Walnut Husk Fly
The adult husk fly is about the size of a housefly and very colorful. A yellow spot just below the area where the wings are attached and a dark triangular band at the tip of the wings distinguishes the husk fly from other flies likely to be found in orchards.
Husk flies have one generation per year and overwinter as pupae in the soil. They emerge as adults from late June until early September. Peak emergence is usually in mid-August. The female deposits eggs in groups of about 15 below the surface of the walnut husk near the stem. Eggs hatch into white maggots within 5 days. Older maggots are yellow with black mouth parts. After feeding on the husk for 3 to 5 weeks, mature maggots drop to the ground and burrow several inches into the soil to pupate. Most will emerge as adults the following summer but some remain in the soil for 2 years or longer.
Adult female husk flies can be distinguished from males by their slightly larger size. The walnut husk fly is a mid- to late season pest. It occurs in all walnut-growing areas in California except in certain parts of the central and southern San Joaquin Valley. A husk fly infestation early in the season (late July to mid-August) leads to shriveled and darkened kernels or may induce mold growth. This damage may also be caused by other pests or environmental stresses. Late infestations do little damage to the kernels but may stain the shells.
Photo courtesy of UC Statewide IPM Project, University of California
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