UNIVERSITY PARK, Pa. — 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.
In a study published recently in Ecology and Evolution, 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. They found that variation in heat tolerance was influenced by size, sex and infection status of the bees.
“Small-bodied, ectothermic — or cold-blooded — insects are considered to be highly vulnerable to changing climate because their ability to maintain proper body temperature depends on external conditions,” said study author Laura Jones, who led the research as a doctoral candidate in ecology at Penn State and is now a postdoctoral fellow at the University of Texas at Austin. “Understanding how organisms tolerate temperature extremes is critical for assessing the threat climate change poses to species’ distribution and persistence.”
Jones noted that there is growing interest in studying the heat tolerance and acclimation capacity of ectotherms in the face of changing abiotic conditions such as ambient temperature.
“But few studies have examined biotic impacts, such as pathogen infection, on thermal tolerance in natural populations in combination with abiotic factors,” she explained. “In addition, physical traits such as body size or fat content can impact how organisms tolerate temperature, so it’s vital to consider individual condition, as well as the abiotic and biotic factors that individuals experience in natural environments, when evaluating the heat tolerance of populations.”
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 males, in contrast, are smaller, forage only for nectar, and buffer themselves from heat by retreating into wilted flowers after they finish foraging at midday. So, males are exposed to more variable ambient temperatures than females during the day and night.
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.
To test these hypotheses, the researchers collected squash bees from 14 sites across Pennsylvania that varied in mean temperature, precipitation and soil texture. They measured individuals’ critical thermal maximum — the temperature above which an organism cannot function — as a proxy for heat tolerance and determined the relative intensities of three parasite groups: protozoan parasite trypanosomes, bacterial pathogen Spiroplasma apis and microsporidian parasite Vairimorpha apis.