Webspinning Spider Mites

Webspinning mites are mites that form webs and cause defoliation. In prunes, they include twospotted spider mites and Pacific spider mites. Twospotted and Pacific spider mites are not easily distinguished: both have two black spots on their yellow-green bodies. In fall they turn orange-red before overwintering. The twospotted mite is most common in the Sacramento Valley and the Pacific mite in the San Joaquin valley. They overwinter as adult females under bark and on weeds. When weeds dry in spring, the mites move to trees and feed on lower leaves towards the middle of the tree first. They eventually become distributed over the entire tree. There are many overlapping generations each summer, with eggs being laid in a fine webbing on the undersurface of leaves.

Webspinning spider mites are the most damaging mite species to prunes. Spider mites feed by sucking the contents out of leaf cells. Such leaf damage reduces tree vitality and can adversely affect fruit size. Defoliation as a result of spider mite damage often allows the tree and fruit to become sunburned. Leaf injury caused by spider mites begins as a mottling and browning of leaves. Defoliation usually follows if control measures are not initiated. Both mites produce abundant webbing on both sides of the leaves.

In many cases biological control is effective at keeping spider mites below economically damaging levels, especially when pesticides that destroy natural enemies have not been used. Miticides may be necessary in some orchards if mite populations reach damaging levels. Minimize sulfur sprays used for rust control by treating only when monitoring indicates a need to help preserve the natural enemies of mites.

Predaceous mites, Typhlodromus caudiglans and Galendromus (= Metaseilus) spp., and the sixspotted thrips, Scolothrips sexmaculatus, feed heavily on webspinning mites and may give complete control. Encourage predators by avoiding the use of disruptive insecticides. These beneficials are also commercially available for release in the orchard. A narrow range oil (e.g., superior) spray will suppress low levels of mites without harming these predators. When predatory mites are present, low rates of the selective miticide, fenbutatin oxide (Vendex), may be used to reduce spider mite populations and hopefully better balance predator/prey ratios.

Vigorously growing trees are much more tolerant to mite attack than trees under stress. Maintain trees with optimum irrigation and fertilization. Reduce dusty conditions in orchards by oiling or watering roads and by maintaining a ground cover. Do not allow the ground cover to dry in mid-summer or mites will move up into the trees.

Biological and cultural control, oil sprays, and the release of predatory mites are acceptable methods of controlling spider mites in organically certified orchards.

Assess webspinning spider mite populations by conducting timed searches. Take weekly samples from June 1 to July 15 until the treatment threshold has been reached and a treatment has been applied. If a treatment is applied or after July 15, monitoring is no longer necessary.

How to Monitor:

1. In each orchard up to 40 acres, conduct a 5-minute search in two separate areas of the orchard, for a total sampling time of 10 minutes.
2. For each 5-minute search, examine at least 2 to 3 leaves on 10 trees. Note presence or absence of spider mites or predators. Sample leaves from both inside and outside the tree.
3. If mite population is spotty, continue to do two 5-minute searches throughout the summer. If you determine the mite population is consistent throughout orchard, one 5-minute search is adequate.
4. Keep records of sample results on a monitoring form (PDF).
5. Use the guidelines below to determine need for treatment.

Mite Ratings (percent of leaves with one or more mites):

• low (1-20%) = an occasional mite on occasional leaf; hard to find.
• low/moderate (21-39%) = mites easier to find but no colonies or webbing and few eggs.
• moderate (40-60%) = some leaves without mites, other leaves with small colonies; eggs easy to find but very little webbing.
• moderate/high (61-79%) = mites on mostleaves, colonies with eggs, and webbing on some leaves.
• high (80-100%) = lots of mites on most leaves; eggs and webbing abundant.

Predator Ratings:

• low = hard to find; less than one predatorper six leaves (only a few leaves will have predators).
• moderate = easier to find; one predator per three leaves (about half the leaves will have predators).
• high = one or more predators per leaf (most leaves will have predators).

Treatment Decisions (Treat if the rating from at least one 5-minute search indicates):

• low/moderate mite rating with low/moderate predator rating, or
• moderate/high mite rating with moderate/high predator rating

• Timed searches form for webspinning mites (PDF)