Classical biological control of ash whitefly: factors contributing to its success in California

The ash whitefly, Siphoninus phillyreae, invaded California in 1988 rapidly spreading throughout the state and infesting several species of ornamental trees and shrubs. Released Encarsia inaron rapidly established populations and spread throughout areas occupied by ash whitefly. We examined whether dispersal and overwintering ability could play a role in the extraordinary success of this parasitoid and we measured the impact of released parasitoids using a new method at a single location in northern California. The dispersal ability of E. inaron was examined by releasing two hundred and fifty adults 25 July 1991 into a single tree in a 1 ha pomegranate orchard near Brentwood, California. Based on yellow sticky card traps, the adult population spread at least 45 m from the release tree within 9 weeks of release. Over the same period of time the parasitoid population increased 64 fold. Impact was determined by measuring the number of ash whitefly adults produced in the absence and presence of E. inaron over 12 months. The production of whitefly adults, measured by the number of pupae entering the adult population, was reduced to very low numbers one year following the establishment of the parasitoid. Production of E. inaron adults increased rapidly, then decreased following the drop in production of ash whitefly adults. Duff of flowering pear trees was collected from under sample trees in mid winter to determine whether ash whitefly could survive on fallen leaves. Over fifty percent of adults emerged from whitefly pupae on leaves within 12 days of collection.