Walnut Husk Fly
Rhagoletis completa
A fly whose maggots tunnel through the husk surrounding developing walnuts, turning it into a soft black mush that stains and degrades the nut inside. It is the main insect pest of walnuts across much of the West, and a single generation a year can ruin a large share of a backyard crop if the husks are not protected.
🔎 How to spot it
Adults are slightly smaller than a house fly, brown with a yellow mark on the back, and their otherwise clear wings carry dark bands, including a dark triangular band toward the wing tip. The white to yellow maggots grow up to about three sixteenths of an inch long and feed inside the blackened husk. The first sign is usually a soft, darkened, sunken area on the green husk.
🥀 Damage it causes
Females lay eggs in the husk in mid to late summer, and the maggots feed inside, destroying the husk tissue so it turns black and slimy. The decaying husk stains the shell and can darken and shrivel the kernel, lowering quality, and heavily infested husks cling to the shell and make hulling difficult. Black walnuts are the favored host, though most English walnuts are also very susceptible.
🛡️ Prevent it
Rake up and destroy fallen, infested nuts and husks through the season and after harvest, since mature maggots drop to the ground to overwinter as pupae in the soil. Begin monitoring by mid-July with yellow sticky traps or green spherical traps to catch the adults and time any treatment. Keeping the ground beneath the tree clean reduces the number of flies that emerge the next year.
🧯 If it is already here
For home trees the main spray option is spinosad, applied as the trap catch shows a sharp or steady rise over a few days, usually in early to mid August, and repeated as the label allows because egg laying is spread over weeks. Bait sprays that combine spinosad with a feeding attractant target the adults before they lay eggs. Sanitation paired with well-timed spinosad gives the best home-orchard control.
💡 Good to know
Walnut husk fly has only one generation a year and overwinters as pupae in the soil, so a single thorough cleanup of dropped nuts pays off the following season. The damage is to the husk and shell rather than a worm in the nutmeat, so early hulling of stained nuts often salvages a usable kernel. The banded wings and the yellow spot behind the head separate it from harmless flies in the orchard.
🌱 Plants it attacks
4 plants in the library can be attacked by this pest
For educational and informational purposes only. Pest control advice is general guidance drawn from university cooperative extension sources; always identify a pest positively and read and follow the label on any product before use, especially around food crops, children, and pets.