Effects of Autumn Temperature and Oviposition Date on Timing of Larval Development and Risk of Parasitism in a Spring Folivore

Abstract

Epirrita autumnata (Lepidoptera, Geometridae) is a periodically fluctuating spring folivore which overwinters in the egg stage. The timing of its larval development can be highly variable, and the synchrony of larvae with parasitoids may affect their risk to be parasitized. We estimated the effects of both oviposition date and site on the timing of larval development by allowing experimental eggs, which were laid on various dates during the oviposition period, to overwinter both above and below snow level, and on both the northern and southern sides of tree trunks. Accumulation of thermal sum was used to quantify the effect of oviposition date on the timing of egg hatch. To assess whether the timing of larval development affected the risk of parasitism, the larval stages susceptible to parasitism and the length of the parasitoid flight period were detennined by exposing host larvae to parasitoids. Unlike previously assumed, the thermal sum required for egg hatching began to accumulate already in autumn, and autumn and spring temperatures additively determined the timing of egg hatch. The first-laid eggs accumulated 70% of the thermal sum required for hatching in autumn whereas the last-laid eggs accumulated less than 3%. Accordingly, because the oviposition period of E. autumnata is long and its timing is variable between years, the period of egg hatching also is long and can be variable between years. However, the flight periods of most parasitoid species exceeded the spread of hatching times and developmental stages of larvae. Only during years of low parasitoid density may the variability in egg hatching times create a temporal refuge for some larvae, contributing to the increase of E. autumnata populations.