Technical Abstract: The alfalfa leafcutting bee (Megachile rotundata) is affected by a fungal disease called chalkbrood. In several species of bees, chalkbrood is more likely to occur in larvae kept at 25-30 C than at 35 C. We found that both high and low temperature stress increased the expression of immune response genes in the alfalfa leafcutting bee, and thus increased disease resistance. Genes associated with pathogen recognition and trypsin-like serine proteases were most highly expressed at the lowest rearing temperature (20 C), while prophenoloxidase, melanization, immune response signalling pathways, effectors, and reactive oxygen species were most highly expressed at the warmest temperature (35 C). In addition, the pathogen appears to be able to affect other host functions, particularly protein synthesis, but differentially at different temperatures, with the lowest rates of protein synthesis occurring at 30°C. We hypothesize that temperature stress heightens the insect immune response before the pathogen has elicited a response in the host, and it is this early activity that prevents infection.