Solitary Bee Life History Traits and Sex Mediate Responses to Manipulated Seasonal Temperatures and Season Length

Abstract

The effects of climate change on solitary bee species, the most diverse and abundant group of wild pollinators, remain poorly understood, limiting our ability to forecast consequences for bee-plant interactions and pollination services. Life history traits, such as overwintering life stage, sex, and body size may influence solitary bee responses to climate change by mediating the effects of temperature on physiological processes spanning fall, winter, and spring. Yet, most studies assessing the effects of temperature on solitary bees have focused on managed species and have isolated the effects of winter temperature. Here, we reared male and female individuals representing eight cavity-nesting solitary bee species that overwinter either as adults (i.e., Osmia spp.) or prepupae (i.e., Megachile spp.). Eight rearing treatments were used, in which we manipulated fall and spring temperature, fall duration, and the timing of spring onset. We measured pre-emergence mortality, pre-emergence weight loss, emergence timing, and post-emergence lifespan. We found that Osmia spp. responded primarily to the timing of spring onset, whereas Megachile spp. responded primarily to spring temperature. Early-spring onset increased both pre-emergence mortality and pre-emergence weight loss and reduced post-emergence lifespan in Osmia spp. In addition, treatments caused unequal shifts in the timing of emergence between male and female Osmia spp. By contrast, warmer spring temperature decreased weight loss, and increased lifespan in Megachile spp. These findings suggest that Osmia spp. may be more vulnerable to negative fitness consequences of climate change compared to Megachile spp., and that climate change may have implications for population-level sex-ratios and mating success in species of Osmia. This work helps build a mechanistic understanding of how life histories may mediate solitary bee responses to climate change, and how these responses may impact solitary bee fitness and plant-bee interactions.

Wild bees as winners and losers: Relative impacts of landscape composition, quality, and climate

“…This result coincides with studies showing that, with earlier spring onset, taxa overwintering as adults had higher pre‐emergence weight loss and mortality and shorter life span post‐emergence (Fründ et al, 2013; Schenk et al, 2018; Slominski & Burkle, 2019). Conversely, species that overwinter as pre‐pupae were more responsive to spring temperature (Slominski & Burkle, 2019) and microclimate, which mediates nest‐site selection (Wilson et al, 2020). This pattern of reduced Andrena and Osmia abundance with warmer winters is concerning for several reasons.…”

“…Physiological differences may be particularly important in determining the response of species to altered climate. For example, compared with wild‐bee species that overwinter as larvae or pupae, species that overwinter as adults have higher metabolic rates during diapause, and, when exposed to warmer winter temperatures, lose more weight, and emerge earlier (Fründ et al, 2013; Schenk et al, 2018; Slominski & Burkle, 2019).…”

“…Similarly, we predicted lower abundance of spring bees overwintering as adults following warm winters. In laboratory studies, compared with species overwintering as larvae, species overwintering as adults have higher mortality with warmer overwintering temperature (Fründ et al, 2013; Schenk et al, 2018; Slominski & Burkle, 2019).…”

Bee phenology is predicted by climatic variation and functional traits

“…The few studies that have examined the phenological response of bees to environmental cues have been limited by practical constraints mostly to small subsets of the total bee community (e.g. Kehrberger and Holzschuh, 2019; Slominski and Burkle, 2019). To understand the full effects of climate change on plant communities, it is imperative to determine the community‐level drivers of bee phenology given the role of bees as the primary pollinators in most ecosystems (Klein et al ., 2007).…”

The Supercooling Responses of the Solitary Bee Osmia excavata (Hymenoptera: Megachilidae) under the Biological Stress of Its Brood Parasite, Sapyga coma (Hymenoptera: Sapygidae)

“…In the orchards, the two species overwinter inside thick reeds, which together with their cocoons provide some insulation from cold air and ice present outside reeds. Only a few studies have investigated the supercooling points of Osmia bees or their natural enemies [ 29 , 30 , 31 ].…”