Effect of forest microhabitat and larval stage on overwintering survival, development, and phenology of Spathius galinae, classical biological control agent of emerald ash borer

Authors: SHERR, ADAM - University Of Delaware; QUIN, NICOLE - University Of Massachusetts, Amherst; TALLAMY, DOUGLAS - University Of Delaware; Duan, Jian

Submitted to: Florida Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/21/2023
Publication Date: 6/25/2023
Citation: Sherr, A., Quin, N.F., Tallamy, D., Duan, J.J. 2023. Effect of forest microhabitat and larval stage on overwintering survival, development, and phenology of Spathius galinae, classical biological control agent of emerald ash borer. Florida Entomologist. 106(2) : 104-109. https://doi.org/10.1653/024.106.0206.
DOI: https://doi.org/10.1653/024.106.0206

Interpretive Summary: The emerald ash borer (EAB) is a serious invasive forest pest that has killed tens of millions of ash trees in the United States. The parasitic wasp Spathius galinae is among several natural enemies introduced for biological control of EAB in the U.S. We conducted experiments to determine the overwintering mortality, dormancy, and adult emergence time of the parasitic wasp in both urban and natural forests. Parasitoid overwintering mortality rates were low and varied among forest types and wasp larval ages. None of the wasps deployed as young larvae entered dormancy in either urban and forests, but nearly a third of old (late instar) larvae did so in both urban and natural forests. In addition, adult wasp emergence was earlier in the urban forest (possibly because of warmer climates) than in the natural forest. Our findings suggest that land use and climate changes may profoundly affect the phenology of this introduced natural enemy, and consequently, EAB biological control.

Technical Abstract: Spathius galinae Belokobylskij (Hymenoptera: Braconidae) is among several parasitoids introduced as classical biocontrol agents against the invasive emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae) in the U.S. Since its introduction in 2015, Spathius galinae has been released in several northern mid-Atlantic states. However, the successful establishment of this parasitoid on and its efficacy in suppressing EAB vary among the release sites and regions. We hypothesized that forest microhabitats and parasitoid overwintering life stages affect parasitoid overwintering survival, development, and subsequent emergence phenology of adults in the spring, which may affect its establishment and efficacy in suppressing EAB in some habitats. To test this hypothesis, we placed logs containing three different larval stages of S. galinae (early instar larva, late instar larva, and mature larva in cocoon) in ventilated jars in two distinct microhabitat sites: urban forest and mature natural forest. The ash logs containing different stages of S. galinae larvae were deployed in late fall of 2020, remained in the field through winter, and emergence of adult parasitoids was recorded in spring through summer of 2021. Parasitoids at the warmer urban site emerged 12 days earlier than parasitoids at the cooler, shaded mature forest site. Overall, overwintering mortality rates were low (16.9– 33.5 %), varied among habitats and larval stages. None of the S. galinae deployed as early-instar larvae diapaused at both urban and forest sites, but 27.1 – 32.1% of late instar larvae diapaused in both habitats. Across overwintering larval stages, the median time of S. galinae emergence was ~24 days in the urban microhabitat, significantly shorter than that (36 days) observed in the mature forest microhabitat. Our findings suggest that land use changes and climate change may profoundly affect S. galinae phenology and, consequently, EAB biological control.