Evaluating species’ thermal tolerance is important now more than ever given the ongoing threats to biodiversity under climate change. Species vary widely in heat tolerance, but the degree of variability in their heat tolerance, and the drivers of this variability among populations, are less understood. We expect that some of this variation in bees may be attributed to microclimate, physical differences among individuals, or differences in their infection status. Among “cold-blooded” animals (internal body temperatures are not regulated), larger organisms tend to be more thermally tolerant, and so size likely explains some heat tolerance variation. For individuals in the wild, other factors may impact thermal tolerance, such as infection.
Bees are cold-blooded animals that pollinate the majority of plants in both natural and agricultural ecosystems. Several bee species are known to be in decline, and stressors such as climate variability and pathogen pressure, among others, are drivers of these declines. However, how these factors impact heat tolerance in bees has not been characterized for wild populations. In this study, we asked, (1) is variation in heat tolerance among populations explained by temperature?, (2) are there size differences among individuals that relate to differences in heat tolerance?, and finally, (3) does pathogen infection reduce heat tolerance?
We evaluated the heat tolerance of squash bee (Xenoglossa pruinosa) populations across a thermally variant gradient in Pennsylvania, USA. To determine a bee’s heat tolerance, we measured its critical thermal maximum (CTmax), which gives us a proxy for the highest temperature that individuals can withstand. We also weighed the bees and screened them for three common parasite groups – trypanosomes (e.g., Crithidia mellificae), Spiroplasma apis (mollicute bacteria), and Vairimorpha apis (microsporidian, formerly Nosema apis) – to see if body size or pathogen load impacted their heat tolerance.
We found that temperature did not predict heat tolerance in our squash bee populations. However, we found a strong association between temperature and population-level variation in heat tolerance. Specifically, sites with higher daily temperatures excluded bees with low or high heat tolerance, suggesting that heat stress reduces variation in this trait. Regarding size, we found that larger squash bees were more heat tolerant in congruence with previous studies, but interestingly, males were twice as sensitive to this size effect compared to females! Male squash bees were 40% smaller than females, and so this finding suggests that smaller individuals may be more vulnerable to heat stress. Lastly, we only found that one parasite group, trypanosomes, reduced heat tolerance in highly infected individuals – and again, this effect was sex-dependent! It seems that female squash bees have a harder time tolerating heat when they are hosting a lot of trypanosome parasites in their guts, whereas male heat tolerance isn’t affected by the extra company.
So, what do our findings mean for understanding variation in heat tolerance among bees and their response to heat stress in the future? First of all, this study provides preliminary evidence that extreme heat can reduce variation in heat tolerance within populations, even if it doesn’t reduce mean heat tolerance at those sites. This is relevant for understanding if populations will be able to buffer themselves against future climate regimes, as it suggests that heat stress is already excluding squash bees with low heat tolerance from sites and reducing heat tolerance for the others that are there. Our results also reveal an important gap in most studies of heat tolerance variation among insects and other taxa – sexes may vary in how size and infection status impact heat tolerance. In our system, we know that the sexes differ physically, behaviorally, and physiologically. For example, female squash bees dig nests underground to lay their eggs, spend their mornings collecting pollen for their young, and go to sleep in their nests by midday. In contrast, the smaller male squash bees do not need to collect pollen for their offspring, and instead look for mates in the morning and then go to sleep in wilted squash flowers. So, the squash bee sexes are expending energy differently (i.e., differences in parental care) and are exposed to different microclimates (i.e., females nest underground). Given the known differences in physical, behavioral, and physiological phenotypes among sexes for many species, the López-Uribe lab will continue to consider sex when evaluating bee thermal tolerance in current and future studies.
For more details, check out the full article here:
Jones, L. J., Miller, D. A., Schilder, R. J., López-Uribe, M. M. (2024). Body mass, temperature, and pathogen intensity differentially affect critical thermal maxima and their population-level variation in a solitary bee. Ecology and Evolution, 14(2), e10945
Contributed post and photography by
Laura J. Jones
Postdoctoral Fellow of Plant-Pollinator Ecology
Department of Integrative Biology at UT Austin
This study was funded by the USDA-NIFA-AFRI Pollinator Health Program, Project 2022-67013-36274
Questions? Contact Laura Jones via email at laura.jones@austin.utexas.edu or the López-Uribe lab at lopezuribelab@gmail.com
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