WINTER MORTALITY OF GYPSY-MOTH (LEPIDOPTERA, LYMANTRIIDAE) EGGS IN MICHIGAN

The northern lower peninsula of Michigan was heavily defoliated by gyp sy moth in 1991 and 1992 despite being in a climate considered margina l for gypsy moth. We investigated the impact of extreme cold events on egg hatch by collecting egg masses from counties in Michigan with act ive gypsy moth populations in 1994 and 1996, the 2 coldest winters in the past 7 yr. Egg hatch from masses located 90 cm or more above groun d was not as complete as egg hatch from masses close to the ground in regions with the coldest temperatures. More successful egg hatch close to the ground may be partially explained by snow cover. In 3 separate data sets, egg hatch was negatively correlated with the number of dap s in January and February when the temperature dropped below -26 degre es C (r(2) = 0.31, 0.62, and 0.54). The heaviest defoliation in the no rthern lower peninsula of Michigan occurred in years following a mild winter when temperatures dropped below -26 degrees C fewer than 4 time s.