Description of the Pest
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.
Organically Acceptable Methods
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.
Monitoring and Treatment Decisions
Begin sampling in May and continue throughout shoot and nut growth.
- Take 5 first-subterminal leaflets (each compound leaf has five leaflets) from 10 trees for a total sample of 50 leaflets.
- Check the upper surface of each leaflet for dusky-veined aphids and the lower surface for walnut aphids.
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 of your observations.
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.
|Common name||Amount to use**||REI‡||PHI‡|
|(Example trade name)||(conc.)||(dilute)||(hours)||(days)|
|Not all registered pesticides are listed. The following are ranked with the pesticides having the greatest IPM value listed first—the most effective and least harmful to natural enemies, honey bees, and the environment are at the top of the table. When choosing a pesticide, consider information relating to air and water quality, resistance management, and the pesticide's properties and application timing. Always read the label of the product being used.|
|(Admire Pro)||1.2—2.4 fl oz||—||12||7|
|MODE-OF-ACTION GROUP NUMBER1:4A|
|COMMENTS: Do not apply prebloom. During bloom, do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Assail 70WP)||1.1—4.1 oz||0.271—1 oz||12||14|
|MODE-OF-ACTION GROUP NUMBER1: 4A|
|COMMENTS: Make no more than four applications per season. Do not exceed 0.72 lb a.i./acre per season. Toxic to bees; do not spray directly or allow to drift onto blooming crops or weeds where bees are foraging.|
|(Imidan 70W)||6 lb||1—2 lb||168 (7 days)||28|
|MODE-OF-ACTION GROUP NUMBER1: 1B|
|COMMENTS: Do not apply after husk split. Has a residual of about 21 days. Buffer to a pH of 5.5—6.0.|
|D.||NARROW RANGE OIL#|
|1%||—||See label||See label|
|MODE OF ACTION: Contact including smothering and barrier effects.|
|COMMENTS: Use of oil during the growing season can be harmful to the aphid parasite, Trioxys pallidus. Helps to suppress aphids. More effective on dusky vein aphid than walnut aphid. Oils should not be used on walnut during the dormant season, between bud break and shoot elongation, or on drought-stressed trees; also, do not apply after husk split. Check with certifier to determine which products are organically acceptable.|
|1||Rotate chemicals with a different mode-of-action group number, and do not use products with the same mode-of-action group number more than twice per season to help prevent the development of resistance. For example, the organophosphates have a group number of 1B; chemicals with a 1B group number should be alternated with chemicals that have a group number other than 1B. Mode-of-action group numbers are assigned by IRAC (Insecticide Resistance Action Committee).|
|**||For concentrate application, use the amount given in 80 to 100 gal water/acre, or lower if the label allows; for dilute application, amount is per 100 gal water to be applied in 300 to 500 gal water/acre, according to label.|
|‡||Restricted entry interval (REI) is the number of hours (unless otherwise noted) from treatment until the treated area can be safely entered without protective clothing. Preharvest interval (PHI) is the number of days from treatment to harvest. In some cases the REI exceeds the PHI. The longer of these two intervals is the minimum time that must elapse before harvest may occur.|
|*||Permit required from county agricultural commissioner for purchase or use.|
|#||Acceptable for use on organically grown crops.|