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Title: Trade-offs in parasitism efficiency and brood size mediate parasitoid coexistence, with implications for biological control of the invasive emerald ash borer

item WANG, XIAOYI - Chinese Academy Of Forestry
item JENNINGS, DAVE - University Of Maryland
item Duan, Jian

Submitted to: Journal of Applied Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2015
Publication Date: 7/18/2015
Citation: Wang, X., Jennings, D.E., Duan, J.J. 2015. Trade-offs in parasitism efficiency and brood size mediate parasitoid coexistence, with implications for biological control of the invasive emerald ash borer. Journal of Applied Ecology. 52(5):1365-2664 doi: 10.1111/1365-2664.12487.

Interpretive Summary: The emerald ash borer (EAB) has become a devastating forest pest in North America, killing hundreds of millions of native North American ash trees since its discovery in Michigan, USA in 2002. A new species of parasitic wasp from the Russian Far East is currently being approved for environmental release for biological control of EAB in the midwest and northeastern USA. Because this new parasitic wasp attacks the same larval stages of EAB as a previously introduced Chinese parasitic wasp, the potential exists for competition between these two wasps in the same habitats, and their competitive interactions need to be investigated before deciding whether or not to release the new Russian parasitic wasp in the USA. Scientists from USDA ARS, the University of Maryland and the Chinese Academy of Forestry conducted laboratory experiments to examine the potential effects of competition between these two parasitic wasps on biological control of EAB. Results from our study showed that the Russian parasitic wasp was more efficient in attacking EAB larvae, but produced fewer offspring per attacked host than the previously introduced Chinese wasp. Our results further showed that few EAB larvae were attacked simultaneously by both the Chinese and Russian wasps when confined in the same cages. These findings indicate that the two species of parasitic wasps can coexist in the same habitat, and introduction of the new Russian parasitic wasp is likely to have negligible effects on the previously introduced biocontrol agent. These findings will assist in the regulatory approval of the introduction of the Russian parasitic wasp for biological control of EAB in the U.S.

Technical Abstract: Parasitoids often are selected for use as biological control agents because of their high host specificity, yet such host specificity can result in strong interspecific competition. However, few studies have examined if and how various extrinsic factors (such as parasitism efficiency) influence the outcome of competition between parasitoids, even though they could have profound effects on the implementation of classical biological control programs. To determine the potential influence of extrinsic competition and coexistence on host suppression efficacy we compared the parasitism of two parasitoids (Tetrastichus planipennisi and Spathius galinae) of the invasive emerald ash borer (EAB), Agrilus planipennis, under different host densities, parasitoid densities, host plant sizes, exposure space, and parasitoid-host ratios. Spathius galinae had significantly higher parasitism efficiency (˜4 times), but significantly lower brood size (>6 times), than that of T. planipennisi. The attack rates of hosts increased significantly with parasitoid density, whereas host density and the spatial scale of exposure cages did not significantly affect the parasitism of both parasitoids. The parasitism rate of T. planipennisi on small host plants was significantly higher than that on large logs, while host plant size had no significant impact on S. galinae parasitism. The multiparasitism rate of hosts attacked by both parasitoids simultaneously was rather low, indicating that intrinsic competition between the two parasitoids might seldom occur in the field. The two parasitoids could therefore coexist in the same habitat, and any adverse effects on the suppression of EAB populations caused by competitive behavior between the two parasitoids would likely be negligible. Our findings suggest that multiple introductions of parasitic natural enemies are feasible for management of invasive species. Coexistence between parasitoids can be mediated by trade-offs in their parasitism efficiency and brood sizes.