Rising Temperatures Force Bees and Wasps to Emerge Earlier Depleting Fat Reserves

A study published in Functional Ecology analysing 14,921 cavity-nesting bees and wasps from 161 sites in Bavaria finds that warmer temperatures consistently advance emergence after hibernation across all five species studied, while higher temperatures accelerate depletion of fat reserves that individuals depend on for survival and reproduction. Summer females of some species lost up to 34 per cent of body mass under warmer treatment conditions.

The research examined five species: Osmia bicornis, Chelostoma florisomne, Trypoxylon figulus, Heriades difformis and Heriades truncorum, under cold, warm and hot post-winter laboratory conditions simulating a temperature range of 5.9°C to 10°C observed across Bavaria. All species emerged earliest under hot conditions and latest under cold treatments. Osmia bicornis showed the strongest response, emerging earliest among spring species under warmer conditions. Heriades truncorum, typically late-emerging in colder environments, showed a counterintuitive pattern by emerging earlier than warm-region counterparts, possibly linked to faster development rates needed to complete its life cycle within shorter summers.

Earlier emergence creates a phenological mismatch risk: if insects emerge before flowers are available or before prey cycles begin, newly hatched individuals may be unable to find food. Mass loss was especially pronounced among summer females, with some losing up to 34 per cent of body weight under warmer post-winter treatments. In Heriades truncorum, females in warmer treatments emerged within a shorter time window and retained higher body mass than those in colder conditions, suggesting reproductive pressure drives sex-specific differences in energy conservation strategies. The effect is critical because fat reserves directly determine pollination performance and reproductive success.

The researchers identify key open questions for future research: how additional days of extreme heat affect emergence timing, how energy reserve depletion influences actual pollination performance in the field and how quickly insect populations can adapt genetically to rapidly changing climatic conditions. The study notes that local adaptation to climate, where insect populations have evolved different thermal sensitivities in cooler versus warmer sites, may affect how populations respond to future warming, making population-level monitoring across temperature gradients essential for biodiversity conservation planning.

Key figure — Up to 34% body mass lost by summer females under warmer conditions