Historical change in the outbreak dynamics of an invading forest insect

The population dynamics and impacts of non-native species often change following their initial establishment, with impacts either increasing or decreasing over time. The reasons why the abundance of an invading species may change are varied but often reflect changes in the way in which populations interact with resident communities. Here we analyze changes in the outbreak dynamics of Lymantria dispar (formerly known to as the "gypsy moth"), a Eurasian foliage-feeding insect that has been established in N. America for ca. 150 years. We find that during the course of this species' presence in N. America, it has continually exhibited population dynamics in which populations reach outbreak levels, resulting in defoliation of large forested areas. However, there is evidence of some changes in both the periodicity and synchrony of these outbreaks. We hypothesize that the accidental introduction of an entomopathogenic nucleopolyhedrosis virus around 1906 resulted in populations shifting from a pattern of sustained outbreaks to oscillatory dynamics with periodic outbreaks synchronized over large distances. We analyze historical L. dispar population data that provide some evidence in support of this hypothesis. There is also evidence that the more recent establishment of the fungal pathogen Entomophaga maimaiga has caused a decrease in the amplitude of L. dispar outbreaks since its emergence in 1989.