Body mass, temperature, and pathogen intensity differentially affect critical thermal maxima and their population-level variation in a solitary bee

Climate change presents a major threat to species distribution and
persistence. Understanding what abiotic or biotic factors influence the
thermal tolerances of natural populations is critical to assessing their
vulnerability under rapidly changing thermal regimes. This study evaluates
how body mass, local climate, and pathogen intensity influence heat
tolerance and its population-level variation (SD) among individuals of the
solitary bee Xenoglossa pruinosa. We assess the sex-specific relationships
between these factors and heat tolerance given the differences in size
between sexes and the ground-nesting behavior of the females. We collected
X. pruinosa individuals from fourteen sites across Pennsylvania, USA that
varied in mean temperature, precipitation, and soil texture. We measured
the critical thermal maxima (CTmax) of X. pruinosa individuals as our
proxy for heat tolerance, and used quantitative PCR to determine relative
intensities of three parasite groups—trypanosomes, Spiroplasma apis
(mollicute bacteria), and Vairimorpha apis (microsporidian). While there
was no difference in CTmax between the sexes, we found that CTmax
increased significantly with body mass, and that this relationship was
stronger for males than for females. Air temperature, precipitation, and
soil texture did not predict mean CTmax for either sex. However,
population-level variation in CTmax was strongly and negatively correlated
with air temperature, which suggests that temperature is acting as an
environmental filter. Of the parasites screened, only trypanosome
intensity correlated with heat tolerance. Specifically, trypanosome
intensity negatively correlated with the CTmax of female X. pruinosa but
not males. Our results highlight the importance of considering size, sex,
and infection status when evaluating thermal tolerance traits.
Importantly, this study reveals the need to evaluate trends in the
variation of heat tolerance within and between populations, and consider
implications of reduced variation in heat tolerance for the persistence of
ectotherms in future climate conditions.