Walnut trees provide valuable nuts and shade, but they are threatened by insect pests that can compromise the health of the tree and the quality of the harvest. Protecting a walnut tree requires understanding the specific pests involved, recognizing the damage they cause, and timing management efforts correctly. Successful management targets the insects at their most vulnerable stages, ensuring the tree remains productive and healthy.
Identifying the Main Pests of Walnut Trees
One of the most widespread pests is the Walnut Husk Fly (Rhagoletis completa), an insect about the size of a common housefly distinguished by its unique wing pattern. The adult fly has a dark body with a prominent yellow spot and three dark, inverted V-shaped bands across the translucent wings. The damaging stage is the larva, a white to yellowish maggot that develops inside the nut’s husk.
The Codling Moth (Cydia pomonella) is another pest, appearing as a small, mottled gray moth with a distinct copper-brown band at the tips of its wings. The larvae are pinkish-white caterpillars with brown heads, growing to about half an inch long. These caterpillars bore into the developing nuts.
Walnut trees are also commonly affected by aphids, primarily the pale yellow Walnut Aphid (Chromaphis juglandicola) and the grayish Dusky-veined Aphid (Panaphis juglandis). The Walnut Aphid feeds on the underside of leaves, while the Dusky-veined Aphid is often found along the mid-vein on the upper leaf surface. Finally, the Walnut Twig Beetle (Pityophthorus juglandis) is a tiny, reddish-brown bark beetle, only about one-sixteenth of an inch long, that bores directly into the tree’s phloem.
Recognizing Signs of Infestation and Tree Damage
The Walnut Husk Fly damages the nut’s exterior when female flies create small, dark “stings” while laying eggs beneath the husk surface. As the maggots feed, the husk tissue becomes soft, dark, and mushy, releasing pigments that stain the shell. Early-season feeding can lead to shriveled, moldy kernels that drop prematurely or are difficult to extract.
Aphid activity is identified by the sticky, sweet residue known as “honeydew” they excrete while feeding on sap. Honeydew promotes the growth of sooty mold, a dark, velvety layer that covers leaves and nuts, blocking sunlight and reducing market value. High aphid populations cause foliage to curl, yellow, and drop, weakening the tree and exposing nuts to sunburn.
Codling Moth damage is indicated by the presence of the larva or the reddish-brown waste material, called frass, pushed out of the entry hole. Early-season damage causes young nuts to drop prematurely. Later generations bore directly into the shell to feed on the kernel, making the nuts unfit for consumption. The Walnut Twig Beetle introduces the fungus that causes Thousand Cankers Disease. External signs include yellowing leaves, a thinning canopy, and the dieback of larger limbs, often resulting in tree death within a few years for susceptible species like black walnut.
Understanding Pest Life Cycles and Timing
Targeting pests during their vulnerable life stages requires understanding their seasonal timing. Walnut Husk Flies emerge from the soil, where they overwinter as pupae, starting in early to mid-summer. Peak adult activity occurs from mid-July through August. Since the eggs and larvae are protected inside the husk, control applications are most effective when timed after the adults emerge but before the females lay eggs.
Codling Moths overwinter as mature larvae in silken cocoons under loose bark or debris at the tree base. The first flight of adult moths begins in early spring, often coinciding with leaf-out, initiating the first generation. Since there can be two to four generations per year in warmer areas, monitoring for subsequent flights in July and August is important. Treatments must target the newly hatched larvae before they bore into the nuts.
Walnut Aphids and Dusky-veined Aphids overwinter as eggs on the twigs, hatching as soon as leaf buds open in the spring. These aphids reproduce rapidly throughout the warmer months, going through many generations in a season. The Walnut Twig Beetle overwinters as both larvae and adults beneath the bark, with flight activity starting in April and often peaking again in September and October.
Effective Management and Control Strategies
Cultural and Biological Controls
Cultural control methods are foundational for reducing pest populations and maintaining tree health. Diligent sanitation is key: removing fallen, infested nuts and debris beneath the tree in the fall eliminates overwintering sites for Codling Moth and Walnut Husk Fly larvae. Proper irrigation and nutrition are also important, as excessive nitrogen fertilizer encourages succulent growth that increases aphid reproduction.
Biological control is highly effective for managing aphids, particularly the Walnut Aphid, which is largely controlled by the introduced parasitic wasp, Trioxys pallidus. The presence of these beneficial insects is indicated by brown, crusty, swollen “mummies,” which are the remnants of parasitized aphids. Conserving natural predators like ladybugs, lacewings, and syrphid fly larvae is a broad strategy for controlling aphid populations. This requires avoiding broad-spectrum insecticides that harm these natural enemies.
Monitoring and Chemical Interventions
Mechanical and chemical interventions are necessary for direct control of destructive pests. For Walnut Husk Flies, placing yellow sticky traps high in the tree canopy monitors the emergence of adults, dictating treatment timing. Insecticide applications, sometimes mixed with a protein bait to attract the flies, should be applied within days of a significant increase in fly catch, before egg-laying occurs.
Codling Moths are monitored using pheromone traps, which attract male moths and help predict when egg hatch will occur. This is the most effective time to apply treatment. Products containing Bacillus thuringiensis (Bt) are a low-toxicity option for controlling the caterpillar stage. For the Walnut Twig Beetle and the associated Thousand Cankers Disease, management is limited to cultural practices like pruning out infected branches and removing infested wood to prevent the spread of the fungus, as chemical treatments are usually ineffective once the disease is established.