BARTHELL, J. F.*; HRANITZ, J. M.; THORP, R. W.; OVERALL, L. M.; GRIFFITH, J. L.; University of Central Oklahoma; Bloomsburg University of Pennsylvania; University of California, Davis; University of Central Oklahoma; University of Central Oklahoma: Heat Stress Responses of an Invasive Leafcutting Bee Species Nesting in the Central Valley of California: A Case of Risky Maternal Behavior?

We examined heat stress responses in egg, larval, prepupal, pupal and adult stages of the invasive solitary bee Megachile apicalis during its nesting season in the Central Valley of California. Artificial nesting cavities were affixed to vertical plywood boards oriented so as to maximize temperature differences: one side faced south (exposed to direct sun) while the other one faced north (shaded). After several weeks we returned, removing nesting females and brood cells containing their offspring from nests during the hottest period of the day. (Temperatures between 45 ^oC and 50 ^oC were recorded in exposed nests.) Levels of a stress protein, heat shock protein 70 (hsp70), were later measured from homogenized specimens using ELISA tests. Eggs and larvae contained significantly higher levels of hsp70 on the exposed sides of boards relative to the same developmental stages on shaded sides. Females associated with nests from the exposed sides of boards showed higher levels of stress protein relative to those associated with nests on the shaded sides. In a subsequent study we found that mortality, attributed to heat stress, was significantly higher for offspring in nests exposed directly to the sun. Our studies demonstrate that despite the stressful (even lethal) conditions within which these female bees nest, their offspring appear to survive in sufficient numbers to inoculate new, high-temperature habitats with incipient populations of M. apicalis. We discuss these patterns in the context of this species� invasion of California and the western USA.