2 TACHINID SPECIES DISCRIMINATE BETWEEN PARASITIZED AND NON-PARASITIZED HOSTS

The abilities of Myiopharus doryphorae (Riley) and M. aberrans (Townse nd) (Diptera: Tachinidae) to discriminate between parasitized and non- parasitized Colorado potato beetle (host) Leptinotarsa decemlineata (S ay) (Coleoptera: Chrysomelidae) larvae, were investigated under labora tory and field conditions. Laboratory experiments showed that both Myi opharus species have a significantly greater frequency for larvipositi ng in non-parasitized hosts over parasitized ones. Direct field observ ations of larvipositional behavior of both Myiopharus species over thr ee wing seasons showed effective restraint from larviposition into par asitized hosts, while larviposition into non-parasitized ones occurred readily. Avoidance of previously-parasitized hosts occurred after the larvipositing flies briefly landed on host larvae without attempting to insert the larvipositor. The low levels of superparasitism which oc curred in the caged experiments and in the field appeared to be due to a breakdown of the larvipositing parasitoids' restraint when they met only parasitized hosts or when many parasitoids competed for reduced numbers of hosts late in the season.