Response of Walnut Husk Fly (Diptera: Tephritidae) to Low Temperature, Irrigation, and Pest-Free Period for Exported Stone Fruits

Abstract

A 1993 and 1994 trapping program in walnuts in 5 counties of the San Joaquin Valley verified the pest-free period for walnut husk fly, Rhagaletis campleta Cresson, between the beginning of stone fruit harvest in the spring and 1 July. First emergence of adults in walnuts occurred after the pest-free period in 7 of 8 trapping sites in both years. A single adult was collected in a trap before 1 July in Fresno County in 1993 and in Tulare County in 1994. Peak populations of walnut husk fly occurred in walnut orchards and road side trees in late August after early, midseason, and most late season stone fruit cultivars had been harvested. The pest-free period and late seasonal emergence of peak populations in walnuts show that the biological risk is negligible for accidental introductions of walnut husk fly into countries where the pest does not occur through shipments of stone fruits from California. No relationship was observed between immersion of walnut husk fly pupae in soil saturated with water and survival to the adult stage. Survival to the pupal stage for 2nd and 3rd instars was significantly lower than controls after exposure to low temperature storage in green walnuts at 1.1-1.7 for 7, 14, and 21 d. Few pupae developed from eggs and 1st, 2nd, and 3rd instars after a 21-d exposure. The loss of food quality of green walnuts after storage probably affected survival of immatures to the pupal stage.