Walnut trees are susceptible to various insect pests that can diminish yields and compromise tree health. Understanding these common “walnut bugs” is important for anyone cultivating walnut trees. Timely identification and appropriate management protect trees and their valuable harvests, contributing to a successful growing season.
Common Walnut Pests and Identification
The walnut husk fly, Rhagoletis completa, is a small, yellow-brown fly with distinctive dark, inverted “V”-shaped bands on its wings. Initial signs of its presence include tiny oviposition punctures on the green husks of developing walnuts. The walnut aphid, Chromaphis juglandicola, is a small, pale yellow-green insect often found on the undersides of walnut leaves, particularly along the veins. Its presence can first be noticed by a shiny, sticky residue on leaves and surfaces below the tree.
The codling moth, Cydia pomonella, is a small, mottled gray-brown moth with a distinctive coppery spot near the tip of its forewings. While adult moths are nocturnal and rarely seen, the first indication of their activity might be tiny entry holes on young nuts, sometimes accompanied by small piles of reddish-brown frass. The walnut caterpillar, Datana integerrima, is most identifiable in its larval stage. Young larvae are reddish-brown with two narrow yellow stripes, maturing into black caterpillars covered with long, whitish hairs. These caterpillars are often observed feeding in large clusters on branches, sometimes creating silken mats.
Damage Caused by Walnut Pests
The walnut husk fly primarily damages the nut’s outer husk. Infested husks develop dark, soft, mushy spots, often turning black and becoming difficult to remove. This discoloration can stain the nut’s shell, reducing market value and sometimes leading to a shriveled or moldy kernel if the infestation is severe. Walnut aphids feed on the sap of walnut leaves. Their feeding can cause leaves to yellow, curl, or become distorted, stunting tree growth and reducing overall vigor.
Walnut aphids excrete honeydew, a sugary substance that coats leaves, branches, and surfaces below the tree. This honeydew becomes a substrate for sooty mold, a black fungus that impedes photosynthesis and makes the tree unsightly. Codling moth larvae directly damage nut kernels. After hatching, larvae bore into the nut, feeding on the developing kernel and filling the cavity with frass. This internal damage often leads to premature nut drop or renders nuts inedible, significantly impacting the harvest.
Walnut caterpillars are primarily defoliators; their communal feeding can rapidly strip a walnut tree of its leaves, often consuming entire leaves and leaving only midribs. While a single defoliation may not severely harm a healthy tree, repeated defoliation over several years can weaken it, reducing nut production.
Life Cycle and Timing of Infestations
Walnut husk flies typically emerge as adults from the soil in mid-summer, from late July through August. They lay eggs just under the surface of green walnut husks. Larvae hatch and feed within the husk for several weeks before dropping to the ground in late summer or early fall to pupate in the soil, remaining there until the following year. Walnut aphids overwinter as small, black eggs laid in bark crevices. In spring, around bud break, these eggs hatch into nymphs, which mature into wingless females that reproduce rapidly.
Multiple generations of walnut aphids can occur throughout the growing season, with winged forms developing in late summer or fall to disperse and lay overwintering eggs. Codling moths typically have two to three generations per year, depending on climate. Larvae from the previous year overwinter in cocoons under bark or debris, pupating in early spring. Adult moths emerge in late spring (May to early June), mate, and lay eggs on developing nuts or leaves. Subsequent generations appear throughout the summer and early fall.
Walnut caterpillars typically have one or two generations annually. Overwintering pupae in the soil or leaf litter give rise to adult moths in early to mid-summer. These moths lay egg masses on the undersides of walnut leaves. The caterpillars hatch and feed communally from mid-summer through early fall before descending to pupate in the soil for the winter.
Effective Management and Prevention
Effective management of walnut pests often begins with cultural practices that promote tree health and reduce pest populations. Sanitation, involving the prompt removal and disposal of fallen nuts, leaves, and other plant debris from around the base of walnut trees, helps eliminate overwintering sites for pests like the walnut husk fly and codling moth, breaking their life cycles. Proper watering and fertilization also contribute to a vigorous tree, which can better withstand pest pressure.
Organic methods offer environmentally sensitive approaches to pest control. Introducing beneficial insects, such as parasitic wasps for aphids or lacewings and ladybugs, can naturally reduce pest numbers. Pheromone traps can monitor codling moth populations, helping to determine intervention timing, while yellow sticky traps can capture adult walnut husk flies. For direct treatment, organic sprays like neem oil or insecticidal soap are effective against aphids and young caterpillars by disrupting their feeding or suffocating them. Spinosad, derived from a soil bacterium, offers another organic option for controlling caterpillars and some other chewing insects.
When pest pressure is high and other methods are insufficient, chemical controls may be considered. Apply these treatments judiciously and only when necessary, following all label instructions regarding dosage, application timing, and safety precautions. For instance, some pyrethroid-based insecticides or those containing carbaryl can be used against walnut husk flies or codling moths. Their use should be timed carefully to target vulnerable life stages while minimizing impact on beneficial insects. Consult local agricultural extension services for specific recommendations.