How to Manage Pests

UC Pest Management Guidelines

DESCRIPTION OF THE PEST

The adult walnut 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. They overwinter as pupae in the soil and emerge as adults from late June until early September (in coastal areas emergence can begin as early as mid-May). Peak emergence is usually from mid-July to mid-August. The female deposits eggs in groups of about 15 below the surface of the husk. 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 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, a pointed abdomen with an ovipositor, and by the color of the first leg segment. On females, the first leg segment is straw colored, whereas on males it is brown to black. This character can be readily seen with the use of a 10X hand lens.

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.

MANAGEMENT

Not every orchard requires treatment for walnut husk fly every year. When chemical treatment is needed, precise timing is critical. Correct timing is not the same in every orchard and varies depending on insecticide and monitoring method used. Husk flies are not a problem after husk split. Growers with previous severe late damage from this pest may want to use ethephon (for more information see USING ETHEPHON) to hasten maturity and husk split.

Organically Acceptable Methods
The use of GF-120 is acceptable in organically certified orchards. The Entrust formulation of spinosad is also organically acceptable but must be mixed with an organically acceptable attractant such as Phyta-Molasses QC.

Monitoring and Treatment Decisions
Trapping. Use an unbaited yellow sticky trap with a lure (Pherocon WHF super-charged with ammonium carbonate) to attract adult flies for monitoring populations. Be sure not to use baited apple maggot traps. Hang traps in the orchard (by June 15 in inland areas and by June 1 in coastal areas) as high as possible within an area of dense foliage on the north side of trees. If they are not hung high enough, they will not accurately detect the first female with eggs. Use at least two traps per 10 acres and place the traps in orchard hot spots: large shaded trees, trees growing in damp areas or near black walnut trees, and trees that were damaged by walnut husk fly the previous season. Monitor traps at least twice a week, and preferably three times a week to avoid damage before the first treatment is applied. Write down the catch each time. As soon as flies are caught or whenever there is a sudden increase in trap catches, monitor for eggs. Husk flies are not a problem after husk split and treatments are not necessary if harvest will occur within 3 weeks. Growers with previous severe late damage from this pest may want to use ethephon to hasten maturity and husk split.

Monitoring for eggs. To monitor egg laying, remove the flies from the trap and place them on a dark-colored surface, which makes it easier to see the white eggs. Using a hand lens, identify the female flies (light-colored first leg segment, pointed abdomen and slightly larger in size) and use a pointed object to press on the abdomen and squeeze out the contents. (This can be easily done on the sticky trap with a blunt pencil.) If eggs are present, they are pearly white and resemble small grains of rice. At the first appearance of eggs, there is one week to spray before egg laying occurs, unless using GF-120 bait, which is applied when the first flies are caught and reapplied weekly as long as flies are present. Because walnut husk fly development is not driven by temperature, each orchard must be monitored separately, and treatment timing based on the monitoring results for that orchard. Keep records of your results (example form—68 KB, PDF).

Other monitoring methods. Another monitoring method is to use trap catch information to time sprays. Once flies are being caught, write down the catch each time traps are checked. When a sharp increase occurs, prepare to spray the orchard within the next few days if using GF-120, malathion, or spinosad (Success, Entrust) whereas the other products should be applied within 7 to 10 days.

Looking for stings is not as accurate as monitoring for eggs but can also be used to time treatments. After flies are first caught in traps, check 10 nuts on the north side of 20 trees for a total of 200 nuts. Females prefer the stem end but may lay eggs anywhere on the nut. Dark juice flows from the puncture, leaving a teardrop-shaped stain. Treat as soon as stings are found.

Insecticides. Use all insecticides with a bait except GF-120, which contains its own bait. By adding bait to the treatment, coverage is not critical and alternate row applications, aerial application, and applications with hand held spray equipment are all effective.

Continued monitoring. Continue to monitor traps weekly after treatment. If the infestation occurred early, a second treatment may be necessary 3 to 4 weeks later. If post-treatment catches from traps placed high in the tree increase, eggs are present in the trapped females, and the spray residue of the first treatment has run out, a treatment will be required if harvest is more than 3 weeks away. Generally a short-residual insecticide plus bait will kill walnut husk fly for 10 days. With the egg development period added to this time, there is about 3 weeks of protection after an application. GF-120 treatments must be applied more frequently (every 7-14 days).

At harvest, collect and crack out 1,000 nuts to assess damage and to plan for next year.

• Summary of Important Links

IMPORTANT LINKS

• Photos of mated female walnut husk flies
• Walnut husk fly monitoring form (68KB, PDF)
• Identifying seasonal pests in harvest samples

PUBLICATION

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471
Insects and Mites
C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
J. A. Grant, UC Cooperative Extension, San Joaquin Co.
J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
W. W. Coates, UC Cooperative Extension, San Benito Co.
R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to the insects and mites section:
W. H. Olson, UC Cooperative Extension, Butte Co.
L. C. Hendricks, UC Cooperative Extension, Merced Co.
G. S. Sibbett, UC Cooperative Extension, Tulare Co.

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