Fitness and interspecific competition of Trissolcus japonicus and Anastatus japonicus, egg parasitoids of Halyomorpha halys

Abstract

Trissolcus japonicus (Ashmead) (Hymenoptera: Scelionidae) and Anastatus japonicus Ashmead (Hymenoptera: Eupelmidae) are the most important egg parasitoids of Halyomorpha halys Stål (Hemiptera: Pentatomidae) in northern China. In this study, we assessed the fitness of these two parasitoid species at different temperatures. We examined the intrinsic competition of these parasitoids inside host eggs by providing each parasitoid species with H. halys egg masses previously parasitized by the other species, at different time intervals between attacks. We also investigated their intraguild competition for H. halys eggs by releasing a single species (one female T. japonicus or five female A. japonicus per cage) or both parasitoid species (one female T. japonicus and five female A. japonicus per cage) simultaneously in exclusion cages stocked with three host egg masses per cage in a kiwifruit orchard. Our results showed that the developmental time from eggs to adults of both parasitoids decreased with increased temperatures, between 15 °C and 30 °C, and T. japonicus developed much faster than A. japonicus during the immature stages in host eggs. Both T. japonicus and A. japonicus were able to parasitize a host already parasitized by the other parasitoid, and sex ratios of their offspring from multi-parasitized hosts were male-biased for both parasitoids. Inside host eggs, T. japonicus outcompeted A. japonicus when the time lags between the first and second attacks were less than or equal to 1 day, regardless of the order of attack by the two parasitoids, whereas A. japonicus began to dominate when the time lag was 2 to 5 d. The host location, exploitation and impact indexes of the two parasitoids were influenced by their exploitation modes and/or releases in the exclusion cage tests in the field. However, regardless of single or simultaneous release, the parasitism rates of the two parasitoids were not so much affected. We concluded that the combined release of T. japonicus and A. japonicus at the given parasitoid densities did not show any advantage compared to the single release of either species. The two parasitoids employed different reproductive strategies contributing to the outcome of their interspecific competition, which should be considered in any future biological control program for the sustainable management of H. halys.