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.