The Eurasian gypsy moth’s inability to find a mate is a mechanism for Allee effects, defined as depressed population growth in low-density populations. Variation in the strength of Allee effects is negatively correlated with range expansion, thus, understanding how landscape affects mate-finding ability may provide insight into the pattern of gypsy moth invasion. Gypsy moth mating occurs when a flighted male follows pheromone trails to a flightless female. In this study we examined whether sloped topography affects mate finding success in male gypsy moths. We replicated release-recapture plots in paired flat and sloped terrain with the sloped plots selected for different aspects. Each plot had two female moths caged in delta traps at the end of eight radial transects extending 24m from a center male moth release point. The slope of each transect, vegetation, and daily maximum temperatures were recorded. Males were released mid-morning. Males able to locate females were found inside the delta traps, which were collected 24 hours after release. The number of males captured was used as a proxy for mate-finding ability. A generalized linear mixed effects model assuming a Poisson-distribution was used to test whether slope affected male moth’s ability to find a mate.
Results/Conclusions
More males were able to locate females in the flat plots than in the sloped plots. Male gypsy moths in the sloped plots were more likely to find females upslope of the release point than downslope. The vegetation effect was not a component of the best model. There was an interaction effect between slope and maximum daily temperature such that the effect of slope was reduced during warmer days. The higher catch rate on flat plots suggests that Allee effects may be weaker in flat topography than sloped topography. In sloped topography, higher catch rates upslope from the release point suggests that Allee effects may be weaker at higher elevations. The interaction effect of maximum daily temperature may be due to hot air carrying pheromones upslope, thus, causing males to follow the pheromone concentration gradient downslope. Overall, these results suggest that range expansion by gypsy moth tends to be greater on flat terrain and moving upslope on hills and mountains. Understanding how topography plays a role in mate finding success, and furthermore, colonization of invasive forest pests could inform management decisions that mitigate range expansion.