Environmental impacts on diapause and survival of the alfalfa leafcutting bee, Megachile rotundata

Authors: WILSON, ELISABETH - North Dakota State University; MURPHY, CLAIRE - North Dakota State University; WONG, COVEY - North Dakota State University; Rinehart, Joe; Yocum, George; BOWSHER, JULIA - North Dakota State University

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/30/2021
Publication Date: 8/3/2021
Citation: Wilson, E.S., Murphy, C., Wong, C., Rinehart, J.P., Yocum, G.D., Bowsher, J.H. 2021. Environmental impacts on diapause and survival of the alfalfa leafcutting bee Megachile rotundata. PLOS ONE. https://doi.org/10.1371/journal.pone.0254651.
DOI: https://doi.org/10.1371/journal.pone.0254651

Interpretive Summary: In the United States and Canada, the primary pollinator of alfalfa is the alfalfa leafcutting bee, Megachile rotundata. The alfalfa leafcutting bees spend 9 to 10 months a year in a state of dormancy known as diapause. Early in the growing season some bees will not enter diapause but will continue development. These nondiapausing bees negatively impact the number of bees the growers will have for the following growing season because they kill siblings as they emerge; spread disease to neighboring nests; and often leave the alfalfa field, meaning their nests cannot be collected by farmers. The reason why some bees fail to enter diapause is currently unknown. Therefore, the Insect Genetics and Biochemistry Research Unit, Fargo, ND, in collaboration with researchers at the North Dakota State University, Department of Biology designed a novel nesting box that enables us to monitor key environmental cues (day length, temperature, and nutrition) known to trigger diapause in other insect species. We determined that it is the mother that determines whether her offspring will enter diapause. The female alfalfa leafcutting bee uses day length as its primary cue controlling whether her offspring will enter diapause, with longer days blocking diapause. This is unusual in that it is far more common for the environmental cues experienced by an individual insect, not its mother, to determine if it will enter diapause. These results provide key insights into possible new nest designs that will decrease the number of non-diapausing bees, thereby improving the over health of the bee population under management as well as increasing the number of bees available for pollination.

Technical Abstract: Megachile rotundata exhibits facultative diapause but the cues are not well understood. Possible cues for diapause include photoperiod and temperature of the nest cavity. M. rotundata spends its entire developmental period in the nesting cavity without the ability to move or change its environment, potentially being subject to stressful temperatures that may affect diapause incidence and survival. To estimate the true impact of experienced cavity temperature on offspring, we designed a 3D printed box that measures individual cavity temperature. We monitored nesting temperature from June 21st -Sept. 22nd and followed offspring through development to measure diapause incidence and mortality. We found that nesting cavity temperature does not influence diapause incidence, and that photoperiod is a strong cue for diapause. Eggs that were laid during long days had lower probabilities of diapause. Females established nests that contained both diapausing and non-diapausing individuals. Some of the non-diapausing individuals had the potential to kill diapausing siblings upon emergence. Nest cavities reached stressful temperatures but those temperatures did not affect survival. Mortality was significantly higher in non-diapausing bees, and the individuals that were laid first in the nest also had higher rates of mortality. In conclusion, we demonstrate a strong maternal effect for diapause that is partially mediated by photoperiod and is independent of temperature. We determine that juvenile M. rotundata can survive temperatures under field conditions that have been demonstrated as stressful in other studies.