Caste-specific developmental and allometric responses to sulfoxaflor in a bumblebee

Social insect colonies depend on the coordinated development and performance of distinct castes, yet these developmental programs may differ in their sensitivity to environmental stressors. Here, we investigated how chronic exposure to the systemic insecticide sulfoxaflor (SFX) affects caste-specific phenotypes in the Common Eastern Bumblebee, Bombus impatiens (Cresson, 1863). Colonies were exposed to field-realistic concentrations of SFX for 8 weeks. We then quantified morphological and behavioral responses across workers, queens, and males. SFX produced significant effects on both body size and morphological scaling. Queens and males from high-exposure colonies had greater wet mass than those from control colonies. Across castes, SFX exposure was associated with altered allometric scaling in 8 of 10 measured morphological traits, indicating broad effects on proportional development. However, these scaling responses were caste-dependent for tibia and tarsus length: high-SFX workers exhibited significantly shallower allometric slopes than control workers, whereas treatment-specific slopes did not differ in queens or males. Despite these developmental changes, caste-typical locomotor, social, and defensive behaviors remained intact under pesticide exposure. At the colony level, SFX exposure was associated with altered reproductive timing, including elevated numbers of queen pupae and reduced proportions of adult queens in exposed colonies, suggesting delayed maturation and shifts in caste allocation. Together, our findings demonstrate that chronic pesticide exposure can broadly alter morphological scaling while producing particularly strong caste-specific effects on worker development, even when behavioral phenotypes remain comparatively robust. These results highlight the importance of caste-specific developmental endpoints for understanding how environmental stressors influence pollinator health and social insect resilience.