Host patch use and potential competitive interactions between two egg parasitoids from the family Scelionidae, candidate biological control agents of Bagrada hilaris (Hemiptera: Pentatomidae)

Authors: Hougardy, Evelyne; Hogg, Brian

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/12/2021
Publication Date: 2/17/2021
Citation: Hougardy, E.H., Hogg, B.N. 2021. Host patch use and potential competitive interactions between two egg parasitoids from the family Scelionidae, candidate biological control agents of Bagrada hilaris (Hemiptera: Pentatomidae). Journal of Economic Entomology. 114(2):611-619. https://doi.org/10.1093/jee/toab014.
DOI: https://doi.org/10.1093/jee/toab014

Interpretive Summary: The bagrada bug is an invasive pest of cole crops (i.e., cabbage, broccoli, cauliflower, kale, arugula, etc.) from southern Asia that appears to lack natural enemies in the US. This study focuses on two species of parasitic wasp from Pakistan that may help control this pest. Both wasps attack bagrada bug eggs, killing them in the process. One wasp species is more promising than the other because of its ability to find eggs beneath the soil surface, where the bagrada bug lays most of its eggs. The second wasp species lack this ability but was recently found in the US and could compete with the more efficient species for eggs deposited on the soil surface. In a series of experiments, we found that both wasps avoided parasitizing eggs that were already parasitized by a female of their own species, but readily parasitized eggs that were previously parasitized by a female of the other species. When two females foraged for eggs simultaneously, the more efficient wasp species seemed to dominate the other to some extent, keeping the other female away from eggs. When an egg was parasitized by the two wasp species in less than one day, both species were equally likely to complete development and emerge from the egg. When a full day had elapsed between the time the first and second wasp species parasitized the egg, the first species that parasitized the egg was usually the one winning the competition. Lastly, the two wasp species appeared to work together to increase egg mortality. These results suggest that the presence of the less efficient wasp species in California is unlikely to reduce the efficiency of the more efficient species but will instead improve control of bagrada bug.

Technical Abstract: Bagrada bug, Bagrada hilaris, is an invasive pest of cole crops that appears to lack effective natural enemies in its invaded range in the US. The parasitoid Gryon gonikopalense is a promising biological control candidate for B. hilaris based on its high overall parasitism rates and its ability to successfully parasitize eggs beneath the soil surface, where B. hilaris lays the majority of its eggs. However, foraging by G. gonikopalense is not restricted to eggs deposited under the soil surface, and therefore, females are likely to compete for eggs with Trissolcus hyalinipennis, another candidate biological control agent from Pakistan that was recently discovered in California. Unlike G. gonikopalense, T. hyalinipennis appears to lack the ability to attack eggs in the soil, and mainly parasitizes eggs on the soil surface. This study first detailed the foraging and oviposition behavior of G. gonikopalense and T. hyalinipennis and assessed their relative foraging efficiency. Then, we investigated possible competitive interactions between the two parasitoids by assessing (1) the occurrence of intra- and interspecific host discrimination abilities, (2) possible mutual interferences between adult females (extrinsic competition), (3) the outcome of multiparasitism (intrinsic competition), and (4) the effect of competition on host suppression. Our results showed that the two species differed in the frequency and intensity of self-superparasitism, leading to longer host patch time and lower foraging efficiency for T. hyalinipennis. Females of both species avoid conspecific superparasitism, but there is no avoidance of multiparasitism. When the two species forage simultaneously, G. gonikopalense seems to be slightly superior in extrinsic competition, while neither species seems to have an advantage in intrinsic competition. Also, neither species can develop as a facultative hyperparasitoid, although they do appear to inflict non-reproductive mortality on eggs containing 4-day old larvae of their competitor. Lastly, host mortality inflicted by G. gonikopalense and T. hyalinipennis when acting together appeared to be additive. These results suggest that the presence of T. hyalinipennis in California is unlikely to reduce G. gonikopalense efficiency but will instead improve the biological control of B. hilaris.