Diapause, pollen ball incidence, and overwintering energetics in the alfalfa leafcutting bee, Megachile rotundata

Authors: SINGH, PREETPAL - North Dakota State University; Rajamohan, Arun; WAYBRIGHT, SARAH - University Of Wyoming; MICHAEL, DILLON - University Of Wyoming; FERRENBERG, SCOTT - New Mexico State University; Rinehart, Joe; BOWSHER, JULIA - North Dakota State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/15/2024
Publication Date: N/A
Citation: N/A

Interpretive Summary: The alfalfa leafcutting bee is the world’s most intensively managed solitary bee, and is used extensively as a pollinator for alfalfa seed production throughout North America. Although best management practices for winter storage have been well established, managing whether a generation of bees will stop developing and prepare for winter by entering diapause, or continue development and create an additional generation before winter arrives, remains elusive. While the overwintering status of bees in the field is crucial to population management, we still don’t understand how this process works. Previous studies have suggested that, like most insects, temperature and daylength both play roles in diapause determination, and that the conditions experienced by the mother influence whether her offspring enter diapause or continue development. However, no direct link has been established. To investigate how environmental factors affect diapause determination, we established nests of this species in Fargo, North Dakota, Laramie, Wyoming, and Las Cruces, New Mexico, and looked at the incidence of overwintering at two-week intervals throughout the growing season in each location. We found that both the daylength and temperatures experienced by the mother are important to diapause determination, and that they were more important than those experienced by the developing offspring. This information will help pollination managers as they develop protocols to manage diapause determination in this important species.

Technical Abstract: Megachile rotundata is a solitary bee used for commercial pollination of alfalfa in the United States. M. rotundata undergoes facultative diapause in the prepupal stage. Early season progeny can emerge as adults to produce a second generation, whereas most late season progeny diapause as prepupae. Prepupae that diapause early in the season are exposed to warm temperatures for a longer period of time than individuals that start diapause closer to fall, which may reduce lipid reserves in bees that diapause early compared to later in a year. Warm temperatures may also contribute to pollen ball incidence, which is when a provision is present but there is no sign of a larva in the brood cell. Our study investigated cues that may regulate diapause and pollen ball incidence, and examined effects of pre-wintering field conditions on post-overwintering energy reserves in M. rotundata. Three replicate nest boxes were installed near Fargo, ND, Laramie, WY, and Las Cruces, NM to expose bees to different photoperiods and thermal regimes. We monitored nesting conditions and diapause and pollen ball incidence throughout the season. Lipids, sugars, and glycogen reserves were measured in adults after overwintering. Our results suggest that diapause incidence is more strongly influenced by the conditions experienced by the mother than those experienced by the progeny. Our study is the first to report that stressful nesting cavity temperatures are directly linked to increased pollen ball incidence. Post-overwintering macronutrient reserves did not vary between early and late season diapausers. Lipid stores increase with more exposure to stressful temperatures during the larval stage. In conclusion, maternal effects and temperature were important factors for diapause and pollen ball incidence in M. rotundata with macronutrient reserves similar for early and late season bees.