Comparison of fluctuating thermal regimes and commercially achievable constant-temperature regimes for short-term storage of the alfalfa leafcutting bee (Hymenoptera: Megachilidae)

Authors: Yocum, George; Rajamohan, Arun; Rinehart, Joe

Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/24/2021
Publication Date: 3/4/2021
Citation: Yocum, G.D., Rajamohan, A., Rinehart, J.P. 2021. Comparison of fluctuating thermal regimes and commercially achievable constant-temperature regimes for short-term storage of the alfalfa leafcutting bee (Hymenoptera: Megachilidae). Journal of Economic Entomology. https://doi.org/10.1093/jee/toab019.
DOI: https://doi.org/10.1093/jee/toab019

Interpretive Summary: The alfalfa leafcutting bee, Megachile rotundata, is the primary pollinator used in the production of alfalfa seed. M. rotundata is also used to pollinate several specialty crops such as carrot, onion, and blueberry. The growers seek to synchronize the peak nesting activity of the females to the crop bloom in order to optimize seed production. Growers seek this optional timing by transferring the developing bees between 29°C and temperatures below which the bees can develop (low-temperature storage) to adjust for changes in the weather. Exposure to low-temperatures storage can be stressful depending on the temperature and duration of exposure. Previously the Insect Genetics and Biochemistry Research Unit, Fargo, ND, in collaboration with researchers at the North Dakota State University, Department of Biology had developed new low-temperature storage protocols that significantly improved survival of the bees. However, these new protocols place significant strain on the refrigeration compressors used by growers to hold the bees at the lower temperature. To address this problem without compromising the bees’ survival we explored variations of our early low-temperature storage regimes. Here we outline recommendations for low-temperature protocols that minimize the mechanical stress on the compressor while maximizing the bees’ survival. These new guidelines will ensure the availability of high-quality bees for the nation’s alfalfa and specialty crops producers.

Technical Abstract: Interrupting the spring incubation of Megachile rotundata (F.) with a period of low-temperature storage for synchronizing the bees’ emergence with crop bloom is an essential part of M. rotundata management protocol. Previously, we demonstrated that a wide range of fluctuating thermal regimes (FTRs) differing in their thermoprofiles have significantly higher survival rates as compared to bees exposed to constant 6°C. But, rapidly changing the temperature in the large thermal mass of bees commonly found in most commercial settings is one of the greatest challenges to moving an FTR based storage protocol into commercial production. To address this challenge, we investigated raising the cryophase of the FTR from 6 to 12 or 15°C and compared the successful storage rates of these bees against bees exposed to commercially relevant constant temperatures. For bees whose spring incubation was interrupted at the eye-pigmented stage the FTRs 6-18°C and the control FTR (6°C with a daily 1 hour pulse at 20°C) had the highest successful storage rates for the two years tested. For the constant temperature storage protocols, constant 15 and 18°C were either equivalent or lower than the control FTR. The 6-18°C FTR had the highest successful storage rates for bees stored as emergence-ready adults. The next highest successful storage rates for the emergence-ready adults were achieved in the constant 15°C and control FTR storage protocols. The constant 15°C and the control FTR had equivalent successful storage rates. Under the current constraints imposed by commercial chambers’ compressors interrupting M. rotundata spring incubation by exposing the developing bees to constant temperature of 15°C or above is currently the best option for commercial operations.