. The beetles apparently never approached Chinese holly. These observations suggest the possibility of a phagodeterrent (or lack of a phagostimulant) in weigela and a repellent in Chinese holly.

Many plant substances attractive to the Japanese beetle, e.g., acetic acid, benzal- dehyde, caproic acid, citral, citronellol, eugenol, and geraniol, occur in various com- binations in highly preferred but diverse plant taxa, e.g., Rosaceae and Lauraceae (Fleming, ’72; Klein, ’81). Other odoriferous species such as broccoli (Cruciferae) release entirely differ- ent volatiles, e.g., allylisothiocyanates (Van Etten and Tookey, ’79). I t is not clear whether the beetles respond to distinctive mixtures of these chemicals, as has been shown for the multicomponent pheromone systems (Birch, ’74), or use as olfactory cues specific chemicals common to many plants. Clarification of the chemical nature of olfactory cues for the Japanese beetle is needed, as is the neurophys- iological basis for the discrimination of poten- tially a multitude of chemosensory information.

In conclusion then, my initial studies have shown that olfaction is the “key” to host loca- tion and possibly also in host choice in the highly polyphagous Japanese beetles. More- over, the observation that impairment of olfac- tion by antennectomy not only reduces the beetles’ ability to recognize and locate host plants readily, but also reduces the extent of feeding, suggests (for the first time) probable integration of olfactory and gustatory inputs in eliciting maximdoptimal feeding response in an insect species. The impairment (or reduc- tion in input to below threshold level) in olfac-

120 S. AHMAD

tion could result in markedly reduced phago- kinesis despite beetles' contact with a highly preferred host plant such as sassafras.

ACKNOWLEDGMENTS I thank Dr. M.L. May for his critical review

of this manuscript and many valuable sugges- tions. This paper is part of the Journal Series, New Jersey Agricultural Experiment Station, Cook College, Rutgers University, New Bruns- wick, New Jersey. This work was performed under NJAES Project No. 08130, supported by Hatch Act Funds.

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