Understanding the role of climatic limits for invasive species is important for determining range expansion and the potential for future spread. The spread of the gypsy moth (Lymantria dispar L.) across wide climatic gradients in North America provides an ideal system for studying the role of thermal limits in invasion. Previous work has shown that variability in spread rates at the southern invasion front (West Virginia and Virginia) is correlated with exposure to supraoptimal temperatures. We tested the effect of heat stress on survival and performance by exposing gypsy moth individuals to either an optimal control (22-28°C) or one of three supraoptimal temperature treatments (30-36°C, 32-38°C, or 34-40°C) at specific developmental stages (first through fourth larval instar, pupae) for either two or seven days. We measured survival over the duration of the heat exposure and long term effects on development time and pupal mass or adult size.
Survival of larvae and pupae generally decreased as treatment temperature increased for all stages. The 34-40°C temperature treatment had the largest effect on larval survival, with seven days of exposure being lethal for all stages except second instar and pupae. The 22-28°C and 30-36°C temperature treatments had more than 80% survival and the 32-38°C temperature treatment showed only slightly lower survival for all larval instar stages measured. For pupae, there was no difference in survival after two days in the heat for the three lower temperature treatments. The long term effects on final pupal mass of the two day exposure to heat during the larval stage were more pronounced for females than males. In general, exposure to heat at later larval stages resulted in larger decreases in pupal mass. Together, these results show negative effects of high temperature on gypsy moth populations and support patterns of spread seen in the southern invasion front. Our work provides important data on the susceptibility of gypsy moth to high temperature across different developmental stages. As global temperatures rise, understanding how temperature can mediate the spread of invasive populations will be a critical component of management decisions.