Walnut Husk Fly

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 early June until early September (in coastal areas emergence can begin as early as mid-May). Peak emergence is usually from 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 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 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 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 Tulare, Hartley, Serr, and and Franquette are very susceptible to husk fly damage; black walnut is also a preferred host. Different cultivars differ in the time of year they are susceptible; for example, earlier damage occurs in Payne, Serr, and Hartley than in Chandler.

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 and pathogens (walnut blight and aphids) and environmental stresses (sunburn and water stress) also may cause this damage. Early walnut husk fly damage can result in a 30% loss in value of the nuts. Late infestations do less damage to the kernels but may stain the shells and make hull removal difficult.

Not every orchard requires an insecticide application 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 a history of severe late damage from this pest may want to use ethephon (for more information see USING ETHEPHON) to hasten maturity and husk split.

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 bait.

Trapping

Use an unbaited yellow sticky trap (Pherocon AM (apple maggot) NB (no-bait)), super-charged with ammonium carbonate to attract adult flies for monitoring. Be sure not to use baited apple maggot traps.

1. Hang traps in the orchard by June 1, 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 provide an accurate assessment of when the flight begins.
2. 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.
3. Monitor traps at least twice a week, and preferably three times a week to avoid damage before the first insecticide is applied. Each orchard must be monitored separately, with the treatment timing based on the monitoring results for that orchard.
4. Write down the numbers caught in traps each time, and keep records (PDF) of your results.
5. Treatment timing can be based on one of three monitoring methods described below. The first two are most effective.

Using Trap Catches to Time Sprays (Overall Trap Numbers)

• In low- to moderate-pressure orchards, spray when a sharp increase occurs in traps.
• In high pressure orchards or if using GF-120, spray when any flies are detected rather than waiting for a sharp increase in catches.

Using Trap Catches to Time Sprays (Detection of Eggs in Trapped Females)

Recently emerged flies only require a short period of time to become sexually mature, mate, and begin laying eggs. As such, it is possible to more precisely time insecticide treatments to match the onset of egg laying by examining female flies caught on traps for the presence of eggs. This is a simple process that requires slightly more time than counting overall trap catches. Only monitor for eggs in orchards that use standard insecticide-plus-bait sprays. For orchards using GF-120, see: Using trap catches to time sprays, above.

Examine female flies caught on traps for the presence of eggs.

1. Remove all flies from the trap and place them on a dark-colored surface, which makes it easier to see the white eggs.
2. 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 with a blunt pencil.) If eggs are present, they are pearly white and resemble small grains of rice.
3. Apply insecticide when the first female with eggs is found. While past guidelines have stated that the treatment window is one week after egg detection, in practice trap checks (even 2 to 3 checks per week) may not be frequent enough to represent initial egg development in the female population and there is often a lag time in getting the treatments. Therefore, treat as soon as possible after eggs are detected to minimize infestation.

Monitoring for Stings

Monitoring for stings on nuts is the least useful method to determine treatment timing, as damage has already occurred. However, examining nuts for stings can provide indication of effectiveness of your insecticide applications. Check periodically for stings (female egg-laying punctures) once flies have begun to emerge and they are detected in traps.

• Carefully inspect at least 10 nuts on the north side of 20 trees, for a total of 200 or more 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.
• The presence of fresh stings, with or without trap captures or a recent insecticide application may be an indication of inadequate control.
• If treatment timing is based on sting detection, apply an insecticide at the first sting. Full cover neonicotinoids that have some ovicidal (egg-killing) activity mixed with an adulticide will provide best control (see table below).

Continued Monitoring

Continue to monitor traps weekly after an insecticide application.

• If the infestation occurred early, a second spray may be necessary 3 to 4 weeks later.
• Short-residual insecticides plus bait will generally kill walnut husk fly for 7 to 10 days. With the egg development period added to this time, there is about 3 weeks of protection after an application.
• Target subsequent applications at 2 to 4 week intervals based on the efficacy of the previous spray. Clean traps the day after an application and check 3 to 4 days later. If the number of flies drops to near zero, the spray was highly effective and a longer treatment interval may be used.
• 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, an insecticide application will be required if harvest is more than 3 weeks away.

Before harvest, sample 100 nuts from each variety, block, or high damage area to assess damage and to plan for next year's management program. Survey areas with the highest damage as sites for next year's traps.

Insecticides

Use all insecticides with a bait except GF-120, which contains its own bait. For low- to moderate-populations, coverage is not critical; partial coverage (e.g., alternate row) or low-volume applications of bait with insecticide (or both) can be effective. However, in high population orchards with extensive previous damage, high-volume, full coverage, or multiple applications of bait with insecticide (or a combination of these approaches) may be necessary to achieve adequate control. If treatments are timed in response to sting detection, full cover neonicotinoids that have some ovicidal (egg-killing) activity mixed with an adulticide will provide partial control of eggs if applied immediately after stings are observed (see table below).

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 often must be applied more frequently.

Common name		Amount to use**		REI‡	PHI‡
(Example trade name)		(conc.)	(dilute)	(hours)	(days)
Pesticide precautions Protect water Calculate VOCs Protect bees
Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.
BAITS (TO BE COMBINED WITH AN INSECTICIDE BELOW)
A.	MOLASSES	4 gal/100 gal spray mixture		NA	NA
B.	CORN GLUTEN MEAL	1–3 pt	1 pt	0	NA
	(NU-Lure Bait)
C.	CORN STEEP LIQUOR	Label rates	—	0	NA
	(Brandt 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.
INSECTICIDES WITH ADULTICIDAL ACTIVITY (TO BE COMBINED WITH A BAIT ABOVE)
A.	ACETAMIPRID
	(Assail 70WP)	2.7–3.4 oz	0.271–1 oz	12	14
	MODE-OF-ACTION GROUP NUMBER1: 4A
	COMMENTS: Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.
B.	BIFENTHRIN*
	(Brigade WSB)	32 oz	2–8 oz	12	21
	MODE-OF-ACTION GROUP NUMBER1: 3A
	COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate.
C.	BETA-CYFLUTHRIN
	(Baythroid XL)	2.4–2.8 fl oz	—	12	14
	MODE-OF-ACTION GROUP NUMBER1: 3A
D.	LAMBDA-CYHALOTHRIN
	(Warrior II with Zeon)	1.28–2.56 fl oz	—	24	14
	MODE-OF-ACTION GROUP NUMBER1: 3A
E.	FENPROPATHRIN
	(Danitol 2.4EC)	21.33 oz	2–5.33 oz	24	3
	MODE-OF-ACTION GROUP NUMBER1: 3A
	COMMENTS: Provides approximately 21 to 28 days of residual protection at the high label rate.
F.	ESFENVALERATE*
	(Asana XL)	9.6–19.2 fl oz	4 oz	12	21
	MODE-OF-ACTION GROUP NUMBER1: 3A
G.	SPINETORAM
	(Delegate WG)	3–7 oz	0.75–1.75 oz	4	1
	MODE-OF-ACTION GROUP NUMBER1: 5
H.	SPINOSAD
	(Success)	4–10 fl oz	1–2.5 fl oz	4	1
	MODE-OF-ACTION GROUP NUMBER1: 5
	COMMENTS: When numbers are high, be sure to use high label rates and shorter treatment intervals. Buffer spray solution of either formulation so that pH is in the range of 6.0 to 8.0.
I.	SPINOSAD
	(Entrust)#	1.25–3 oz	0.3–0.75 oz	4	1
	MODE-OF-ACTION GROUP NUMBER1: 5
	COMMENTS: When numbers are high, be sure to use high label rates and shorter treatment intervals. For organic growers use Entrust with an organically acceptable bait. Buffer spray solution of either formulation so that pH is in the range of 6.0 to 8.0.
J.	PHOSMET
	(Imidan 70W)	5 lb	1.25 lb	168 (7 days)	28
	MODE-OF-ACTION GROUP NUMBER1: 1B
	COMMENTS: Do not apply after husk split. Buffer to a pH of 5.5-6.0.
K.	MALATHION 8	1.5–2.5 pt	0.4–0.625 pt	24	7
	MODE-OF-ACTION GROUP NUMBER1: 1B
	COMMENTS: Can increase mite problems. Resistance to malathion may be present in some areas.
NEONICOTINOIDS WITH OVICIDAL ACTIVITY (TO BE COMBINED WITH A BAIT AND ADULTICIDE ABOVE IF STINGS ARE OBSERVED)
A.	IMIDACLOPRID
	(Admire Pro)	Label rates	—	12	7
	MODE-OF-ACTION GROUP NUMBER1:4A
	COMMENTS: Apply as a full coverage spray tank mixed with a bait and any adulticide above (except Assail) when treatment timing is based on sting detection and ovicidal activity is desired. Review and follow the California neonicotinoid regulations effective January 1, 2024. Permissible application rates of this insecticide may be lower than label rates if applying more than one neonicotinoid active ingredients or using more than one application method in the same season.
B.	CLOTHIANIDIN
	(Belay)	3–6 fl oz	—	12	21
	MODE-OF-ACTION GROUP NUMBER1:4A
	COMMENTS: Apply as a full coverage spray tank mixed with a bait and any adulticide above (except Assail) when treatment timing is based on sting detection and ovicidal activity is desired. Review and follow the California neonicotinoid regulations effective January 1, 2024. Permissible application rates of this insecticide may be lower than label rates if applying more than one neonicotinoid active ingredients or using more than one application method in the same season.
INSECTICIDE PLUS BAIT COMBINATION (PRE-MIX)
A.	SPINOSAD
	(GF-120 NF Naturalyte Fruit Fly Bait)#	20 fl oz	—	4	0
	MODE-OF-ACTION GROUP NUMBER1: 5
	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/acre and apply as a spot spray. Continue treatments until fly numbers begin to drop in traps or 3 weeks before harvest. May be more effective in the Central Coast than in hot areas with low humidity.

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 concentrate application, use the amount given in 80 to 100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300 to 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 (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) 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.

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