Life history factors
Known differences in social and solitary bee life histories can offer insights into potential mechanisms of climate resilience. Principal among these differences is the seasonal life cycle of social bees. Solitary bees are active as adults for narrow windows of time, averaging about one month per year, but often for much shorter (Michener, 2007). This restricted activity period could render solitary bees particularly vulnerable to phenological mismatch as host plants advance their flowering times under climate change (Kudo and Ida, 2013; Visser and Gienapp, 2019). Longer activity periods, however, present other challenges. Social bees are typically multivoltine (i.e., they produce multiple broods per year), with an adult activity period spanning several months (Michener, 1974). The breadth of social bee species’ activity windows may increase their likelihood of being impacted by resource gaps (Kaluza et al., 2018; Requier et al., 2020). However, these disadvantages of a long colony lifespan may be counterbalanced by behavioral adaptations common to social species, including polylecty and resource storage, that enhance survival when floral resources are depleted.
Social and solitary bees also differ markedly in their overwintering life cycles. Most solitary bees spend most of the year in diapause in the pre-pupal stage. Eusocial bees, by contrast, typically overwinter as fertilized adult female gynes. Several studies have demonstrated that bees overwintering as adults are more sensitive to increased winter temperatures, leading to increased mortality and weight loss relative to species overwintering in the prepupal stage (Fründ et al., 2013; Kammerer et al., 2021; Slominski and Burkle, 2019). Importantly, not all solitary bees conform to the typical, likely ancestral pattern of prepupal winter diapause, and communal bees exhibit a mix of adult and pre-pupal overwintering life histories. For the eusocial bees, however, many of which are adult-wintering, warming winter temperatures may be disproportionately challenging.