As climate warms, temperatures are more likely to exceed organisms’ upper thermal limits, especially in already hot and arid regions. Species whose life histories expose them to temperature extremes will experience the hottest temperatures, and unlike broader-scale climate patterns, microclimate changes are harder to predict. But microclimates are often the proper scale for considering temperatures of insect habitats. Despite the importance of this fact in predicting whether insects will survive extremes, in-nest temperatures experienced by developing bees are largely unknown. The desert carpenter bee (Xylocopa californica) is an aboveground-nesting bee and a common pollinator in desert ecosystems. We asked what temperatures carpenter bees experience inside their nests, and whether the nest substrate insulates against temperature extremes. To test this, we installed 8 paired thermocouple probes inside and outside 60 active carpenter bee nests in southern Arizona and compared internal and external temperatures. We also compared temperatures within nest substrates to air temperatures from a nearby weather station. Larval CT_max was determined using flow-through respirometry. Nest temperatures were measured during 2020, the hottest summer on record in southern Arizona, and are presented in the context of larval thermal tolerance.