Distributions and interactions of tachinid (Diptera) parasitoids of leafrollers (Lepidoptera: Tortricidae) affecting tree fruits in central Washington

Nemorilla pyste (Coquillett) and Nilea erecta (Walker) (Diptera: Tachinidae) are among the most important parasitoids of the leafrollers Choristoneura rosaceana Harris, and Pandemis pyrusana Kearfott (Lepidoptera: Tortricidae), which are economic pests in apples and cherries. The relative frequencies and geographic distributions of the two parasitoids were determined from leafrollers collected over five field seasons. A degree-day model for C. rosaceana was used as the physiological scale to determine phenology, and host lifestage preferences were determined by leafroller headcapsule size at the time of collection. The flies were found to be widely distributed across the study region, and N. pyste was the dominant species. Usually the two flies occurred together, but if not, N. pyste was often the species collected independently. The flies had overlapping phenologies and host preferences, and they sometimes attacked the same hosts. However, emergent broods were limited to one species. Field observations suggested that these flies occupy very similar niches, and prompted an investigation of endoparasitic life history. Intra- and interspecific competition was manipulated in vitro. Maggot development rates were similar, suggesting that selective starvation could be a competitive mechanism only for specific parasitization timings. Respiratory strategies of the maggots were distinct, and precluded the possibility of competition for oxygen. Morphology of maggots was also distinct; cephalopharyngeal structures were much larger in N. erecta, and evidence was found that third (last) instar maggots use the enlarged mandibles to prey on competitors. Implications for biological control and possible evolutionary origins of this unique biology will be briefly discussed.