Effects of winter temperatures on gypsy moth egg masses in the Great Lakes region of the United States

Accurate prediction of winter survival of gypsy moth ( Lymantria dispar L.) eggs and phenology of egg hatch in spring are strongly dependent on temperature and are critical aspects of gypsy moth management programs. We monitored internal temperatures of egg masses at three heights aboveground level and at the four cardinal aspects on oak tree stems at two different locations in Michigan during the winter seasons of 1997/1998, 1998/1999 and 1999/2000. Effects of aspect were more strongly associated with observed egg mass temperatures than height above the ground surface. Instantaneous differences between egg mass temperatures on sunny days were as high as 30 °C greater on the southern aspect vs. egg mass temperatures on other aspects, resulting in substantial differences in pre-hatch growing degree accumulations for egg masses on a single tree. Egg masses on southern and western aspects, where solar loading and temperatures were greatest, experienced substantial mortality. Mean survival of eggs averaged across the three seasons was less than 25% on southern and western aspects, compared with averages of 53 and 73% on eastern and northern aspects, respectively. Linear regression of mean monthly egg mass–air temperature differences (between north and south aspects) and mean daily solar flux density resulted in slope coefficient estimates of 0.13 and 0.21 °C MJ m −2, and correlation coefficients of 0.81 and 0.82 at the two field locations, respectively. Using a simple egg mass hatch phenological model, such dissimilarities in temperature and growing degree day accumulation resulted in differences in estimated egg hatch dates of up to 25 days. Snow cover moderated egg mass temperature, with extreme seasonal minimum winter temperatures under snow cover as much as 7.1 °C warmer than those without snow cover.