How to Manage Pests

UC Pest Management Guidelines


Walnut Husk Fly

Scientific Name: Rhagoletis completa

(Reviewed 12/07, updated 8/13)

In this Guideline:


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.


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.


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.

Monitoring and Treatment Decisions

Use an unbaited yellow sticky trap (Pherocon AM (apple maggot) NB (no-bait), 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 (PDF) of your results.

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 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.


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.

Common name Amount to use** R.E.I.+ P.H.I.+
(trade name) (conc.) (dilute)
(hours) (days)

Calculate impact of pesticide on air quality
Bee precaution pesticide ratings
The following materials are listed in order of usefulness in an IPM program, taking into account efficacy and impact on natural enemies and honey bees. When choosing a pesticide, also consider information relating to environmental impact.
A. CORN GLUTEN MEAL 1–3 pt 1 pt 0 NA
  (NU-Lure Bait)
  . . . or . . .
  CORN STEEP LIQUOR Label rates 0 NA
  (Monterey Insect Bait)
  COMMENTS: Baited sprays are the preferred treatment and are aimed at killing adults before eggs are laid. Baits attract flies to spray material and enhance control. If significant egg laying has occurred before treatments, however, adequate control will not be attained. Generally the residual period of the bait is about 7 to 10 days.
  . . . PLUS . . .
  (Assail 70WP) 4.0–8.0 oz 0.271–1 oz 12 14
  COMMENTS: Make no more than 4 applications/season. Do not exceed 0.72 lb a.i./acre/season.
  . . . or . . .
  (Brigade WSB) 32 oz 2–8 oz 12 7
  COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate. Do not exceed 0.5 lb a.i./acre/season.
  . . . or . . .
  (Danitol 2.4 EC) 21.33 oz 2–5.33 oz 24 3
  COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate. Do not exceed 0.8 lb a.i./acre/season.
  . . . or . . .
  MALATHION 8 1.5–2.5 pt 0.4–0.625 pt 24 7
  COMMENTS: Can increase mite problems.
  . . . or . . .
  (Asana XL) 1 pt 4 oz 12 21
  . . . or . . .
  (Lorsban) 4EC 4 pt 1 pt 24 14
  COMMENTS: Make no more than 2 applications per season. Levels in surface waters of this material that are high enough to be toxic to certain aquatic invertebrates have occurred following rains; avoid runoff into surface waters. Certain formulations emit high amounts of volatile organic compounds (VOCs); use low-VOC formulations (PDF). Regulations affect use for the San Joaquin Valley from May 1 to October 31, 2015 and 2016. Review the Department of Pesticide Regulation's updated fact sheet (PDF).
  . . . or . . .
  (Delegate) WG 3–7 oz 0.75–1.75 oz 4 1
  . . . or . . .
  (Entrust)# 1.25 oz 0.3–0.75 oz 4 1
  (Success) 4 fl oz 1–1.25 fl oz 4 4
  COMMENTS: When population levels are high, be sure to use high label rates and shorter treatment intervals. For organic growers use Entrust with an organically acceptable attractant. Buffer spray solution of either formulation so that pH is in the range of 6.0 to 8.0. Do not use more than 29 fl oz of Success or 9 oz of Entrust per acre per crop.
  . . . or . . .
  (Imidan) 70WP 5 lb 1.25 lb 7 days 28
  COMMENTS: Do not apply after husk split. Do not apply more than 8.5 lb/acre/application or more than 5 times per season. Buffer to a pH of 5.5-6.0.
  (GF-120 NF Naturalyte Fruit Fly Bait)# 20 fl oz 4 0
  COMMENTS: A pre-mixed spinosad and bait formulation. Check label and organic certifying agency for organic acceptability. Start applications at first fly emergence and repeat every 7 to 14 days, shortening the interval during rainy periods and as fruit ripens. Use in 30 to 80 oz of water per acre and apply as a spot spray. Continue treatments until fly populations begin to drop in traps.

1 Rotate chemicals with a different mode-of-action Group number, and do not use products with the same mode-of-action Group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a Group number of 1B; chemicals with a 1B Group number should be alternated with chemicals that have a Group number other than 1B. Mode of action Group numbers are assigned by IRAC (Insecticide Resistance Action Committee). For additional information, see their Web site at
** For concentrate application, use the amount given in 80–100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300–500 gal water/acre, according to label. Much lower rates of water/acre have been used with handgun applications of bait sprays.
+ Restricted entry interval (R.E.I.) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (P.H.I.) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.
* Permit required from county agricultural commissioner for purchase or use.
# Acceptable for use on an organically certified crop.
NA Not applicable.




[UC Peer Reviewed]

UC IPM Pest Management Guidelines: Walnut
UC ANR Publication 3471

Insects and Mites

  • C. Pickel, UC IPM Program/UC Cooperative Extension, Sutter/Yuba counties
  • J. A. Grant, UC Cooperative Extension, San Joaquin County
  • W. J. Bentley, UC IPM Program/Kearney Agricultural Center, Parlier
  • J. K. Hasey, UC Cooperative Extension, Sutter/Yuba counties
  • W. W. Coates, UC Cooperative Extension, San Benito County
  • R. A. Van Steenwyk, Insect Biology, UC Berkeley
Acknowledgment for contributions to Insects and Mites:
  • W. H. Olson, UC Cooperative Extension, Butte County
  • L. C. Hendricks, UC Cooperative Extension, Merced County
  • G. S. Sibbett, UC Cooperative Extension, Tulare County

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