ASH WHITEFLY (HOMOPTERA, ALEYRODIDAE) OVERWINTERING AND BIOLOGICAL-CONTROL BY ENCARSIA INARON (HYMENOPTERA, APHELINIDAE) IN NORTHERN CALIFORNIA

The ash whitefly, Siphoninus phillyreae (Haliday), was introduced into northern California in 1989-1990. It developed through two or more ge nerations from fall through spring (overwinter) on toyon, Heteromeles arbutifolia Roemer, a native broadleaf evergreen shrub. In spring, whi teflies migrated to ash, Fraxinus velutina Torrey, and other deciduous hosts; whiteflies were absent or not detected on toyon during each of the three summers from 1990 to 1992. Ash whitefly densities were incr easing before natural enemy introductions. About 98% of ash leaves wer e infested in 1991 (before establishment of introduced natural enemies ) in comparison with <1% of leaves infested in 1992 (after natural ene my establishment). Whitefly densities declined by two to four orders o f magnitude on both ash and toyon after the introduction and establish ment of Encarsia inaron (Walker) (=partenopea) and Clitostethus arcuat us (Rossi) (Coleoptera: Coccinellidae). E. inaron is apparently the mo re important species. Unlike ash whitefly, which must migrate from dec iduous hosts to overwinter on evergreen plants, E. inaron can survive the deciduous host-free period as adults or by slowly developing as im matures on fallen ash leaves, as well as by parasitizing whiteflies on evergreen plants. Biological control may be enhanced because overwint ering parasitoids can already be present in the deciduous-host (ash) h abitat when whiteflies migrate from toyon back to ash during the sprin g.