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Atanycolus cappaerti
(Hymenoptera: Braconidae)

by Ian Lane , Michigan State University

The emerald ash borer (EAB) is a serious invasive insect pest that was first discovered in southwest Michigan in 2002 and affects ash species in North America. The high rate at which the emerald ash borer has been destroying ash trees has spurred research on the potential for biological control. In particular, researchers have been searching for parasitic wasps that could be raised in large numbers and released to provide suppression of EAB.

Biological control efforts have focused primarily on identifying and releasing egg and larval parasitoids imported from EAB’s home range in Asia. The three most important natural enemies to date are the larval parasitoids Tetrastichus planipennis (Hymenoptera: Eulophidae) and Spathius agrili (Hymenoptera: Braconidae); along with an egg parasitoid named Obius agrili (Hymenoptera: Encyrtidae). In 2008 a native larval parasitoid Atanycolus cappaerti was discovered in Michigan. While it has been well documented that a species of the same genus (A. hicoriae) parasitizes the EAB, this newly described species is the first to attack at rates high enough to be considered significant for biological control. While studies are still ongoing, the use of the native parasitoid A. cappaerti for augmentative biological control would be desirable from an environmental perspective as it is already established and has a niche in the ecosystem.

Appearance

While distinguishing A. cappaerti from A. hicoriae species is difficult, narrowing it down to one of these two species is easily done by sight. The female ranges from 5-7 mm and has a ovipositor that can be as long as 6 mm.  The male is around 4 mm. The head and thorax in both species are jet black, while the abdomen is bright red (Figure 1). The wings are colored dark grey.

Habitat

Atanycolus cappaerti has so far only been identified at two sites in Michigan. Its status as a new species and the difficulty of distinguishing it from A. hicoriae mean its range is largely unknown.

Pests Attacked (Host Range)

Atanycolus cappaerti’s full host range is not yet fully known. It has been documented parasitizing the larvae of two native beetles in the genus Agrilus in addition to EAB. These include the two-lined chestnut borer (Agrilus bilineatus) and the poplar borer (Agrilus liragus). These beetles can cause mortality in trees that are under stressed conditions like drought, but do not seem to attack healthy trees.

Life Cycle

Atanycolus cappaerti is a solitary ectoparasitoid. This means that the parasitoid lays only a single egg, which hatches into a larva that feeds externally on its host.  The female adult uses her ovipositor to drill through the outer layer of bark and sap wood to parasitize wood boring larvae in their 2nd, 3rd, and 4th instars that are feeding on the phloem tissue of the tree (Figure 2); once the larvae move deeper into the wood to pupate, A. cappaerti can no longer parasitize them . Once a wasp identifies the larvae and drills down, she paralyzes it with a sting and deposits one egg on the outside of the host. This egg will hatch into a larva, which remains on the outside of its host and feeds (Figure 3) until it is ready to detach itself and pupate in the EAB gallery (Figure 4). This whole process takes around two weeks. In its cocoon A. cappaerti will pupate for about another 2 weeks and then emerge after chewing an exit hole through the bark, completing its life cycle after a total of approximately 30 days.

Figure 3. A. cappaerti Larva feeding on the outside of an emerald ash borer larva. PHOTO: David Cappaert - MSU - Bugwood.org		Figure 4. A cappaerti cocoon in an emerald ash borer gallery PHOTO: David Cappaert - MSU - Bugwood.org

The wasp pupae overwinter in cocoons in borer galleries. They emerge as adults in May before EAB and parasitize the larvae that have overwintered in their galleries. These wasp larvae then develop and emerge around July to parasitize borer larvae that hatched from eggs laid in May. This cycle will repeat until October when temperatures decline and overwintering begins again.

Relative Effectiveness and Conservation

Research into the effectiveness of A. cappaerti is ongoing but promising. Parasitism rates have been highly variable, ranging from 9 -71% among study sites.

Pesticide Susceptibility

To date there has been no research on the effect of pesticides on A. cappaerti. It would be reasonable to assume that like most natural enemies it would be highly susceptible as an adult. The host gallery and cocoon may afford it some protection as a larva, from bark-applied insecticides, though any residual topical pesticide applied to the tree’s bark may kill the newly pupated adult as it emerges. The main method of controlling EAB has been soil-applied and injected systemic insecticides. These chemicals are ingested by EAB as they eat the phloem tissue though which these pesticides are being transported. This could have serious consequences for A. cappaerti due to the death of its host or secondary consumption of the pesticide.

Commercial Availability

Currently A. cappaerti is not being reared for commercial release. The U.S. Forest Service and APHIS are currently rearing other EAB parasitoids. These should be available in the near future for release to control this pest.

Acknowledgements

I would like to acknowledge Jason Hanson, Deb Miller, Leah Bauer, and Deb McCullough for their contributions to my knowledge. I would also like to acknowledge David Cappaert for the photographs of EAB parasitoids.

References

Cappaert, D., and D.G. McCullough. 2009. Occurrence and seasonal abundance of Atanycolus cappaerti (Hymenoptera: Braconidae), a native parasitoid of emerald ash borer. Agrilus planipennis (Coleoptera: Buprestidae). Gt. Lakes Entomol. 42: 16-29

Marsh, P.M., J.S. Strazanac, S.Y. Laurusonis. 2009. Description of a new species of Atanycolus (Hymenoptera: Braconidae) from Michigan reared from the emerald ash borer. Gt. Lakes Entomol. 42: 8-15

Dunn, J.P., T.W.  Kimmerer, G.L. Nordin. 1986.  The role of host tree condition in attack of white oaks by the twolined chestnut borer, Agrilus bilineatus (Weber) (Coleoptera: Buprestidae). Oecologia 70: 596-600

Bauer L.S., H. Liu, D. Miller, J. Gould. 2008. Developing a classical biological control program for Agrilus planipennis (Coleoptera: Buprestidae), an invasive ash pest in North America. Newsletter of the Michigan Entomological Society 53: 38-39

Herms D.A., D.G. McCullough, D.R. Smitley, C.S. Sadof, R.C. Williamson, P.L. Nixon, 2009. Insecticide options for protecting ash trees from emerald ash borer. North Central IPM Center Bulletin. 12 pp. http://www.emeraldashborer.info/files/multistate_eab_insecticide_fact_sheet.pdf

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