REGIONAL DIFFERENCES IN THE DISTRIBUTION OF THE PYRALID MOTH URESIPHITA-REVERSALIS (GUENEE) ON FRENCH BROOM, AN INTRODUCED WEED

The factors influencing the distribution of the moth Uresiphita revers alis (Guenee) on French broom, Genista monspessulana (L.), its major h ost plant in California, were investigated. Within an intensively sear ched 7000 km(2) area around the San Francisco Bay, larvae were abundan t on broom in the region east of the Bay, but consistently absent nort hwest of the Bay. In held experiments there were no differences in egg mortality, larval development, pupal weight, predation, plant chemist ry, average ambient temperatures or solar radiation between the two re gions which might account for the distribution of the insect. Survivor ship of experimentally placed larvae was dramatically reduced in field sites northwest of the Bay compared to sites east of the Bay during a rainy winter season. Winter precipitation was consistently twice as h igh northwest of the Bay compared to east of the Bay, and large losses of larvae were associated with rainfall. After three years of drought in California, larvae were found in a site northwest of the Bay for t he first time. It is postulated that heavy rain is a major source of m ortality for these insects, which, along with other unknown factors, m ay preclude establishment in particular microclimates.