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.