COS 156-6 - Patterns in intraspecific variation in host plant populations contributes to altered tri-trophic interactions for a range-expanding insect

Abstract: Phytophagous insects are expanding their ranges poleward in response to climate change, resulting in altered tri-trophic interactions with their host plants and natural enemies. Altered direct and indirect, bottom-up and top-down interactions may result in “high-niche opportunities” for range-expanding species, leading to “ecological release” or increased performance in new locations. Intraspecific variation in some host plant populations declines towards the poles. As a result, range-expanding insects may experience a more homogenous resource base, which could affect their performance and interactions with higher trophic levels. We study a recent poleward range-expanding oak gall wasp, Neuroterus saltatorius, in Quercus garryana-dominated ecosystems in western North America. N. saltatorius is outbreaking in its expanded range, where oak patches have higher frequencies of infested trees. Here, we investigate if intraspecific variation in host plant traits, including foliar quality, differs between the native and expanded range and how differences contribute to patterns in the abundance and distribution of N. saltatorius and its natural enemies. We selected 16 trees per site (n = 11) across the native and expanded ranges. We collected leaves to assess foliar quality before and after gall formation, to assess how quality influences galls and natural enemies, and the effect of galls on host plant quality. Using imaging analysis, we calculated foliar gall damage, and measured foliar moisture, carbon, and nitrogen content. As predicted, we found higher variation in foliar damage among trees in the native range, along with higher variation in foliar quality (moisture content prior to gall formation). Overall damage and impacts on foliar quality (moisture and nitrogen loss, post-gall formation) were higher in the expanded range. We collected mature galls excluded from parasitoids to assess gall performance, measuring survival and gall size. We found greater variation in host performance, in the native range, with the gall size being larger and less variable among trees in the expanded range. We are identifying parasitoid wasps reared out of galls collected on the same trees. We will present data on additional leaf traits and impacts of variation in traits on the parasitoid community, which we predict to have lower a and b (among-tree) diversity in the expanded range. Understanding how intraspecific variation in host plant quality affects range-expanding phytophagous insects and their associated enemy community is important to uncover the outcome of biotic interactions under short-distance, poleward range expansions.