Thermal history of alfalfa leafcutting bees affects nesting and diapause incidence

Variable spring temperatures may expose developing insects to sublethal conditions, resulting in long-term consequences. The alfalfa leafcutting bee, Megachile rotundata, overwinters as a prepupa inside a brood cell, resuming development in spring. During these immobile stages of development, bees must tolerate unfavourable temperatures. In this study, we test how exposure to low temperature stress during development affects subsequent reproduction and characteristics of the F1 generation. Developing male and female M. rotundata were exposed to either constant (6°C) or fluctuating (1 h/day at 20°C) low temperature stress for one week, during the pupal stage to mimic a spring cold snap. Treated adults were marked and released into field cages, and reproductive output was compared to that of untreated control bees. Exposure to low temperatures during the pupal stage had mixed effects on reproduction and offspring characteristics. Females treated with fluctuating low temperatures were more likely to nest compared to control bees or those exposed to constant low temperature stress. Sublethal effects may have contributed to low nesting rates of bees exposed to constant low temperatures. Females from that group that were able to nest had fewer, larger offspring with high viability, suggesting a trade-off. Interestingly, offspring of bees exposed to fluctuating low temperatures were more likely to enter diapause, indicating that thermal history of parents, even during development, is an important factor in diapause determination.