MONITORING SOLITARY BEES IN MODIFIED WILDLAND HABITATS - IMPLICATIONSFOR BEE ECOLOGY AND CONSERVATION

The long-term goal of this study was to use solitary bees to assess th e impact of advancing Africanized honey bees (Apis mellifera scutellat a Lepeletier) on the native California bee fauna. Cavity-nesting, soli tary bee species were systematically monitored in wooden trap nests at 6 sites in northern central California from 1990 to 1992. Three sites were in the San Joaquin Valley in wildlife refuges or preserves; 3 we re in coastal mountain or Sierran foothill parks or reserves. Differen ces in frequencies of nesting bees were observed and recorded among si tes and habitats through time. A Poisson regression indicated that all explanatory variables and their Ist order interactions were highly si gnificant. There were significant differences among bee taxa, sites an d years, and their interactions (bee x site; bee x year; site x year). Megachile species were the predominant cavity nesters in the 3 valley sites; Osmia species were the common group in coastal mountain/foothi ll sites. In a computer simulation, using a variation of the Poisson r egression, several significant differences were also observed between yearly frequency counts for certain bee species at the same site. Poss ible reasons for year to year changes included differential natural mo rtality, extreme annual weather patterns, use of marginal habitats, an d negative impacts from exotic solitary bee species. Three exotic mega chilids (Megachile rotundata (F.), M. apicalis Spinola, and M. concinn a Smith) have successfully invaded and become established in Californi a. Possibly, M. apicalis has affected other cavity nesting bee species , including the exotic M rotundata in our Central Valley sites.