Asian gypsy moth (Lymantria dispar L.) populations: Tolerance of eggs to extreme winter temperatures.

Gypsy moth Lymantria dispar (GM) is a polyphagous insect and one of the most significant pests in the forests of Eurasia and North America (U.S. and Canada). Accurate information on GM cold-hardiness is needed to improve methods for the prediction of population outbreaks, as well as for forecasting possible GM range displacements due to climate change. As a result of laboratory and field studies, we found that the lower lethal temperature (at which all eggs die) range from -29.0 °C to -29.9 °C for three studied populations of L. dispar asiatica, and no egg survived cooling to -29.9 °C. These limits agree, to within one degree, with the previously established cold-hardiness limits of the European subspecies L. dispar, which is also found in North America. This coincidence indicates that the lower lethal temperature of L. dispar is conservative. Thus, we found that the Siberian populations of GM inhabit an area where winter temperatures go beyond the limits of egg physiological tolerance, because temperatures often fall below -30 °C. Apparently, it is due to the flexibility of ovipositional behavior that L. dispar asiatica survives in Siberia: the lack of physiological tolerance of eggs is compensated by choosing warm biotopes for oviposition. One of the most important factors contributing to the survival of GM eggs in Siberia is the stability of the snow cover.