Webspinning 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 plums. 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 keeps spider mites under control. Miticides may be necessary in some orchards in summer, but only when mite populations reach damaging levels, which often occurs after pesticides have been used that disrupt natural enemies. Use timed searches from June 1 to July 15 to assess need for treatment.

Predaceous mites, Typhlodromus caudiglans and Galendromus spp., and the sixspotted thrips, Scolothrips sexmaculatus, feed heavily on webspinning mites and may give complete control in the orchard. Control is most successful when the presence of predators and mites per leaf is close to a ratio of 1:1. The presence of these predators can be encouraged 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 (Carzol), 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 and oil sprays are organically acceptable management tools.

Soon after bloom, start looking for spider mites and predatory mites on first emerging leaves on scaffolds. Use this information to map out areas of concern for future monitoring in the fruit development season.

Take weekly samples using the timed search method below, from June 1 to July 15 until the treatment threshold has been reached and a treatment has been applied. Record observations on a monitoring form (PDF). If a treatment is applied on or after July 15, monitoring is no longer necessary. Continue to monitor untreated orchards until harvest.

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 the monitoring form.
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

• Photos of natural enemies of pest mites
• Photos of spider mites
• Timed searches form for webspinning mites (PDF)