How well bees tolerate temperature extremes could determine their ability to persist in a changing climate. But heat tolerance varies between and within populations, so a research team led by Penn State entomologists examined bee physical traits -- such as sex differences in body mass -- to understand how these traits interact with environmental conditions, pathogens and other factors. The researchers measured body mass, local climate and pathogen intensity to assess how these factors influence heat tolerance and its population-level variation among individuals of the species Xenoglossa pruinosa, commonly known as the hoary squash bee. Understanding how organisms tolerate temperature extremes is critical for assessing the threat climate change poses to species' distribution and persistence.

A pollinator of cucurbit crops such as squash and pumpkin, the hoary squash bee is a solitary species that exhibits sex differences in physiology and behavior, according to the researchers. Females are larger than males and collect pollen for offspring in the morning through midday. The females nest underground, which buffers them from variations in air temperature, although soil texture may affect the degree of thermal buffering as sandy soils have a lower heat capacity. The researchers hypothesized that the bees' heat tolerance would increase with body size; that male heat tolerance would increase with ambient temperatures above ground whereas female heat tolerance would increase with sandier soils; and that parasite infection would reduce heat tolerance.

However, the results did show that where average maximum temperatures were highest, the variation in heat tolerance among individuals was lower, suggesting that extreme temperatures were "filtering out" individuals with high and low critical thermal maximums, the researchers said. Overall, the study contributes to growing evidence that small-bodied invertebrates' ability to adapt or acclimate their heat tolerance to local climate conditions is limited and depends on several factors. Given this, it is critical to identify the populations that are at risk under future climate scenarios, they suggest that future research assesses the thermal tolerances of populations across a species' distribution to identify those that are most vulnerable to local extinction.