A variety of insects attack the shoots and foliage of black walnuts (Juglans nigra), but a limited number feed on the nuts. Their larvae bore into the hulls, eating the interior or causing the nuts to drop prematurely. There are pesticides registered for controlling them, although there may be restrictions where you live. You can grow black walnuts in U.S. Department of Agriculture plant hardiness zones 4 through 9.

Walnut Curculio

The walnut curculio (Conotrachelus retentus) is the most common insect that eats its way into black walnut hulls. This reddish brown insect, about 1/5 inch long, has two small white spots on its wing covers and a long snout. Its dirty white larvae are legless. Adult female curculio lay their eggs on young nuts in late spring and early summer. Their larvae bore into the developing nuts, making some drop and causing smaller kernels in those that remain on the tree. They leave a small hole on the side of the nut hull.

Walnut Husk Fly

The walnut husk fly (Rhagoletis completa) is the second insect likely to eat its way into black walnut husks. Their maggots spend winters in the ground and emerge as flies in mid to late summer. Adult husk flies lay eggs in the husks of black and Persian walnuts (Juglans regia), hardy in USDA zones 5b through 6b. The legless maggots feed on the hulls, staining the shells. The light brown flies, smaller than a housefly, have dark bars crossed on their two transparent wings. Their legless, pale yellow larvae grow up to 1/2 inch long.

Pecan Nut Casebearer

While the pecan nut casebearer (Acrobasis nuxvorella) typically infests pecans (Carya illinoensis) hardy in USDA zones 5b through 9a, hence its name, it can also infect black walnuts. A small, brownish-gray moth about 1/3 inch long, it has a tuft or ridge of dark scales across the middle of its front wings. It spends the winter in a cocoon at the base of nut buds and emerges in spring to lay eggs on developing nuts. Its larvae enter the stem end of the hulls and eat the interior.

Control

To control walnut curculio, spray your tree with a pesticide containing the active ingredient esfenvalerate, lambda-cyhalothrin, carbaryl or phosmet. Do this just after your tree yields slim, cylindrical clusters of flowers, signifying that is ready to receive pollen. Home growers typically ignore the walnut husk fly because they don’t affect the nutmeats. If you do wish to use a pesticide, University of California entomologists recommend one containing the active ingredient spinosad. For the pecan nut casebearer, Texas A&M entomologists recommend the organic Bacillus thuringiensis or a chemical pesticide containing the active ingredient carbaryl, malathion or spinosad. State registration for chemical pesticides change from time to time as do application rates and how they should be applied. Consult your local agriculture extension agent before using them.

Cautions:

Bacillus Thuringiensis: (B.T.) A natural microbe found in the soil, Bacillus thuringiensis produces proteins that are toxic to young, immature insects (or larvae). There are countless types of Bt, each targeting different groups of insects. Few studies have been conducted on the chronic health effects, carcinogenicity, or mutagenicity of B.t. People exposed to B.t. have complained of respiratory, eye, and skin irritation, and one corneal ulcer has occurred after direct contact with a B.t. formulation.

Spinosad: toxic to dogs, bees. For humans:

• Itching, burning, flaking, or redness of the scalp
• redness of the eye or skin

Esfenvalerate: The compound may produce nausea, vomiting, headache, temporary nervous system effects such as weakness, tremors and incoordination in domestic animals if accidentally ingested. Esfenvalerate is a strong eye irritant, producing tearing or blurring of vision .

Carbaryl: Ingestion of carbaryl affects the lungs, kidneys and liver. Inhalation will also affect the lungs (14, 17). Nerve damage can occur after administration of high doses for 50 days in rats and pigs (12). Several studies indicate that carbaryl can affect the immune system in animals and insects.

Malathion: Short-term exposures to high levels of malathion can affect the nervous system causing a variety of symptoms, including headaches, nausea, dizziness, weakness, cramps, diarrhea, excessive sweating, blurred vision and increased heart rate.

Cyhalothrin Lambda-cyhalothrin is toxic if swallowed. It is also toxic if it comes in contact with skin. It can cause eye irritation, and it may cause respiratory irritation. Lambda-cyhalothrin can damage organs through prolonged or repeated exposure. It is also very toxic to aquatic life, with long-lasting effects.

Phosmet: Typical of other organophosphates , phosmet is an inhibitor of the enzyme cholinesterase. Symptoms of acute phosmet poisoning include nausea, vomiting, abdominal cramps and diarrhea. Acute exposure at high levels may result in muscle spasms, loss of muscle coordination, mental confusion and drowsiness.