Effect of forest microhabitat and larval stage on overwintering survival, development, and phenology of Spathius galinae (Hymenoptera: Braconidae), biological control agent of emerald ash borer, Agrilus planipennis (Coleoptera: Buprestidae)

Spathius galinae Belokobylskij (Hymenoptera: Braconidae) is among several parasitoids introduced as classical biocontrol agents for control of the invasive emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), in the US. However, the successful establishment of this parasitoid and its efficacy in suppressing emerald ash borer vary among the release sites and regions. We hypothesize that forest microhabitats and parasitoid overwintering life stages affect parasitoid overwintering survival, development, and emergence phenology of adults in the spring. To test this, we placed logs containing 3 different larval stages of S. galinae (early instar larva, late instar larva, and cocoon) in ventilated jars in 2 distinct microhabitat sites, i.e., urban forest and mature natural forest. These were deployed in late fall of 2020 and remained in the field through winter. The emergence of adult parasitoids was recorded in spring through summer of 2021. Parasitoids in the warmer urban site emerged more quickly, emerging 12 d earlier than parasitoids in the cooler mature forest site. None of the S. galinae deployed as early instar larvae diapaused in either urban or forest sites. In both habitats, 27.1 to 32.1% of late instar larvae diapaused. Across overwintering larval stages, the median time of S. galinae emergence was about 24 d in the urban microhabitat, significantly shorter than that (36 d) observed in the mature forest microhabitat. Our findings suggest that environmental factors profoundly affect S. galinae phenology, and consequently emerald ash borer biological control.