Voltinism of a solitary bee was influenced by temperature but not provision size.

Abstract

Changes in the timing and duration of life cycles are distinctive fingerprints of environmental change. Yet, the biotic and abiotic cues underpinning phenology and voltinism, i.e., number of generations per year, are poorly understood. Here, I experimentally test how temperature and provision size influence voltinism and survival to emergence in a solitary bee Colletes validus, and how temperature influences voltinism in the brood parasite Tricrania sanguinipennis. Within the same population, univoltine individuals emerge after 1 year (1-year form), whereas semivoltine individuals enter prolonged dormancy and emerge after 2 years (2-year form). I reared field-collected bees under 2 × 2 factorial experiments with cool (18.5 °C ± 0.5 °C) vs. warm (24 °C ± 0.5 °C) temperature treatments (bees and beetles) and no supplement vs. supplemental food treatments (+ 20% ± 5% pollen provision by mass); beetles were reared under temperature treatments only. Cool temperatures consistently increased the proportion of 2-year bees regardless of provision size, a finding that was consistent with three years of field observations. There was a demographic cost to prolonged dormancy in that both 1- and 2-year bees survived to emergence as adults, but survival of 2-year bees was approximately 50% lower than 1-year bees. Two-year beetles were produced under cooler temperatures, but unlike bees, beetles had nearly perfect survival to emergence in all treatments. This experiment advances our mechanistic understanding of the environmental drivers of voltinism in diverse insect taxa and underscores the importance of considering cryptic life stages when interpreting responses to environmental change.