Thermal history of alfalfa leafcutting bees affects nesting and diapause incidence

Authors: EARLS, KAYLA - North Dakota State University; PORTER, MONIQUE - Pennsylvania State University; Rinehart, Joe; GREENLEE, KENDRA - North Dakota State University

Submitted to: Journal of Experimental Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/20/2021
Publication Date: 11/26/2021
Citation: Earls, K., Porter, M., Rinehart, J.P., Greenlee, K. 2021. Thermal history of alfalfa leafcutting bees affects nesting and diapause incidence. Journal of Experimental Biology. 224(22). https://doi.org/10.1242/jeb.243242.
DOI: https://doi.org/10.1242/jeb.243242

Interpretive Summary: The alfalfa leafcutting bee is critical to the production of alfalfa seed, meaning it is vital to the $9.9 billion alfalfa industry. Alfalfa leafcutting bees are an intensely managed species, with many aspects of its lifecycle carefully controlled. Of particular interest to many bee managers is the spring incubation, which requires a one month incubation at warm temperatures before adult emergence occurs. When weather events lead to a delay in crop bloom, the bees must be delayed as well, which is often accomplished by a short storage period at a low temperature. However, while survival of these low temperatures has been well established, other implications that could cause a reduction in insect quality are less clear. In this study, we exposed bees undergoing spring incubation to two different one week long low temperature treatments: a static thermal regime (STR) where the temperature remained constant, and a fluctuating thermal regime (FTR) where the cold exposure was interrupted by a daily pulse of high temperature. Upon emergence, the bees were then assessed for their performance under field conditions, as well as how the cold treatment affected the resulting offspring. Our results indicate that bees stored under FTR were more likely to nest than those stored under STR, perhaps partially due to sublethal effects such as damaged wings that was observed in the STR treated bees. The resulting offspring were affected as well, with those from FTR treated bees being significantly more likely to enter an overwintering stage than both the STR treated bees and the controls that did not undergo storage. These results demonstrate that many factors should be considered when implementing cold storage protocols for this important alternative pollinator.

Technical Abstract: 1. Variable spring temperatures may expose developing insects to sublethal conditions. Exposure to unfavorable conditions may have long-term consequences that affect offspring quantity and quality. The alfalfa leafcutting bee, Megachile rotundata, overwinters as a prepupa and resumes development in the spring. During immobile stages of development, bees must endure temperature conditions wherever the nest is located. Megachile rotundata develop inside structures built from leaves, called brood cells. Female M. rotundata determine various offspring characteristics including sex ratios, individual body size, and possibly diapause incidence. 2. The purpose of this study was to determine if exposure to low temperature stress during development reduces future reproductive fitness, including offspring quality and quantity. To test this, developing male and female Megachile rotundata were exposed to either a fluctuating (FTR) or constant (STR) low temperature stress for one week, during the prepupal stage. After adult emergence, bees were marked, released into a field cage in an alfalfa field, and observed to measure reproductive output. Offspring were brought into the lab to measure diapause incidence, sex ratios, and dry weight. 3. FTR-treated females were more likely to nest compared to control or STR-treated bees. However, the number of offspring did not statistically differ among groups. Interestingly, offspring of FTR-treated bees were more likely to enter diapause, indicating that thermal history of parents during development is an important factor in diapause determination. 4. Low nesting rates of STR-treated parents may be due to sublethal effects that affected wing morphology and thus, ability to fly and mate or make nests. Females that were able to nest had fewer and larger offspring with high viability suggesting a trade-off between quantity and quality of offspring.