Spatial spread of introduced infectious diseases following a colonizing invasive host

Background/Question/Methods

The interactions between spatially structured host populations and the transmission dynamics of their macro- and microparasites can result in a range of spatial and temporal dynamic behavior. With increasing effects of globalization resulting in increasing biological invasions, there is much interest in understanding the spread of non-native species and their macro- and microparasites that follow, whether parasites are introduced with their hosts or are subsequently introduced as biological control agents. Moreover, changing climates could cause changes in the dynamics between interacting species. We studied interactions between the gypsy moth, Lymantria dispar, a non-native forest defoliator, and the entomophthoralean fungus Entomophaga maimaiga, the baculovirus Lymantria dispar nucleopolyhedrovirus (LdNPV), and larval parasitoids as they tracked newly-established gypsy moth populations along the leading edge of its range in central and southern Wisconsin. We collected over 4800 larvae and cadavers from a total of 37 field sites between 2005-2007, which we used to determine rates of larval infection or parasitization. We then related the presence of pathogens and parasitoids to the prior year’s background gypsy moth abundance, long-term history of gypsy moth, past deliberate introductions of natural enemies, and climate.

Results/Conclusions

Host abundance in the prior year was related to pathogen presence although E. maimaiga moved into host populations of lower densities than LdNPV. Purposeful releases of pathogens were not associated with pathogen presence, suggesting that pathogen (and not anthropogenic) dispersal was more likely responsible for the presence of pathogens in our sites. The number of years that male moth density exceeded 100 per trap was a significant predictor of both pathogens, while the number of years that moth density exceeded 10 per trap was a significant predictor for only E. maimaiga. In contrast, rates of larval parasitism were highest when gypsy moth history was the shortest. Increases in rain during gypsy moth egg hatch was associated with increased rates of E. maimaiga infection, while temperature decreases during the period of early instars was associated with increased infection rates. Increased temperatures in June, and April and June, were associated with increased E. maimaiga and LdNPV infection, respectively, possibly due to increased pathogen developmental rates, which could facilitate transmission. We show the extent to which spread of infectious diseases in response to spread of a non-native host species along a leading population front is influenced by host population dynamics and climate.