Transgenerational effects of heavy metal contamination on two Trichogramma egg parasitoids and potential impacts on biological control.

The increased basic knowledge of biological control agents' bio-ecology under environmental stress provides useful information for optimizing integrated pest management. With an increasing awareness of cadmium (Cd) pollution in agroecosystems, this study evaluated the transgenerational effects and host preference of two widely distributed and utilized egg parasitoids, Trichogramma dendrolimi and Trichogramma japonicum through bottom-up effects mediated by Cd. The host rice moth Corcyra cephalonica was reared on a diet with 20 mg/kg Cd exposure for 1 generation; eggs were collected, and then exposed to Trichogramma. The survival rate, longevity, daily number of parasitized eggs, and fecundity of second generation (F1) females, emergence rate, and female adult proportion of the third generation (F2) were tested. After emerging from Cd-exposed hosts, the survival rate and longevity of F1 T. dendrolimi female adults were significantly increased, but the daily number of eggs parasitized by, and fecundity of, F1 T. japonicum were significantly decreased. Cd exposure significantly resulted in a preference for hosts exposed to Cd in both F1 T. dendrolimi and T. japonicum, whereas no such host preference was observed when the first generation (F0) parasitoids were not exposed to Cd during their development. Our study shows the transgenerational effects and host preference on natural enemies under heavy metal exposure stress in a species-specific way. This highlights the potential synergetic effects of heavy metal contamination on biological control. Specifically, the release of T. dendrolimi, rather than T. japonicum, should be favored at Cd-contaminated sites, where T. dendrolimi is a more suitable biocontrol agent.