Costs and benefits of maternal nest choice: tradeoffs between brood survival and thermal stress for small carpenter bees

Nest site selection is a crucial decision for bees because where mothers construct their nests influences the developmental environment of their offspring. Small carpenter bees (Ceratina calcarata) nest in sun or shade, suggesting that maternal decisions about nest sites are influenced by thermal conditions that influence juvenile growth and survival. We investigated the costs and benefits to mothers and their offspring of warmer or cooler nest sites using a field experiment in which mothers and newly founded nests were placed in sunny or shady habitats. Maternal costs and benefits in sunny and shady treatments were quantified by comparing brood provisioning behaviour, nest size, number of brood cells, and offspring survival rates. Juvenile costs and benefits were quantified as body size, high temperature tolerance (CTmax), metabolic rate, and pupal duration. The major maternal benefit of nesting in sun was significantly lower rates of total nest failure (caused by predation, parasitism or abandonment), which led to sun mothers producing 3.2 brood on average, while shade mothers produced only 2.9. However, sun nesting entailed costs to brood, which were significantly smaller, less likely to survive to adulthood and had significantly elevated CTmax. This suggests that juvenile bees in sun nests bees experienced thermal stress during development, causing them to shunt resources from growth to thermoprotection, at the cost of smaller size and higher mortality. Pupae raised in a thermal-gradient "BeeCR" machine developed significantly faster at warmer average temperatures, which may be an additional benefit of sun nesting. Overall, our results highlight a tradeoff between maternal benefits and offspring costs when mothers choose nest sites, in which maternal fitness is enhanced by nesting in sun, despite significant physiological costs to offspring, due to the necessity for thermoprotective responses. Thinking through pandemic researchThe first lockdowns of the COVID-19 pandemic began as we prepared to enter the second field season of this study in 2020. Student research halted overnight. Lab access and travel were restricted. With limited access to field sites and no access to lab equipment, we brainstormed alternative approaches that would repeat, if not replicate, our main experiments of 2019 and fulfill degree requirements for JL de Haans MSc in a satisfying way. Our 2019 results had provided convincing evidence developmental temperature has long-term impacts on C. calcarata physiology, so we thought about which physiological measurements would be feasible outside the lab. Authors MH Richards and GJ Tattersall suggested collecting more measurements of CTmax: the Peltier plate device required running water, but a portable water pump and a bucket allowed the apparatus to be set up anywhere. No calibration of instruments was required, and the only maintenance was to change the water in the bucket after a few hours of use. Thus, a students home basement became a laboratory. To investigate how temperatures affect developmental rate, we needed to raise bees in controlled environments, but incubators were not available. Author A Skandalis suggested repurposing a gradient PCR unit as a portable insect incubator ("The BeeCR"). The idea was tested successfully at home in 20202, so a larger study was done by J Maretzki in 2021 when undergraduate lab access was permitted again. Two outcomes of our pandemic pivot produced long-term benefits for our research. The BeeCR is a flexible, inexpensive, easy-to-use incubator perfectly suited for raising small insects at multiple simultaneous sets of variable temperatures. And the ease with which "field" sites could be established in our backyards demonstrates how amenable small carpenter bees are to field manipulations, suggesting this is a model species for addressing a variety of ecological and physiological questions.