Cold tolerance of field-collected soybean gall midge (Diptera: Cecidomyiidae) under laboratory overwintering...

Soybean gall midge, Resseliella maxima Gagné, was first described in 2019 following widespread outbreaks in soybean across the midwestern United States. Larval feeding inside stems causes lesions, lodging, and yield loss. Third instars overwinter in the soil within silken cocoons, but their cold tolerance, an important factor for survival in the temperate midwestern United States, is unknown. In 2022 and 2023, late-season soybean stems with R. maxima were collected from southwestern Minnesota. Third instars were retrieved and transferred to vials of sand, where they successfully burrowed and formed cocoons. Vials with cocoons were stored at 13 and 3 °C, and larval supercooling points and lower lethal temperatures (LTs) were measured after 1 and 2 months. All individuals remained larvae by the time of measurements, suggesting they were in diapause. Supercooling points and lower LTs were primarily between -20 and -25 °C. Alignment between these measurements suggests that R. maxima is freeze-intolerant. There were no consistent effects of storage time or temperature, indicating a lack of further acclimation. The temperature causing 50% mortality (LT50) was -19.8 °C in both 2022 (95% CI: -26.4 to -13.2 °C) and 2023 (95% CI: -26.6 to -13.0 °C). Historical soil temperature data reveal the coldest temperature recorded from 2015 to 2024 near the northern portion of R. maxima's known range (-17.7 °C) would cause <50% mortality to R. maxima populations, while average coldest temperatures were substantially warmer (-3.1 to -8.5 °C). Short-term exposures to winter soil temperatures may not cause significant mortality to overwintering R. maxima in the midwestern United States.