Precipitation effects
Social strategies can also help mitigate environmental stressors associated with precipitation and water availability. Many of these effects are intrinsically bound to thermal effects, working in concert with temperature variation to determine whether conditions are favorable or unfavorable for foraging and brood rearing. Annual variation in precipitation can dictate activity periods, either by inhibiting foraging in times of extended rainfall or by creating floral dearth periods in times of drought. Indeed, for the facultatively eusocialAugochlorella aurata , drought conditions reduced brood sizes by two to three offspring (Packer, 1990). Conversely, Schürch et al. suggest that an increase in spring rainfall under climate change could reduce the frequency of social nesting in Halictus rubicundus , by delaying provisioning and reducing the time window to produce a second brood (2016). Likewise for Halictus ligatus , high rainfall created conditions unfavorable for worker production, with consequences for social organization (Richards and Packer, 1995). Finally, precipitation can pose direct survival risks that may affect the frequency of social nesting. Heavy rainfall and flooding threaten brood survival, which can reduce worker recruitment, decreasing colony size and restricting opportunities for social nesting.
Precipitation can also alter features of the physical environment that determine the costs and benefits of social nesting. For ground nesting bees, nest excavation may be particularly costly when soils are hard, especially during drought. Danforth suggests that the high energetic costs of excavating dry, hard-packed soils favors communal nesting for the desert-adapted bees Perdita portalis Timberlake, 1954 (1991) and Macrotera texana Cresson, 1878 (1996). Drying of soils is a predicted consequence of climate change in many regions, which may raise the costs of nest excavation for ground nesting bees, thereby increasing the benefits of cooperative nest excavation. Indeed, Bohart and Youssef observed an increase in the incidence of social nesting during drought conditions for the typically solitary sweat bee Lasioglossum lusorium Cresson, 1872 (1976). In addition to energetic costs, excavation of dry soils could entail increased cuticular wear, potentially increasing risk of desiccation. However, the physiological and behavioral consequences of dry soil excavation remain to be tested empirically.