Emerald Ash Borer:

the Complexities of a Catastrophic Invader

Deborah G. McCullough, Professor

Dept. of Entomology & Dept. of Forestry

Michigan State University

Fraxinus sp.

www.emeraldashborer.info

≈1990: EAB becomes established in southeast Michigan.

2002: EAB “discovered” & identified as Agrilus planipennis

2017: EAB in 31 states & 3 Canadian provinces

EAB is native to China, Korea, Mongolia & E. Russia.

Likely arrived in solid wood packing material from Asia.

Beetles feed on ash foliage throughout their

3-6 week life span. Oviposition begins 2-3

weeks after emergence. Probably 50-60

eggs per female; range is 2 to 268 eggs.

EAB adults select hosts for feeding & egg-laying

Egg hatching

Larvae feed on phloem & cambium in S-shaped galleries

from mid summer to fall. Complete 4 instars. Most larvae

overwinter as prepupae in outer sapwood or outer bark.

Prepupal

larvae L2, L3 & L4

larvae in Sept. Gallery

Pupation occurs in spring.

New adults emerge from

D-shaped exit holes

beginning around 450

DD50 F (mid May to June).

Pupation (≈14 days)

As larval density builds, galleries disrupt translocation.

Woodpeckers prey on larvae (mostly in winter) & holes are

often the 1st sign of EAB. Eventually, foliage thins, branches

die, bark cracks & epicormic sprouts appear.

EAB is already the most destructive forest insect to ever

invade North America. Hundreds of millions of ash

(Fraxinus spp.) in landscapes & forests have been killed.

Sept. 2017: Ash is “red-listed” by IUCN.

Very difficult to detect, delimit & survey EAB

1. No external signs or symptoms at low larval densities.

2. Two-year EAB life cycle in healthy, newly-infested trees.

3. No long range pheromones.

4. Girdled & debarked ash most effective, but not often used.

5. Canopy traps & lures are not highly effective.

USDA APHIS prism &

funnel traps in ash trees Double-decker Girdled trees - debarked

N

S

W E

SE

NW NE

SW

0

0 - 900

900 - 2000

2000 - 3598

Ash Phloem Area (m2)

400 400 0 800 m

N

EW

S

N

W E

S

Adult EAB dispersal: Most females lay eggs within 100

meters of their emergence point. But… a few females

(maybe 1 to 5%?) disperse 1-5 km (?). This “long range”

dispersal is unpredictable.

Mercader et al. 2009, 2011, 2012

McCullough et al. 2011; Siegert et al. 2010

Pro

bab

ility n

ot d

etected

Years after establishment

Actual EAB distribution is likely 4-6 km or more beyond the

detection threshold, even 6 years after establishment. Satellite

populations “simmer” for at least 4 years before detection.

Black ash

Adult EAB host preference varies among ash species

Green ash Blue ash White ash

Less preferred Preferred &

vulnerable

Anulewicz et al. 2007; Rebek et al. 2007; Chen & Poland 2010,

Tanis & McCullough 2012, 2015; Robinett & McCullough 201X

Core: Southeast, near EAB origin

Crest: Central; Densities peaking

Cusp: Southwest; EAB more recent

Green ash condition at 3 stages of the EAB invasion

Core - SE Crest - central Cusp - SW

Surveyed EAB density, overstory &

regeneration in 1 ha areas in 2010 &

2011 (8 sites per stage).

Green ash comprised 20-50% of

the overstory in all 24 sites.

Average percent of green ash basal area that was alive

declined as EAB invasion progressed (n=8 sites per stage)

Core Crest Cusp

2010 2011 2010 2011 2010 2011

8% 5% 63% 45% 95% 85%

0

2

4

6

8

Core Crest Cusp

Live

as

basa

l are

a (m

² pe

r ha

)

2010

2011

a

z

y

x

c

b

Core Crest Cusp

Live basal area (1035 ash trees) No ash seedlings with

cotyledons in Core sites

Burr & McCullough. 2014. Can J For Res.

PAR intercepted by other overstory species in Core sites

Photosynthetically active radiation (PAR) was lower in

Core & Cusp than in Crest sites.

Lateral in-growth of non-ash tree canopies filled most

canopy gaps & intercepted PAR in Core sites.

Ash recruits require sun to reach the overstory.

Green ash persistence?

0%

4%

8%

12%

16%

20%

Core Crest Cusp

PA

R a

s a

per

cen

tag

e o

f fu

ll s

un

Mean ( SE) PAR - 2011

a

bb

Green ash were inventoried (by DBH class) in 2007 in 2

sites, each 16.2 ha, then re-surveyed in 2015. From 2007 to

2013, ≥ 8 ash were felled & debarked annually to monitor

EAB larval density.

Jasper: Long-Term Evaluation of EAB & Green Ash

400 m

800 m

50 x 50 m grids

Density of EAB larvae increased exponentially.

Few green ash >10 cm DBH were alive in 2015.

0

10

20

30

40

50

60

70

80

90

2007 2008 2009 2010 2011 2012 2013

Average (+SE) Live EAB larvae per m2

0

500

1000

1500

2000

2500

J3 J5

No. live green ash inventoried

2007

2015

Green ash saplings & recruits are abundant.

2015 J3 J5

No. ash recruits per ha 784 1393

Percent alive in 2015 46% 58%

Size & persistence of canopy gaps, plus EAB dynamics,

will determine whether young green ash reach the overstory.

Green ash could be functionally lost from many sites.

Current options for managing EAB & ash trees

A. Remove infested trees (or pre-salvage ash timber)

B. Insecticides (systemics)

C. Girdled trees: Function as “sinks” to attract EAB adults

then debarked, sectioned or destroyed to kill larvae.

D. Natural enemies & biological control

E. Integrated management – combinations of A-D

Ash trees in landscapes generally killed if not protected

with effective insecticides. Dead trees are hazards. Tree

removal is costly & unpleasant.

Ohio

Minnesota

Lansing, MI

Saginaw, MI Shields, MI

Systemic insecticide products & application methods are

much improved compared to the early days of EAB

TreeAzin

Safari basal trunk spray

Imidacloprid soil

drench; (2x rate) TREE-äge

TREE-äge

Emamectin benzoate (TREE-äge): Trunk injection in

spring provides 3 years of nearly 100% EAB control,

even at the lowest application rate.

TREE-äge

Control

Relatively new systemic insecticide options

Products (active ingredient) Treat

TREE-äge - trunk injection

(emamectin benzoate)

2-3 years

TreeAzin - trunk injection

(azadirachtin)

1-2 years

Azasol - trunk injection

(azadirachtin)

1-2 years

Safari, Transtect – basal trunk spray

(dinotefuran)

1 year

Imidacloprid products - 2x rate

spring soil drench or soil injection

1 year

Information on insecticides for EAB & ash protection

available free at www.emeraldashborer.info

Effects of EAB insecticide treatments on

pollinators. EAB University Webinar, Oct.

2017. Dr. Reed Johnson, Dept of Ent., OSU

Avoid neo-nic applications to soil if flowering plants are near the

tree. Otherwise, few non-target impacts of systemic insecticides

used on ash. All aspects of insecticide registration are regulated

by EPA under FIFRA.

Risk assessment of imidacloprid use in

forest settings on the aquatic macroinvert.

community. Environ. Tox. Benton et al.

2017.

Economics favor treating ash trees; much less costly &

disruptive than removals.

Kovacs et al. 2010; 2014; McCullough & Mercader 2012;

McKenney & Pedlar 2012; Sadof et al. 2017; Vannatta et al. 2012

Treating mature landscape trees retains ecosystem

services & property values.

Allows for long-term planning & staged replacement of

ash trees in municipalities (e.g., 20-30 year period).

Forested settings: systemic insecticides are rarely used.

Widespread use limited by costs & per acre restrictions.

EmBen - PA DCNR protects white ash in seed orchards.

EmBen - some Native American tribes protect individual

black ash trees for seed collection (cultural resource).

Insecticide use on private forest land occurs occasionally;

usually a few mature trees are protected.

EA’s (environmental assessment)

were prepared for national forest lands

(e.g., for SLAM Pilot Project).

Some insecticide use by NPS, state

DNRs, etc. to protect individual trees

in campgrounds, walkways, etc.

SLAM Pilot Project: 2008-2012 (> 390 km2 area)

Tiny proportion of ash trees along roads were injected with

EmBen. Treatment slowed EAB population growth 2+ years.

Grids of girdled trees used for EAB detection & as “sinks.”

Girdled trees slowed EAB pop. growth & spread for 1 year.

McCullough et al. 2015

Mercader et al. 2013, 2015, 2016

McCullough SLAM webinar - 2015

Natural Enemies & Biological Control

WP predation is the major source of EAB

mortality but rates are highly variable.

WPs prey on late instar EAB larvae &

prepupae in winter & early spring.

WP holes are often the first sign of EAB;

Flecking occurs on heavily infested trees.

No evidence that WPs can slow EAB

population growth or rates of ash mortality.

Woodpecker Predation

Behavioral response to EAB by WP

populations but numerical response = ?

Native parasitoids, particularly

Atanycolus spp., are increasingly

common.

Native parasitoids are clearly

learning to search ash trees for

EAB larvae.

Highest parasitism rates usually

in heavily infested trees (attraction

to stress volatiles).

Potentially more important in the

EAB Core, post-invasion?

Native Parasitoids A. cappaerti

Phasgonophora sulcata

A. cappaerti

USDA APHIS, ARS & USFS are spending $ millions on

imported parasitoids. First releases were in MI in 2007.

APHIS releasing >280,000 wasps annually since 2012.

Asian parasitoids - Classical biocontrol for EAB

Tetrastichus planipennisi Spathius agrili

Oobius agrili

(egg parasitoid)

S. galinae

Research begins in native range to assess life history, hosts,

reproduction, develop rearing methods, etc.

Insects are imported & held in quarantine labs in US for

further host range testing with North American species.

EAB parasitoids - Environmental Assessment & Federal

Register announcement in 2007

Large scale parasitoid rearing occurs at USDA APHIS

facility in Brighton, MI

State approval typically requested for releases

Ideally, releases are followed by evaluations to assess

establishment (MapBiocontrol website) but… doesn’t

always occur.

Classical biocontrol for EAB

Classical biocontrol for EAB

Spathius agrili – Cold tolerance poor. Host specificity?

Tetrastichus planipennisi – Good disperser but tiny ovipositor

Oobius agrili – Difficult to study in the field

Spathius galinae – Too soon to assess

Issues: Rearing sustainability? Requires infested ash logs

for EAB rearing; No adequate artificial diet.

Parasitoids have not slowed EAB population growth or ash

mortality rates in newly infested sites or post-invasion sites.

Little evidence of subcortical insect populations that are

regulated by predators or parasitoids.

But… few other options for ash in North American forests.

Additive or even superadditive effects could occur:

(1) Tactics target different life stage of the pest

(2) Insecticides do not interfere with natural enemies

(3) Natural enemies engage in non-random searching for

their prey or hosts.

Barclay & Li. 1991. Theor. Pop. Ecol.

Berec et al. 2007. Trends Ecol. Evol.

Sucking et al. 2012. Environ. Entomol.

Integrating tactics for EAB – systemic insecticides &

biocontrol are compatible

Focus on the insect or the host? Many Fraxinus species occur in North America (≥ 18

species) & Europe. Host preference or host suitability varies,

but EAB probably can develop on all ash species. In its native range, EAB is a secondary pest. Co-evolved

resistance breaks down - stressed trees are attractive &

vulnerable to EAB. Reducing EAB success (e.g., host-finding, reproduction,

dispersal ability?) would presumably benefit all ash species.

Other questions from the Committee

Focus on the insect or the host?

Contrast with hemlock woolly adelgid (HWA) (Adelges picea) HWA: Passive dispersal - no flight; Immature life stages

dispersed via wind, birds); No host selection;

No mating (parthenogenic) Two hosts of concern (Tsuga canadensis, T. caroliniana), while

western hemlock (T. heterophylla) is relatively resistant. Insect predators help control HWA populations in Pacific NW.

Some amount of host resistance is generally needed for

effective biocontrol.

Other questions from the Committee

Other questions from the Committee

Altering the host(s) to enhance HWA resistance (and

perhaps biocontrol) seems more likely to be successful than

modifying the pest.

HWA mutation rate: “On average, a mutation arises each

generation at every base 10 pair in the entire A. tsugae genome

within infested areas of only 40 ha” (Butin et al. 2005).

Selection for cold tolerance observed in New England