Sustained warming alters Bombus impatiens foraging, colony performance, and pollination of zucchini (Cucurbita pepo)

Climate change is intensifying heat exposure for managed pollinators, yet most bumble bee heat stress studies use short-term heatwave designs. We exposed Bombus impatiens micro-colonies and Cucurbita pepo plants to three sustained temperature treatments (control (29.5°C day/21°C night), + 1.5°C, and + 3°C) in growth chambers for approximately ten weeks, with three colonies per treatment. Queen weight was significantly lower at + 3°C; adult colony size did not differ. Fanning rate increased approximately 20x at + 1.5°C, while brood-care declined significantly under both warming levels. Within-hive spatial clustering was disrupted at + 3°C, with significantly reduced aggregation in brood-adjacent and entrance-adjacent zones. Nectar volume nearly doubled at + 3°C, and bee visitation was approximately 70% higher, yet zucchini fruit set per female flower was lowest under moderate warming. Colony responses were non-linear: moderate warming elicited active behavioral compensation, while extreme warming exceeded compensatory capacity and disrupted spatial organization. Elevated visitation reflected plant-mediated nectar increases rather than improved colony condition and did not translate into proportional yield gains. These results indicate that sustained warming can decouple pollinator activity from crop production, and that caste-specific physiological costs and floral reward conditions require monitoring in commercially managed bumble bee systems under climate change.