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Walnut Husk Fly

Rhagoletis completa[3]

Walnut husk fly maggots blacken husks and stain shells, but in backyards most people can ignore them because the nutmeat is usually fine.[1]

Group
Insect
Order
Diptera
Family
Tephritidae
US records
10 states
Sources
3 cited
Updated
2026-10-14
Walnut husk fly adult with three dark bands on each wing, a yellow spot on its back and greenish eyes
Walnut husk fly adult with three dark bands on each wing, a yellow spot on its back and greenish eyes. (c) Don Loarie, some rights reserved (CC BY) — via iNaturalist, CC BY 4.0
On this page
  1. Identification
  2. Look-alikes
  3. Where it lives
  4. Biology
  5. Damage and signs
  6. Health risk
  7. Control
  8. Related profiles
  9. Common questions
  10. Sources

How to identify it

  • Adults are about the size of a house fly, colorful, with a yellow spot behind the wings and iridescent green eyes.[1]
  • Each wing has three dark bands, one bending around into a V.[1]
  • Dark, soft blotches on ripening husks are the main clue; hard, dry blotches are walnut blight instead.[1]

Look-alikes

Look-alikeHow to tell them apart
Apple maggotApple maggot is another banded-wing fruit fly; its body color and wing pattern set it apart from the walnut husk fly, and it attacks apples, not walnuts.[2]

Where it lives

It infests walnuts in most California walnut-growing areas, on black walnut and all English walnut varieties.[1]

US map: public records from 10 states since 1990 (GBIF).

Biology and life cycle

Pupae overwinter in soil; adults emerge around July 1, peaking mid-July through mid-August.[1]

Females lay eggs in groups of about 15 under the husk surface; maggots hatch within five days.[1]

After three to five weeks in the husk, maggots drop and pupate a few inches deep; some wait two or more years to emerge.[1]

Damage and signs

  • Shells are stained and husks stick to them; early infestations can leave shriveled, moldy kernels.[1]

Health risk

It feeds in walnut husks only; UC IPM mentions no risk to people.[1]

Control

Sanitation

Remove damaged nuts promptly and seal infested husks in bags; a tarp under the tree in July and August can stop maggots reaching the soil.[1]

Traps and physical control

Soak damaged nuts in a damp burlap bag for a few days to loosen stuck husks.[1]

Pesticides

For early or severe infestations, spinosad mixed with a bait beginning in July is the only home-use option UC IPM lists, and it may be only partly effective.[1]

Common questions

Are walnuts with black husks safe to eat?
Usually; the husk fly generally doesn't affect the nutmeat, though early infestations can shrivel kernels.[1]
When do walnut husk flies appear?
Mostly from July, peaking mid-July to mid-August.[1]

Sources

  1. University of California Statewide IPM Program, “Pest Notes: Walnut Husk Fly (Publication 7430).” ipm.ucanr.edu/PMG/PESTNOTES/pn7430.html (accessed 2026-10-06)
  2. University of California Statewide IPM Program, “Apple Maggot (Pests in Gardens and Landscapes).” ipm.ucanr.edu/PMG/GARDEN/FRUIT/PESTS/applemaggot.html (accessed 2026-10-06)
  3. iNaturalist, “Rhagoletis completa taxon page (order and family).” www.inaturalist.org/taxa/52202 (accessed 2026-10-06)

Updated 2026-10-14. Built from sourced records. Spot an error? Tell us.