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 parasitoi ds using a new method at a single location in northern California. The disp ersal 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 B rentwood, California. Based on yellow sticky card traps, the adult populati on spread at least 45 m from the release tree within 9 weeks of release. Ov er the same period of time the parasitoid population increased 64 fold. Imp act 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 populat ion, was reduced to very low numbers one year following the establishment o f the parasitoid. Production of E. inaron adults increased rapidly, then de creased following the drop in production of ash whitefly adults. Duff of fl owering pear trees was collected from under sample trees in mid winter to d etermine whether ash whitefly could survive on fallen leaves. Over fifty pe rcent of adults emerged from whitefly pupae on leaves within 12 days of col lection.