Adapted from the article “Continue Watching for Aphid and Spider Mite Infestations” by Bill Krueger, UCCE Farm Advisor Emeritus, Glenn County in the August 2009 Sacramento Valley Walnut News.
There are two aphid species that damage walnuts, the walnut aphid (a small yellow aphid usually found on the lower surface of the leaf) and the dusky veined aphid (larger yellow aphid with dark banded spots that feeds near the mid vein on the upper surface of the leaf). In recent years, a white form (morph) of the walnut aphid has been found mostly in the lower Sacramento Valley.
Populations of the white morph tend to build later in the season than the normal yellow aphids. Aphid feeding produces honeydew and a sooty mold growing on the honeydew turns the leaves black. Aphid feeding can reduce tree vigor, yield, nut size and quality. The honeydew is also attractive to walnut husk fly as a sugary food source.
Walnut aphid will usually be controlled by the introduced parasite, Trioxys pallidus, evidenced by the presence of brown mummified aphids with circular exit holes. Trioxys can be disrupted by sprays to control other pests or by hyperparasitism (parasitism of the parasites). In-season oil sprays can also disrupt Trioxys. Treatments that control other pests such as codling moth and walnut husk fly will normally control hyperparasites but may increase spider mite problems.
Monitoring for Aphids
Sampling for aphids should begin in May and continue through shoot and nut growth. Collect 5 sub-terminal leaflets from 20 trees. Treatment is recommended if there are more than 15 healthy walnut aphids per leaflet. Treatment of dusky veined aphid is recommended if 10% of the leaflets have colonies of 6 or more aphids.
Spider Mites (Web spinning mites)
Spider mites may develop high populations in the late spring through summer as temperatures rise. Light bronzing of the leaves is an indication of an increasing population. As the population develops, clusters of brown leaves are noticed. Heavy feeding results in webbing-over of the leaves and, ultimately, the defoliation of the infested leaves.
Spider mites are usually kept below damaging levels by natural enemies unless those enemies are disrupted by broad spectrum pesticides or mites are favored by dusty conditions or water stress. Use selective materials whenever possible when treating other pests and minimize dust and water stress.
The most dependable natural enemy is the western predatory mite which can be seen by using a hand lens. It is generally clear and pear-shaped and will be moving more rapidly than the spider mites. Six spotted thrips can also be an effective predator, but may come into the orchard too late to control the pest before economic damage occurs.
Monitoring for Spider Mites
Starting in late May or early June and continuing through August at weekly intervals, randomly select ten trees in the orchard and check ten leaflets per tree (5 low and 5 high). Look for web spinning mites, predator mites and six spotted thrips.
Treatment thresholds where organophosphate or pyrethroid insecticides are not used.
- 30-40% infested leaflets if predators are on less than 10% of the leaflets.
- 40-50% infested leaflets if predators are on 40-50% of the leaflets.
- If predators are on 50% or more of the leaflets, a treatment should not be necessary.
Where organophosphates or pyrethroid insecticides are used.
- 10% infested leaflets if predator mites are on less than 10% of the leaflets.
- 20% infested leaflets if predators are on more than 20% of the leaflets.
There are a large number of materials available for controlling spider mites with different modes of action and characteristics. Select a material to fit your situation. Avoid using materials in the same mode of action group more than 2 times per year to reduce the risk of resistance development.