Webspinning Spider Mites

Pacific and twospotted mites overwinter as reddish orange, mature females in protected places on the tree, in soil, and in trash on the ground. During warm weather in spring, overwintered females begin feeding on walnut leaves and ground cover in the orchard. During periods of active feeding twospotted mites have a dark spot on each side of the body. Pacific mites have a second pair of dark spots near the posterior end. Often, however, the spots are barely visible or may coalesce to large dark areas, making it difficult to distinguish the two species.

Colonies develop on the underside of leaves and also on the upper sides when heavy numbers build up. Eggs are spherical and translucent when first laid, becoming opaque soon before hatching. Immature mites molt three times before becoming adults. The first stage mites have six legs; later stages and adults have eight legs. These mites reproduce rapidly in hot weather and may become numerous in June or July. They produce many generations per year. If temperature and food supply are favorable, a generation can be completed in 7 days.

Mite feeding causes stippling and browning of leaves. Clusters of brown leaves are often the first sign of an increasing mite numbers. High numbers produce copious webbing, and their feeding causes leaves to desiccate and drop. Defoliation early in the season can reduce nut yield and quality; defoliation late in the season will interfere with harvest.

Natural enemies usually keep spider mites below damaging levels unless broad-spectrum pesticides are used. Use selective pesticides when treating other pests and monitor carefully for mites, predators, and mite damage.

The most dependable mite predator of spider mites is the western predatory mite, Galendromus (=Metaseiulus) occidentalis. Under optimal conditions, this predatory mite can produce a generation in 7 days. Because walnut orchards often lack an alternate food source such as European red mites early in the season, the western predatory mite sometimes may be late in building to numbers sufficient to control webspinning mites, which become abundant later in the season.

Sixspotted thrips can be very effective in reducing high numbers of webspinning mites. Thrips, however, usually do not move into orchards until mite populations are high. Both the adults and the small, yellowish larvae prey on mites.

The spider mite destroyer, Stethorus picipes, is a small lady beetle that feeds on mites. The larvae are small, dull black and have a velvety appearance. The spider mite destroyer, like the sixspotted thrips, generally does not become numerous until spider mite numbers are high. But it is an active feeder and can reduce mite numbers quickly when it is abundant. Minute pirate bugs also feed on spider mites.

Orchard management practices can reduce mite problems. Minimize dust by oiling orchard roads and maintaining a ground cover. Well-irrigated, vigorous trees are less troubled by mite infestations. Choose selective pesticides when controlling other pests and try to avoid pyrethroids, organophosphates, and carbamates until later in the season.

Biological and cultural controls and sprays of narrow range oil are acceptable for use in an organically certified crop. Caution should be taken when applying oils in walnuts to avoid injuring trees (see narrow range oil comments in the table below).

Begin looking for spider mites in late spring. Map out areas of concern for summer monitoring.

In June or early July start sampling for spider mites once a week through August:

• Select randomly 10 trees in the orchard and pick 5 leaflets from low branches and 5 leaflets from high branches in each tree.
• Look for the presence of webspinning spider mites, predator mites, and sixspotted thrips and record your observations (PDF) separately for low and high branch leaves. If predaceous mites are present on at least half of the leaflets that have mites, then there is a good probability that natural enemies will suppress the population unless broad-spectrum sprays are used for codling moth management. Monitor again in one week.
• If predaceous mites are not present on at least half the leaflets that have mites, monitor again in 3 to 4 days (3 days if weather is hot) to determine if the webspinning mite population is increasing or declining. If mites do not build up in walnuts by mid-August, then a pesticide application may not be warranted.

Treatment Thresholds

Take into consideration populations of predators and whether or not you plan to apply an organophosphate or pyrethroid treatment against other pests later in the season, before you make a treatment decision.

For orchards where NO organophosphate or pyrethroid applications are used:

• Predators present on fewer than 10% of mite-infested leaves: spray if 30 to 40% of leaves are mite infested.
• Predators present on 20 to 50% infested leaves: spray if 40 to 50% of leaflets are mite infested.
• Predators present on more than 50% mite-infested leaves: don't spray.

For orchards where organophosphate or pyrethroid applications are used: (If these materials are used, a tank-mix with a miticide may be warranted in warmer, dry areas on light soils.)

• Predators present on fewer than 10% of mite-infested leaves: spray when 10% of leaves have spider mites.
• Predators present on more than 10% of mite-infested leaves: spray when 20% of leaves have spider mites.

• Mite monitoring form (PDF)
• Photos of natural enemies of mites
• Photos of pest mites and their damage