Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 1, 1995
Publication Date: N/A
Interpretive Summary: Peaches and nectarines are once again being shipped to New Zealand following a suspension on exported fruit since 1989 due to potential infestations of a domestic fruit fly, walnut husk fly. Research at the USDA-ARS, Horticultural Crops Research Laboratory, Fresno, California, in cooperation and support from the California Tree Fruit Agreement, a commodity group, immediately developed a pest-free period for walnut husk fly when stone fruits could be shipped to foreign countries without the risk of infestation. The pest-free period was verified in 1990-1994 through an extensive trapping program in walnuts in five counties, Merced, Madera, Fresno, Tulare, and Kern in the San Joaquin Valley of California. Field and laboratory investigations showed that peaches and nectarines were poor hosts for walnut husk fly and unlikely to be infested. Other quarantine control strategies were also developed to minimize the risk for accidental introductions of the pest including a methyl bromide fumigation and low temperature storage of the fruit. In July 1995, the USDA-APHIS and the New Zealand MAF regulatory agencies resolved the problem of walnut husk fly and other pests of concern and approved a workplan to allow shipments of peaches and nectarines to New Zealand from the five counties in the San Joaquin Valley of California. New Zealand market potential has been estimated at $1 million annually.
Technical Abstract: A 1993 and 1994 trapping program in walnuts in five counties of the San Joaquin Valley verified the pest-free period 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 seven of eight trapping sites in both years. A single adult was collected in a trap prior to 1 July in Fresno County in 1993 and in Tulare County in 1994. Peak populations of walnut husk fly occurred in walnut orchards and roadside 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 of pupae to the adult stage. Exposure to low temperature storage at 1.1-1.7C for 7, 14, or 21 d was detrimental to walnut husk fly eggs, and first, second and third instars in green walnuts. Survival to the pupal stage for second and third instars was significantly lower than controls for all exposure periods tested. Fewer pupae developed from eggs and first and second instars than third 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.