1. Acclimation responses are normally assumed to increase the fitness of an organism, but supporting evidence is generally lacking, especially under field conditions. Even where an advantage arises from acclimation, this can be offset by other fitness costs. Heat hardening is a well-known form of acclimation in many invertebrates where exposure to high but sub-lethal temperatures protects against subsequent heat-induced death.
2. Previous laboratory work has shown that hardening occurs in wasp egg parasitoids of the genus Trichogramma because pretreatment with a mild temperature 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 heat resistance in the laboratory. Moreover, hardening enhanced adult fitness in the field under hot conditions. No costs of acclimation were detected under mild field and laboratory conditions. Conditions leading to hardening without costs were different from another Trichogramma species.
4. Hardening can therefore have fitness benefits without costs under field and laboratory conditions, and this process can be used to enhance parasitism rates in inundative commercial releases of Trichogramma against moth pests.