Effects of dispersal mediated by host plant quality on outbreaking winter moth (Lepidoptera: Geometridae) populations
Winter moth (Operophtera brumata) has long been a study organism for research addressing central questions in population ecology, including work conducted by Varley and Gradwell in the 1950's pioneering the use of life tables. Extensive research on winter moth in its native range combined with multiple biological control projects in its invasive range in North America have elucidated the complex role of natural enemies in the population regulation and dynamics of winter moth. Recent work in Europe has alternately suggested the centrality of top-down (specialist parasitoids) or bottom-up regulation of winter moth and closely related autumnal moth (Epirrita autumnata) population cycles, while a recent review has suggested that both top-down and bottom-up factors are likely to affect forest lepidopteran population dynamics. To address the cause of regulation of winter moth populations in the absence of natural enemies in northeastern North America, we investigated survival, dispersal and host plant interactions of the larval stage. We used a combination of observational and manipulative field and laboratory studies to assess the response of larval dispersal and survival to conspecific density, and to previous herbivory, total phenolic content, and phenolic oxidative activity of host plant foliage.
We found a significant increase in mortality due to early larval dispersal in response to conspecific density(p<0.001), and a higher rate of dispersal of larvae from previously defoliated leaves in comparison to controls (un-defoliated leaves)(p<0.001). However, neither total phenolic content or total oxidative phenolics significantly predicted survival (total phenolics: p=0.93, oxidative phenolics: p=0.96). Our results suggest that winter moth populations in the absence of their native natural enemies are regulated by density dependent larval dispersal, which is in turn triggered by an induced decline in host plant acceptability, though likely not due to induction of phenolics. Our work joins relatively few studies documenting bottom-up regulation of leaf chewing insect herbivores due to dispersal and possibly induced host defense rather than food limitation; this type of regulation is common and well documented among sap sucking insects. Detailed investigation of the major factors driving the population dynamics of outbreaking herbivores and invasive pests such as in the present study is essential to the successful implementation of biological control programs in specific cases, and more generally to understand why some exotic species become pests and others do not.