Is aggregative oviposition beneficial to emerald ash borer fitness?

Background/Question/Methods

A breadth of ecological, behavioral, and environmental factors affect the oviposition decisions faced by phytophagous insects, thus better understanding oviposition behavior may lead to more effective approaches for managing invasive insects. Emerald ash borer (EAB, Agrilus planipennis) is one such invasive insect that is decimating ash (Fraxinus spp.) populations across eastern North America, and understanding female oviposition behavior could lead to novel management techniques that slow its spread. By selecting host trees already infested with larvae, females could increase offspring fitness, due to the reduction in host tree defenses by previous herbivory; however, this result is not consistent in the literature. We investigated the association between EAB larval biomass and density (number per m² phloem) in white ash (F. americana) trees in a fragmented eastern deciduous forest. Randomly selected EAB-infested trees (n=4) were debarked (1.25-1.75 m from ground) to expose and collect developing larvae, and available phloem diameter was measured at 1.5 m. The azimuth for each larval gallery was recorded to detect spatial orientation on the stem. 

Results/Conclusions

A majority of the larvae were congregated on the north-northeast side of the stem, with a positive linear relationship between larval biomass and density (R=0.84, p=0.16), despite the small sample size. These results indicate that aggregative ovipositioning by female EABs does occur and may increase offspring fitness, as suggested by another study with an invasive phytophagous beetle. Aside from directly influencing larvae fitness, aggregative oviposition may reduce the ovipositioning time for female conspecifics, while increasing their larvae’s ability to effectively feed on the host tree. Many existing EAB distribution models only use geographic and forest features as variables affecting EAB dispersal; however, our results suggest that oviposition behavior has major implications regarding actual EAB population dynamics. These results should therefore be considered as future distribution models are developed to help better understand and control EAB’s infestation in North America.