Cadmium triggers hormesis in rice moth Corcyra cephalonica but different effects on two Trichogramma egg parasitoids

Increasing awareness of cadmium (Cd) pollution in agroecosystems has highlighted the need for a better understanding of the cascading effects heavy metals on the fitness of herbivores and their natural enemies, as well as the potential impact on the efficacy of integrated pest management (IPM). In this study, we assessed the bottom-up effects mediated by Cd on two important egg parasitoids, Trichogramma japonicum and T. dendrolimi via their factitious host, the rice moth Corcyra cephalonica. Results showed that the bottom-up effects mediated by Cd are complex. Cd exposure at 20 mg/kg in artificial diets of rice moth larvae enhanced the weight of eggs laid once they have reached adulthood, increased the proportion of females produced, and stimulated reproduction, thus suggesting a hormesis response. A stimulation effect on T. dendrolimi fitness was also found (when developing from contaminated rice moth eggs), but T. japonicum instead had a reduced fitness. In addition, unexposed adult parasitoids (reared from uncontaminated hosts) were not able to discriminate between uncontaminated or contaminated eggs (produced by rice moths reared on Cd-diet). Our results show that Cd contamination should not interfere with the effectiveness of inundative releases of T. japonicum, but the capacity of the subsequent F1 generation of parasitoids in controlling pest populations could be affected by Cd contamination. This suggests that inundative releases should be favored over inoculative releases in Cd-contaminated sites, and mixed-species releases of Trichogramma could increase the cost effectiveness of pest biological control.