Acclimation for heat resistance in Trichogramma nr. brassicae:: can it occur without costs?

1. It is well known that animals can increase their stress resistance by prior exposure to sub-lethal conditions, but this acclimation process is often accompanied by deleterious fitness effects. These may reflect a cost of acclimation, or more general costs arising from damage due to sub-lethal exposures. 2. Acclimation for increased adult resistance to a 40 degrees C heat shock was tested in the egg parasitoid, Trichogramma nr. brassicae, following exposure of immature stages to 33 degrees C for different periods. Prepupal exposure did not increase heat resistance, but daily exposures at the pupal stage for 2 h day(-1) and 3 h day(-1) over 4 days increased survival of the shock. 3. Pre-pupal exposure for 3 h day(-1) over 4 days led to decreased parasitism reflecting general damage costs. Pupal exposure for 3 h day(-1) and 4 h day(-1) also decreased parasitism rates, but there were no deleterious effects when pupae were acclimated for 2 h day(-1). 4. These findings indicate that acclimation can occur without costs in Trichogramma nr. brassicae and suggest a general phenotypic approach for separating acclimation costs from other fitness costs caused by sub-lethal exposures. The potential for using acclimation to increase field parasitism success of this important group of parasitoids is discussed.