none 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 parasitization 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 subpopulations parasitized, indicating a clustered distribution of Apanteles over the population of its host.