Location: Invasive Species and Pollinator HealthTitle: Parasitism by Gryon aetherium on Bagrada hilaris eggs in northcentral California
|GRETTENBERGER, IAN - University Of California, Davis|
|BORKENT, CHRISTOPHER - California Department Of Food And Agriculture|
Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/9/2023
Publication Date: 8/31/2023
Citation: Hogg, B.N., Grettenberger, I.M., Borkent, C.J. 2023. Parasitism by Gryon aetherium on Bagrada hilaris eggs in northcentral California. Journal of Economic Entomology. https://doi.org/10.1093/jee/toad172.
Interpretive Summary: Bagrada bug is an invasive stinkbug from Asia that attacks cole crops such as cabbage, broccoli, cauliflower, and kale. A parasitic wasp was recently found in California that can dig in the soil where bagrada bug lays most of its eggs. In this study, we measured parasitism levels by this wasp on naturally-laid bagrada bug eggs in northcentral California, including the Salinas Valley where most cole crops in the US are grown. Parasitism was assessed by leaving soil-filled trays under plants infested with bagrada bug, then removing the eggs and holding them for emergence of wasps. The parasitic wasp occurred at 11 of the 12 sites that were sampled, and emerged from 1,518 of the 17,729 eggs (8.8%) collected in 2021 and 2,654 of the 31,759 eggs (8.4%) collected in 2022. Parasitism levels varied widely and were generally higher inland, and never exceeded 15% at several coastal sites in the Salinas Valley. Results suggest that the wasp can locate patches of eggs efficiently, but is less efficient at finding buried eggs within patches. This wasp is clearly widespread in northcentral California, where it is likely helping to control bagrada bug, particularly in warmer inland areas.
Technical Abstract: Bagrada bug, Bagrada hilaris (Burmeister) (Hemiptera, Pentatomidae), is an invasive pest of cruciferous crops (cabbage, broccoli, cauliflower, kale). The parasitoid Gryon aetherium Talamas (Hymenoptera: Scelionidae) is a promising biological control agent for B. hilaris because it can forage in the soil where B. hilaris deposits most of its eggs. In this study, we assessed parasitism by Gryon aetherium on naturally-laid B. hilaris eggs in northcentral California, including the Salinas Valley where most cruciferous crops in the US are grown. Parasitism was documented by leaving soil-filled trays under infested plants for 7-14 days, then removing eggs and holding them for emergence of parasitoids. Gryon aetherium accounted for over 99% of emerged parasitoids, and occurred at 11 of the 12 sampled sites. Of the 17,729 and 31,759 B. hilaris eggs collected in 2021 and 2022, 1,518 (8.84%) and 2,654 (8.36%) were parasitized by G. aetherium, respectively. Parasitism rates varied widely among sites, and were generally higher inland. Overall parasitism rates at sites where G. aetherium was collected ranged from 3.64 to 44.93% in 2021 and from 1.01 to 23.04% in 2022, and never exceeded 15% on any sample dates at several coastal sites in the Salinas Valley. Discovery efficiency (a measure of the ability of parasitoids to locate egg patches) reached 80% or higher at all but one site, but exploitation efficiency (a measure of the ability of parasitoids to exploit the egg patch after it has been discovered) was generally < 20%, suggesting that G. aetherium can locate egg patches efficiently, but is less efficient at finding buried eggs within patches.