Aphids

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. 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.

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.

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, nut size, yield, and quality. High numbers of aphids may lead to leaf drop, exposing nuts to sunburn, which darkens or shrivels the kernels and increases nut susceptibility to other pests and pathogens. Aphids excrete honeydew. Sooty mold growing on the honeydew turns the husk surface black, also increasing the chance for sunburn on exposed nuts.

Over 15 walnut 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 numbers.

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 an insecticide application may be required in some orchards in some years. A monitoring program is available below for assessing the numbers of both aphid species and detecting damaging levels that may require an insecticide application.

The introduced parasitic wasp, Trioxys pallidus, has reduced the need for insecticide applications for walnut aphid. When the parasite is disrupted by applications of broad-spectrum pesticides that are used to control other pests found in walnut orchards, walnut aphid sprays may be required. The use of oil during the growing season has also been shown to be destructive to Trioxys.

The female Trioxys wasp lays eggs inside the small walnut aphid. Eggs hatch and the parasitic larva consumes the insides of the aphid, which turns tan 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. Although the dusky-veined aphid is occasionally parasitized, rates of parasitism are not high enough to effectively control this aphid; however, predators can be effective.

Trioxys can be reduced 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 insecticide applications than other walnut varieties. Pesticides used for other pests, such as codling moth and walnut husk fly, that may reduce hyperparasite numbers include: esfenvalerate (Asana), phosmet (Imidan), and spinosad (Entrust, Success). However, all of these pesticides also harm the primary parasitoid Trioxys as well as predatory mites, leading to an increase in spider mite numbers.

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. Predators also feed on walnut aphid, but Trioxys keeps the numbers of walnut aphid so low that predators seldom build up to large numbers on walnut aphids alone.

Reliance on biological control is the main management method in an organically certified crop. Oil sprays may suppress aphids, but are also harmful to natural enemies. In addition, be aware of phytotoxicity with oils especially when temperatures are high.

Begin sampling in May and continue throughout shoot and nut growth.

1. Take 5 first-subterminal leaflets (each compound leaf has five leaflets) from 10 trees for a total sample of 50 leaflets.
2. Check the upper surface of each leaflet for dusky-veined aphids and the lower surface for walnut aphids.

Walnut Aphid

Walnut aphid numbers often increase rapidly if chemicals are applied that interfere with biological control or if the hyperparasites are numerous. Consider an insecticide application for walnut aphid if the average number of healthy (non-parasitized) aphids found on the underside of subterminal leaflets of early, heavy-bearing varieties is over 15 per leaflet. Keep records (PDF) of your observations.

Dusky-Veined Aphid

Consider an insecticide application for dusky-veined aphid when an average of 10% of the subterminal leaflets have dusky-veined colonies of six or more feeding on their upper surface along the midvein.

• Aphid monitoring form (PDF)
• Photos of aphids
• Photos of natural enemies of aphids