Competitive interactions between two candidate biological control agents of Bagrada hilaris

Gryon sp. nr. gonikopalense Sharma (Hymenoptera: Scelionidae) is a promising candidate biological control agent for Bagrada hilaris Burmeister (Hemiptera: Pentatomidae) based on its high overall parasitism rates and its ability to successfully parasitize eggs beneath the soil surface. However, G. sp. nr. gonikopalense foraging ability is not restricted to eggs deposited under the soil surface, and therefore, females are likely to compete for eggs with Trissolcus hyalinipennis Rajmohana & Narendran (Hymenoptera: Scelionidae), another biological control candidate from Pakistan that was recently discovered in California. Unlike G. sp. nr. gonikopalense, T. hyalinipennis appears to lack the ability to attack eggs in the soil, and mainly parasitizes eggs on the soil surface.
This study investigated possible competitive interactions between the two species. Intrinsic (competition between parasitoid larvae) and some aspects of extrinsic competition (adult interference and host discrimination ability) were investigated for both species. Our results showed that, when the two species forage simultaneously, G. sp. nr. gonikopalense might be slightly dominant in extrinsic competition (adult interference), while no dominant competitor was observed at the intrinsic level. No interspecific host discrimination ability was detected. We conclude that the presence of T. hyalinipennis in California is unlikely to reduce G. sp. nr. gonikopalense efficiency. In fact, mortality inflicted by T. hyalinipennis likely would be additive and would improve the biological control of B. hilaris.