Seasonal Variation in Parasitoid Attack of Tyria Jacobaeae By Apanteles Popularis

SEASONAL VARIATION IN PARASITOID ATTACK OF TYRIA JACOBAEAE BY APANTELES POPULARIS by L.L. SOLDAAT (Department of Population Biology, Research Group Ecology of Plants and Herbivores, University of Leiden, P.O. Box 9516, 2300 RA Leiden, The Netherlands) SUMMARY Seasonal variation in parasitization of Tyria jacobaeae L. was studied because it could help explain the maintenance of genetic variation in adult emergence time. A sharp decline in the percentage parasitization of larvae of Tyria by the wasp Apanteles popularis Hal. was observed during the season. In years with high levels of parasitiza- tion a strong selection pressure for late emergence of adult moths may exist. The decline in parasitization was caused by a decline in the number of Tyria subpopula- tions parasitized, indicating a clustered distribution of Apanteles over the population of its host. KEY WORDS: Apanteles popularis, parasitism, seasonal variation, Tyria jacobaeae. INTRODUCTION Variation in food quantity is an important factor in the population dynamics of the monophagous cinnabar moth, Tyria jacobaeae L. In most areas where Tyria occurs, the larvae frequently defoliate their food plants (Senecio jacobaea L.) completely, followed by mass starvation (DEMPSTER, 1971; VAN DER MEIJDEN, 1979; MYERS, 1979). In years of food shortage most offspring of late emerging females will die. There- fore, seasonal variation in food quantity may exert a strong selection pressure for early emergence of adults. For selection to act on emergence time it should have a genetic basis. RICHARDS & MYERS (1980) found significant differences in emergence time between egg batches of Tyria. Under laboratory con- ditions RosE (1978) found that larvae from egg batches from different females have different adult emergence times. Both these studies are indicative of genetic influences on adult emergence time of Tyria. Rose also found considerable variation in emergence time within the pro- geny of a single female. This variation in emergence time suggests that other factors than food shortage may select for late emergence time. MYERS (1979) studied the emergence time of Tyria in North America.. She hypothesized that although food shortage would select for early emergence time, seasonal variation in temperature and food quality