The effect of urbanization on host-parasitoid interactions of a forest defoliator
The recent increase in the frequency and scale of fall cankerworm (FCW) defoliation, particularly near the city of Richmond VA, begs the question as to whether urbanization plays a role and, if so, what is the mechanism? One hypothesis is that urbanization may alter the trophic interactions between FCW and their parasitoids. In this study we investigated the spatial patterns in abundance of the FCW and their parasitoids, and how they are affected by urbanization. We hypothesized that rates of parasitism are reduced in urban areas due to higher sensitivities to urbanization. FCW abundance was obtained by placing sticky banding material on oak trunks to trap flightless females as they ascended to mate and lay their eggs. Eggs laid on the bands were collected in the spring and examined under a microscope to determine parasitism rates. We fit the female counts to a second-order autoregressive model for direct and delayed density dependent effects of the two previous years’ outbreak intensities. We also examined the relationships between FCW abundance and parasitism rates across an urban gradient, as measured by human population density.
We found a direct density dependent effect, but no delayed density dependent effect on FCW population dynamics. This suggests that the FCW and its parasitoids are not strongly coupled as in classic host-parasitoid models. This allowed us to analyze parasitism rates as a function of female abundance without delayed effects. Contrary to our hypotheses, we found no significant change in FCW abundance across an urban gradient and a positive relationship between parasitism rates and urbanization. One possible explanation for this is that, because the parasitoids are generalists, their population size and rates of parasitism are likely affected by the abundance of many Lepidopteron species across the active season; thus, the higher parasitism rates in urban areas may reflect greater diversity in Lepidopteran life histories in urban areas. We did not find greater outbreaks of FCW in the area around Richmond, VA, nor a trophic mechanism for such a pattern. Whether the recent FCW outbreaks near Richmond were due to urban effects or some other effect remains unresolved.