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

Enemy-free space is a potentially important factor affecting host plant use by phytophagous insects. In particular, escape from parasitoids, predators and competitors could mediate a successful host shift by compensating for decreased feeding performance on a new plant. Here I investigate the effects that two braconid wasp parasitoids (Opius lectus and Biosteres melleus) have on sympatric host races of Rhagoletis pomonella (Diptera: Tephritidae) infesting apple (Malus pumila) and hawthorn (Crataegus spp.) at a field site near Grant, Michigan, USA. Levels of parasitoid attack were much lower for fly larvae infesting the derived host apple (13%) compared to the ancestral host hawthorn (46%). At least three factors contributed to apples being a relatively safe haven from parasitoids for maggots. First, apples were larger (mean fruit width = 5.2 cm) than hawthorns (mean width = 1.6 cm). Apples therefore provided greater physical protection for fly larvae than hawthorns because the ovipositors of female wasps were too short to reach a majority of maggots feeding within apples. Second, apples had an earlier fruiting phenology than hawthorns and the seasonal distribution of O. lectus parasitoids was asynchronous with the development of larvae in apples. Third, hawthorn fruits were infested by various species of caterpillars (tortricids and an agonoxenid) and a curculionid weevil to a much greater degree than apples (65% compared to 11%). These heterospecific insects forced fly larvae to feed near the surface of hawthorn fruits where they were more prone to detection and attack from braconids (73% of larvae feeding in hawthorns with heterospecific insects were parasitized compared to 34% of larvae infesting hawthorns without caterpillars and weevils). Data from other Rhagoletis and tephritid species suggest that escape from parasitoids may be a common theme in host range expansion and race formation for these flies.