Microclimate temperatures impact nesting preference in Megachile rotundata

Authors: WILSON, ELISABETH - North Dakota State University; MURPHY, CLAIRE - College Of William & Mary; Rinehart, Joe; Yocum, George; BOWSHER, JULIA - North Dakota State University

Submitted to: Environmental Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/23/2020
Publication Date: 2/28/2020
Citation: Wilson, E.S., Murphy, C.E., Rinehart, J.P., Yocum, G.D., Bowsher, J.H. 2020. Microclimate temperatures impact nesting preference in Megachile rotundata. Environmental Entomology. https://doi.org/10.1093/ee/nvaa012.
DOI: https://doi.org/10.1093/ee/nvaa012

Interpretive Summary: The alfalfa leafcutting bee is a solitary, cavity nesting insect and is the primary pollinator used in the production of alfalfa seed in North America. Like most cavity nesting bees, adverse conditions inside the nesting cavity, such as high temperatures, can decrease offspring fitness. However, we don’t know if females choose nest cavities that mitigate their offspring’s exposure to stressful temperatures. This study aimed to understand how cavity temperature impacts the nesting preference and fitness of this bee under field conditions. We designed a 3D printed nesting box with nesting cavities facing in four different directions while the temperature of each cavity was recorded throughout the day. Our study found that temperature significantly varied by direction the nest box faced and by the position of the cavity within the nest box, and that bees preferred to nest in cavities with lower average temperatures during foraging hours. Also nesting females filled cavities on the northeast and northwest facing sides faster than cavities on the south facing sides, and the direction of the cavity impacted number of offspring per nest, with southwest facing cavities having fewer offspring than nests laid on the cooler northeast side. Our study indicates that the nesting box acts as a microclimate, with temperature varying by position and direction of the cavity and variation in cavity temperature impacted individual bee choice and fitness.

Technical Abstract: Nest microclimates may have significant impacts on maternal nest choice and offspring fitness. In cavity nesting bees, high cavity temperatures decrease offspring fitness. Females may choose nest cavities that mitigate their offspring’s exposure to stressful temperatures. This study aims to understand how cavity temperature impacts the nesting preference and fitness of the solitary bee M. rotundata under field conditions. We designed a 3D printed nesting box that measured the temperatures of 432 cavities. Nest boxes were placed along an alfalfa field in Fargo, ND facing northeast, northwest, southeast, and southwest and were observed daily for completed nests. Our study found that temperature significantly varied by direction the nest box faced and by the position of the cavity within the nest box. The southwest sides recorded the highest maximum temperatures while the northeast sides recorded the lowest maximum temperatures. Nesting females filled cavities on the northeast and northwest facing sides faster than cavities on the south facing sides. The bees preferred to nest in cavities with lower average temperatures during foraging hours. Direction of cavity impacted number of offspring per nest. The southwest facing cavities had fewer offspring than nests laid on the cooler northeast side. Our study indicates that the nesting box acts as a microclimate, with temperature varying by position and direction of the cavity and variation in cavity temperature impacted individual bee choice and fitness.