Diapause regulation by environmental cues in the alfalfa leafcutting bee, Megachile rotundata

The alfalfa leafcutting bee, Megachile rotundata F. (Hymenoptera: Megachilidae), is an intensively managed and an important solitary pollinator for North American agriculture. M. rotundata undergoes facultative diapause in which early season progeny complete development to produce a second generation whereas most late season progeny diapause as prepupae. The aim of this study was to determine the environmental cues that may regulate diapause incidence in M. rotundata. Three replicate nest boxes were installed near Fargo, ND, Laramie, WY, and Las Cruces, NM to expose the bees to different photoperiods and thermal regimes. Diapause incidence did not differ significantly across sites, with a majority of individuals diapausing throughout the season. Photoperiod did not influence diapause incidence, but accumulated degree day units experienced by progeny, starting from the day each egg was laid until the end of the season, favor the diapause. Variation in diapause incidence was explained primarily by a maternal effect, which predicted 98% of the variation in diapause incidence. Additionally, the position of individual in the nest influenced diapause incidence, with high diapause rates in the first few cell positions. Our results suggest that diapause incidence, within a nest cohort, is more strongly influenced by the conditions experienced by the mother than those experienced by the progeny.