THE EFFECTS OF PARASITOIDS ON SYMPATRIC HOST RACES OF RHAGOLETIS-POMONELLA (DIPTERA, TEPHRITIDAE)

Enemy-free space is a potentially important factor affecting host plan t use by phytophagous insects. In particular, escape from parasitoids, predators and competitors could mediate a successful host shift by co mpensating for decreased feeding performance on a new plant. Here I in vestigate the effects that two braconid wasp parasitoids (Opius lectus and Biosteres melleus) have on sympatric host races of Rhagoletis pom onella (Diptera: Tephritidae) infesting apple (Malus pumila) and hawth orn (Crataegus spp.) at a field site near Grant, Michigan, USA. Levels of parasitoid attack were much lower for fly larvae infesting the der ived host apple (13%) compared to the ancestral host hawthorn (46%). A t least three factors contributed to apples being a relatively safe ha ven from parasitoids for maggots. First, apples were larger (mean frui t width = 5.2 cm) than hawthorns (mean width = 1.6 cm). Apples therefo re provided greater physical protection for fly larvae than hawthorns because the ovipositors of female wasps were too short to reach a majo rity of maggots feeding within apples. Second, apples had an earlier f ruiting phenology than hawthorns and the seasonal distribution of O. l ectus parasitoids was asynchronous with the development of larvae in a pples. Third, hawthorn fruits were infested by various species of cate rpillars (tortricids and an agonoxenid) and a curculionid weevil to a much greater degree than apples (65% compared to 11%). These heterospe cific insects forced fly larvae to feed near the surface of hawthorn f ruits where they were more prone to detection and attack from braconid s (73% of larvae feeding in hawthorns with heterospecific insects were parasitized compared to 34% of larvae infesting hawthorns without cat erpillars and weevils). Data from other Rhagoletis and tephritid speci es suggest that escape from parasitoids may be a common theme in host range expansion and race formation for these flies.