Field and laboratory evidence for acclimation without costs in an egg parasitoid

1. Acclimation responses are normally assumed to increase the fitness of an organism, but supporting evidence is generally lacking, especially under f ield conditions. Even where an advantage arises from acclimation, this can be offset by other fitness costs. Heat hardening is a well-known form of ac climation in many invertebrates where exposure to high but sub-lethal tempe ratures protects against subsequent heat-induced death. 2, Previous laboratory work has shown that hardening occurs in wasp egg par asitoids of the genus Trichogramma because pretreatment with a mild tempera ture shock can increase survival at high temperatures. This system allowed fitness benefits and costs of hardening under the more stringent conditions in nature to be tested. 3, Heat hardening in Trichogramma carverae was considered at the pupal and adult stages and it was shown that hardening had a beneficial effect on hea t resistance in the laboratory. Moreover, hardening enhanced adult fitness in the field under hot conditions. No costs of acclimation were detected un der mild field and laboratory conditions. Conditions leading to hardening w ithout costs were different from another Trichogramma species. 4, Hardening can therefore have fitness benefits without costs under held a nd laboratory conditions, and this process can be used to enhance parasitis m rates in inundative commercial releases of Trichogramma against moth pest s.