Beneficial effects of exposure to fluctuating temperatures during interrupted development in insects

Many insects overwinter in diapause, a state that protects them from chill injuries due to exposure to low temperatures. Those insects resume development in the spring, leaving behind the protection of the diapause state. Spring cold snaps and variable weather, which are expected to increase with climate change, make spring development risky for some insects. Insects exposed to constant low temperature stress while they are undergoing pupation have been shown to have increased mortality, wing deformities, and flight defects. Interrupting pupal development with recurring pulses of heat instead of a constant low temperature abrogates these effects. However, the mechanisms underlying the beneficial effects of fluctuating temperatures are largely unknown. Most evidence points to the warm period as a time of repair, but the specific systems that are being repaired remain unknown. To broadly explore what systems are involved, we used the alfalfa leafcutting bee, Megachile rotundata. This species is used in agriculture, commercially reared, and has been shown to exhibit benefits of exposure to fluctuating temperatures during development. Bees undergoing metamorphosis were exposed to constant (6C) or fluctuating (1 hour a day at 20C) low temperature stress. After one week in the temperature treatment, bees were flash frozen and assayed for macromolecule concentrations. We measured total lipid, sugar, and stored glycogen and trehalose concentrations. We predicted that total lipid content and glycogen would be higher in bees interrupted with fluctuating low temperature stress. Bees exposed to constant low temperature stress had 40% lower total lipid concentration compared to bees exposed to fluctuating stress, suggesting that not only lipid stores and possibly cell membranes may be compromised in those bees. Understanding the mechanisms underlying beneficial effects of fluctuating temperatures is important for several reasons. In M. rotundata, farmers routinely interrupt development with constant low temperature stress, resulting in bee loss and subsequent decreases in pollination services. Furthermore, many studies are carried out at constant temperatures, and insects are frequently reared at constant temperatures, possibly creating stressful conditions that could negatively affect an insect's physiology