Spatiotemporal variability in effects of weather on rates of invasive spread by the gypsy moth

Clare A. Rodenberg and Jonathan A. Walter, Department of Environmental Sciences, University of Virginia, Charlottesville, VA, Kyle J. Haynes, Blandy Experimental Farm, University of Virginia, Boyce, VA

Background/Question/Methods
Few studies have examined the effects of yearly variation in weather conditions on rates of invasive spread or how these effects vary geographically in species that have successfully expanded their ranges into climatically distinct regions. Along the southern range edge of the gypsy moth (Lymantria dispar), a nonnative forest pest insect in northeastern North America, range contractions have occurred in locations with higher frequencies of supraoptimal temperatures during the larval stage. However, the potential for weather to contribute to spatiotemporal variation in rates of invasion by the gypsy moth are unknown. In this study, we examined the effects of annual variation in temperature and precipitation during the springtime larval period on rates of spread (km/year) in four different ecoregions. We estimated annual spread rates within each ecoregion by determining the distance between 10-moth population abundance isoclines for pairs of successive years. We used robust spatiotemporal data on gypsy moth abundances, temperature, and precipitation for the period of 1989-2019. The effects of springtime temperature and precipitation on gypsy moth spread rates in each ecoregion were examined using generalized additive models.
Results/Conclusions
We found that the effects of springtime temperature and precipitation on gypsy moth spread rates vary with space. For the southernmost ecoregion, the Southern United States Coastal Plain, the gypsy moth displayed lower rates of spread in years with higher temperatures. This is consistent with previous findings of the effects of supraoptimal temperatures based on spatial variation in spread and climate. In the Appalachian Forest ecoregion, rates of spread were highest in years with above-average temperature and below-average precipitation, possibly because warm and dry conditions are unfavorable for the gypsy moth’s specialist fungal pathogen Entomophaga maimaiga. In the two northernmost ecoregions of this study, temperature and precipitation did not have a significant effect on the rate of invasive spread. Previous research suggests that northern range expansion may be limited by wintertime temperatures, which may explain the lack of an effect of weather during the larval period (spring). The findings of this study have implications for how climate change may alter the spread of the gypsy moth. For example, expected increases in drought may accelerate the gypsy moth invasion.