Thermoprofile parameters affect survival of Megachile rotundata during exposure to low-temperatures

Authors: Yocum, George; Rinehart, Joe; Rajamohan, Arun; BOWSHER, JULIA - North Dakota State University; Yeater, Kathleen; GREENLEE, KENDRA - North Dakota State University

Submitted to: Integrative and Comparative Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/28/2019
Publication Date: 7/4/2019
Publication URL: https://handle.nal.usda.gov/10113/6759788
Citation: Yocum, G.D., Rinehart, J.P., Rajamohan, A., Bowsher, J.H., Yeater, K.M., Greenlee, K.J. 2019. Thermoprofile parameters affect survival of Megachile rotundata during exposure to low-temperatures. Integrative & Comparative Biology. 59(4):1089-1102. https://doi.org/10.1093/icb/icz126.
DOI: https://doi.org/10.1093/icb/icz126

Interpretive Summary: The primary pollinator used in the production of alfalfa seed in North America is the alfalfa leafcutting bee (Megachile rotundata). Besides alfalfa, the alfalfa leafcutting bee is employed to pollinate a number of specialty crops such as carrot, onion, and blueberry. To maximize both pollination and the production of new bees for the following year, bee nesting activity must be synchronized with peak crop bloom. Currently, this is accomplished by first predicting the bloom date (using a mathematical model) and then transferring the overwintering prepupae to 29-30°C approximately a month prior to the expected bloom to initiate development for adult bees to emerge on time. This process can be complicated by environmental conditions that could change, thereby delaying the bloom, and causing pollination managers to interrupt the high temperature incubation with a period of low-temperature incubation to slow the bees’ development. Previously, we demonstrated that using a fluctuating temperature regime, in which insects are given a daily high temperature pulse, improves both bee survival and pollinator health as compared to the standard practice of constant temperature storage protocols. However, fluctuating temperature regimes can be defined by a number of components. So an investigation was carried out to determine how these various components affect bee survival during low-temperature storage. Survival was not related to the total amount of time spent at higher temperatures, but to several specific aspects of the fluctuating temperature regime. These results can serve to develop novel low-temperature procedures, resulting in improved low-temperature storage survival and a subsequent increase in pollinator availability.

Technical Abstract: Fluctuating the temperature during the exposure of insects to sub-optimal temperatures increases their survival and can also block the development of sub-lethal effects as compared to insects exposed to constant temperatures. How the temperature fluctuates impacts the rate of survival and the induction of sub-optimal effects is poorly understood. One reason for this dearth of information is that the wide range of temperature parameters are rarely compared in a single study. In this study two developmental stages of Megachile rotundata, eye-pigmented pupae and emergence-ready adults, were exposed to one of eight thermoprofiles. All the thermoprofiles had a base of 6°C, and an upper temperature of either 12°C or 18°C (peak temperature). The duration at peak temperature varied depending on the shape of the thermoprofile, either square or wave form. Two other treatments acted as controls, a constant 6°C and a thermoprofile with a base temperature of 6°C that was interrupted daily by a single one hour pulse at 20°C (FTR). Compared to constant 6°C all the test thermoprofiles significantly improved survival. A more complex picture emerges when the thermoprofiles are examined against the FTR control. The thermoprofiles subdivided into two groups based on the peak temperature with the 18°C profiles outperforming the 12°C profiles. In the eye-pigmented stage the 18°C profiles subdivided into two groups based on the shape of the profile with the square profiles yielding higher survival rates than that of the wave profiles. In the emergence-ready stage all the 18°C profiles had significantly higher survival rate than the FTR controls. Only the two 18°C square form thermoprofiles were able to block the induction of the delay in the time-to-emergence seen in all other test thermoprofiles. Counter to expectations the least ecologically relevant thermoprofiles (square) had the highest survival rates and blocked the development of sub-lethal effects (delayed emergence).