Assessing the Repellency of an Insecticide-Fungicide Combination to Bees Using Bioacoustics

Soybeans (Glycine max (L.) Merr.) and bees have a mutually beneficial relationship where nectar is exchanged for pollination service, which may enhance seed yield. However, pesticide use on soybeans during flowering could disrupt this relationship by harming bees and therefore reducing the yield benefits from pollinatiors. This study examined how pesticide applications during flowering affect bee foraging and soybean yield, focusing on application timing (mid-morning vs. mid-day) and flowering stage (R3-full bloom vs. R5-late/end of bloom), compared to an untreated control. To do this, an experiment was conducted in central Ohio in 2023 and 2024, where soybean plots were sprayed with a bee-toxic mixture of a pyrethroid insecticide (0.9% alpha-cypermethrin) and a sterol biosynthesis-inhibiting fungicide (41.8% propiconazole) using a backpack sprayer. Bee activity was monitored using audio recorders placed in each plot and analyzed with the machine learning tool buzzDetect. Bioacoustic analysis showed no effect of pesticide application on bee activity, regardless of timing. Bee activity was significantly higher during the R3 growth stage, when soybeans were at peak bloom, compared to the R5 growth stage, when flowers were less abundant. Bee activity was significantly higher in mid-day than in mid-morning. No significant differences in soybean defoliation or yield were detected across pesticide treatments. These results provide insight into how bee foraging varies across the soybean growing season and throughout the day, which can inform pesticide application recommendations to minimize bee exposure.