Walnut husk fly

Walnut husk fly (Rhagoletis completa Cresson), The walnut husk fly is about the size of a housefly and very colorful. It has a yellow spot just below the areas where the wings are attached and iridescent, greenish eyes. The wings have three prominent dark bands, one of which extends around the wing to form a V-shape. The banded wings distinguish it from other flies found in the walnut orchard.

Larvae feed in groups within the husk, but you won’t see them unless you remove the skin of the damaged husk. Dark, soft blotches on maturing husks are a good clue to husk fly presence. Blotches that are hard and dry are caused by blight disease and should not be confused with husk fly damage.

Hosts : The walnut husk fly’s primary host is walnuts, but it may also attack late maturing varieties of peaches near infested walnuts. Like the cherry fruit fly, the walnut husk fly remains on a single walnut tree or group of trees as long as there are plenty of nuts. It is a peach pest primarily in urban areas where peaches and walnuts are grown side by side, rather than in commercial orchards.

Life Cycle: This fly has one generation per year. Walnut husk flies overwinter as pupae in the soil and emerge as adults in some areas as early as May but generally around July 1. Peak emergence often occurs mid-July through mid-August.

The female fly deposits eggs in groups of about 15 below the surface of the husk. Usually the first sign of an infestation is a small, stinglike mark on the husk caused by this depositing of eggs. At first these areas are difficult to see, but they soon darken and appear as little, black spots on the husk, usually near the stem end of the husk and often on the shaded side of the nut.

Eggs hatch into white maggots within 5 days. The maggots feed inside the husk, enlarging the black area, which remains soft, unsunken, and smooth. The outer skin of the husk usually remains intact, but its fleshy parts decay and stain the nutshell.

Older maggots are about 1/4 inch long and are yellow with black mouthparts. 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 emerge as adults the following summer, but some remain in the soil for 2 or more years.

Damage: The walnut husk fly is a mid- to late season pest. It occurs in all walnut-growing areas in California. Black walnut and all cultivars of English walnut are suitable hosts for the husk fly. Some cultivars, such as Ashley, escape serious damage in most years. Other cultivars such as Eureka, Hartley, Franquette, Mayette, Chandler, and Tulare are very susceptible to husk fly damage; black walnut is also a preferred host.

The first signs of an infestation are small stings caused by females depositing eggs in the husk. After hatching, the maggots feed inside the husk, turning it very soft and black. The outer skin of the husk usually remains intact, but its fleshy parts decay and stain the nutshell. These stains cannot be removed by normal bleaching procedures, and the nut is therefore unsatisfactory for in-shell sale. A husk fly infestation early in the season late July to mid-August leads to shriveled and darkened kernels, increased mold growth, and lower yields. Other pests walnut blight, aphids or environmental stresses sunburn, water stress also may cause this damage. Late infestations do little damage to the kernels but may stain the shells and make hull removal difficult.

Monitoring : A yellow sticky trap developed to monitor apple maggot works well for walnut husk flies. A trap can be made from a piece of yellow cardboard, about 5-1/2 by 9 inches 14 by 23 cm, coated with Tanglefoot adhesive, and with a small bottle of ammonium carbonate attached to the bottom.

Yellow rectangle traps are not specific to husk fly and will catch many other flies. A more specific trap is a green sticky spherical trap the same size used to monitor apple maggot. However, the green sphere trap is less efficient than the yellow rectangle. At the beginning of July, hang traps at least 6 feet above the ground in a shady part of the tree. If husk flies were a problem the previous year, hang traps in trees that were infested.

Controls: Remove fallen infested fruit and remove the source of the infestation, probably a nearby walnut tree. Apply an insecticide recommended for walnut husk fly that is also registered on peaches within 10 days after trap catches show a sharp increase over a 3-day period. This will usually be from late July to mid-August. Apply again in 10 days if the husk fly was a problem the previous year. A third application may be needed 3 to 4 weeks later if flies continue to be caught in traps. A protein bait can be used to attract the flies to the insecticide to make it more effective.

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