CANCELLED - Evaluating how intraspecific variation in host plant quality influences patterns of abundance in a range-expanding gall forming wasp

Background/Question/Methods

Phytophagous insects are experiencing poleward range expansions faster than their host plants and are encountering host plant populations with lower intraspecific diversity at the poles. Intraspecific variation in host plant quality influences the abundance and distribution of insects, and in turn, insects induce changes in host plant quality. We examine how variation in host plant quality of a dominant oak in western North America, Quercus garryana, affects the abundance and distribution of a range-expanding gall wasp and how this species induces changes in host plant quality. Neuroterus saltatorius expanded its range from mainland USA to Vancouver Island, BC, where it is outbreaking. In the expanded range there are higher frequencies of infested trees, leading to high gall abundances that cause foliar necrosis. We investigated if variation in Q. garryana foliar quality between native and expanded range contributes to variation in the frequency of tree infestation. In addition, we investigated if N. saltatorius has different effects on hosts of varying quality. We collected leaves before and after gall formation from 16 trees per site (n = 9) across the native and expanded range. Using imaging analysis, we calculated foliar gall damage post gall formation and measured water, carbon, and nitrogen content.

Results/Conclusions

We found that host plants in the native range are less susceptible to the gall wasp, with lower abundances on infested trees and more variation in the frequency of infested trees compared to the expanded range. Sites in the native range had higher variability in water and C:N content in pre gall formation leaves. Lower wasp abundances in the native range could be a result of lower foliar quality, and a more patchy distribution could be due to greater variation in foliar quality among trees. Post gall formation, we found a negative relationship between foliar damage and moisture content in both the native and expanded range and a positive relationship between leaf damage and C:N in both ranges. N. saltatorius caused more foliar damage in the expanded range thus it causes a greater reduction in foliar quality. Our results suggest more homogenous host plant populations could be causing lower variation in host plant quality and higher frequencies of or less patchy distribution of infested trees in the expanded range. Understanding how intraspecific variation in host plant quality affects phytophagous insects experiencing range expansions is important to uncover the outcome of biotic interactions under range expansions.