How to Manage Pests
UC Pest Management Guidelines
Two aphid species that may damage walnut trees are the walnut aphid and the dusky-veined aphid. Their seasonal development is very similar, but their appearance and behavior are quite different. The walnut aphid was once a major pest in walnuts but now is mostly controlled by an introduced parasitic wasp. Since the biological control of the walnut aphid, the dusky-veined aphid has become a pest in some orchards.
Walnut aphids are easily distinguishable from dusky-veined aphids. They are much smaller and are typically found scattered on the lower side of leaves. Dusky-veined aphids feed in rows along the midvein on the upper leaf surface. During spring and summer, adult females are commonly winged, and their wings have distinctive dusky markings along the veins. Nymphs of the dusky-veined aphid have dark, banded spots on the back. These spots are much less pronounced or absent on the nymphs of the walnut aphid. In the last few years, a white form (morph) of the walnut aphid has been found in the Sacramento Valley. Populations of the white morph build later in the season than normal-colored ones.
The life cycle of these two species is basically the same. Both aphids overwinter in the egg stage on twigs. Eggs hatch as soon as leaf buds of early cultivars begin to open. These aphids settle on the leaflets, mature, and reproduce without mating, giving birth to live nymphs. The aphids pass through many generations a year, depending upon temperature. In fall, wingless females mate with smaller, winged males and lay the overwintering eggs.
Aphid feeding can reduce tree vigor and nut size, yield, and quality. Aphids excrete honeydew. Sooty mold growing on the honeydew turns the husk surface black, and increases the chance for sunburn on exposed nuts. High populations of aphids may lead to leaf drop, exposing more nuts to sunburn, which darkens or shrivels the kernels.
Walnut aphid populations of over 15 aphids per leaflet early in the season reduce nut yield and quality and cause an increase in nuts with perforated shells. An infestation in summer lowers the nut quality. Some late cultivars, such as Franquette, may tolerate heavy populations.
Feeding by dusky-veined aphids causes the midribs of leaves to turn black. A correlation has been established between infestation of dusky-veined aphids and nut quality. If 10 to 15% of the leaflets are infested for 3 to 4 weeks before shell hardening, nut size is decreased. The same level of infestation during late summer will result in shriveled kernels at harvest time.
In most orchards, walnut aphids are kept below damaging levels by an introduced parasitic wasp in combination with other naturally occurring biological control agents. However, if broad-spectrum insecticides are applied to control other pests such as codling moth, outbreaks of walnut aphid may occur. Predation often effectively controls the dusky-veined aphid as well, but it may require treatment in some orchards some years. A monitoring program is available for following the populations of both aphid species and for detecting damaging levels that may require treatment.
The adult female parasite lays eggs inside the small walnut aphid. Eggs hatch and the parasitic larva consumes the insides of the aphid, which turns tan in color and becomes "mummified." The presence of aphid mummies is an indication the parasite is present. After the parasite pupates, the adult wasp emerges from the aphid mummy by chewing a small exit hole.
Populations of Trioxys have been found to be affected by native hyperparasites. Hyperparasitism (the parasitism of a parasite) has been found to be greatest in unsprayed orchards and orchards with codling moth-tolerant tree varieties that require fewer treatments than other walnut varieties. Treatment materials used for other pests, such as codling moth and walnut husk fly, that effectively control this hyperparasite are chlorpyrifos (Lorsban), esfenvalerate (Asana), phosmet (Imidan), and spinosad (Entrust, Success), but all of them have a negative impact on spider mites.
Predators such as lady beetles, including the Asian multicolored lady beetle and ashy gray lady beetle, lacewings, and flies play an important role in the natural control of the dusky-veined aphid. They also feed on walnut aphid, but the parasite keeps the population of that aphid so low that predators seldom build up to large numbers on walnut aphid populations.
Monitoring and Treatment
UC IPM Pest Management Guidelines: Walnut
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