Spread rates do not necessarily predict outbreak dynamics in a broadly distributed invasive insect

Abstract

Biological invasions by insects entail several processes including spread and outbreaks that can cause widespread tree mortality. These processes are commonly viewed as a linear sequence because we often lack the mechanistic understanding needed to partition them. We explored an invasion that occurred over a large enough spatial scale and time frame to examine the conditions under which spread rates may be uncoupled from outbreak severity. Lymantria dispar is an invasive generalist folivore that has spread continuously from its initial point of introduction in North America at variable rates. Since its establishment in Wisconsin, USA in 1998, L. dispar has spread more rapidly there than any other region, but has never irrupted into regionwide outbreaks. In contrast, L. dispar spread more slowly elsewhere, in part due to strong mate-finding Allee effects, but periodically undergoes highly damaging, landscape-scale outbreaks in susceptible forest types. We conducted a 10-year study across 45 stands in northern Wisconsin with a high composition of favorable hosts. We quantified population trends of life stages, natural enemy abundance and impacts, and radial tree growth increment, and assessed biotic and abiotic factors as putative drivers of the decoupling between spread rates and outbreak severity. We then compared the results from these rapid-spread, low-impact sites with those from slow-spread, high-impact sites in Michigan and Pennsylvania, USA. The abundance and richness of natural enemy communities in Wisconsin were either equivalent to or less than those communities reported elsewhere, reducing the likelihood that greater top-down control explains the absence of outbreaks. We also found no differences in larval developmental time or drought that could increase the effects of natural enemies among the different regions. Rather, higher overwintering mortality due to more extreme and frequent lethal minimal temperatures in Wisconsin appeared most responsible for the lack of outbreaks. Overall, it appears that the combination of strong mate-finding Allee effects and low overwintering mortality elsewhere is a more substantial hindrance to spread than are weak mate-finding Allee effects and high overwintering mortality in Wisconsin. Once populations are established, the net contributions of these constraints become reversed: mate-finding limitations become only a minimal challenge whereas regional differences in winter survival best explain differences in outbreak frequency. These results suggest that spread rates of invasive insects should not be used as a proxy for predicting their local or overall impact, and that management efforts should be tailored to underlying processes determining their region-specific relative importance.