Webspinning Spider Mites

Pacific and twospotted spider mites overwinter as adult females in protected places on the tree or in the litter, trash, and weeds on the orchard floor. The overwintering forms of both species are reddish orange. The mites become active in early spring soon after trees leaf out and begin feeding on weeds or in the lower part of the trees. Both species are favored by hot, dry conditions, and as the weather becomes warmer, they increase in numbers and move up the center of the tree until the entire tree is infested.

Adult females are about 0.03 inch long. Active summer females are greenish or pale yellow with large dark spots on each side of the body. The Pacific mite may also have a second pair of spots near the posterior end of the body, which help distinguish it from the twospotted mite. Females can complete a generation in as little as 10 days during the hot part of summer. Eggs are spherical and almost translucent when first laid. They are generally deposited on the underside of leaves. As heavy populations build up, eggs may be deposited on both surfaces. There may be from 8 to 18 generations per year depending on temperature.

Stone fruits can tolerate some mite damage, particularly on water sprouts in the center of trees. Twospotted mites generally feed on the lower leaf surface, but Pacific mites may be found on both leaf surfaces. Feeding by both species causes a mottling of the leaves, and under severe conditions, can cause heavy leaf drop. Both species produce heavy webbing. If defoliation happens early in the season, fruit fails to size properly, and limbs and fruit may be exposed to sunburn.

Successful mite management requires regular monitoring both for pest mites and predators as well as good cultural practices to maintain healthy trees that are not stressed for water. In many orchards with adequate predator populations, no treatments for spider mites are necessary. It is especially important to monitor mites, however, in orchards where insecticides (eg., pyrethroids, organophosphates, and carbamates) that destroy mite predators are used during the growing season. In all orchards, use timed searches from May through August to assess the need for treatment.

Predators are very important in regulating pest mite populations in orchards. The most dependable predator is the western predatory mite, Galendromus occidentalis. This mite is about the same size as a spider mite and is generally translucent, but may be shaded in various colors of red to brown, depending on its food source. Galendromus occidentalis is pear shaped, somewhat shiny, and generally moves faster than plant-feeding mites. Under optimum conditions, this predator can produce a generation in 7 days, which allows it to build up rapidly and in many cases control plant-feeding mites.

Sixspotted thrips, Scolothrips sexmaculatus, are often responsible for the sudden disappearance of Pacific and twospotted mite populations. Adult sixspotted thrips are tiny, brownish, slender insects characterized by three dark spots on each forewing. Both the adults and small yellowish larvae are predaceous on mites. If sixspotted thrips are present on most mite-infested leaves, a treatment is rarely needed.

The spider mite destroyer, Stethorus picipes, is a small lady beetle that feeds on spider mites. Adults are about the size of a pinhead, jet black, with inconspicuous silver hairs covering the body. The hairs can be seen with a hand lens. The elongated larvae are small, dull black, and covered with numerous hairs, giving them a velvety appearance. Plant-feeding mite populations sometimes increase to damaging levels before Stethorus brings the populations under control; however, they are voracious feeders and can control populations quickly once they become abundant.

The above predators are adversely affected by certain materials applied for control of other pests such as oriental fruit moth and thrips. Every effort should be made to use pesticides that have the least adverse effect on these predators.

Management of twospotted and Pacific mites depends on a number of factors. Mite buildups are encouraged by hot, dry, and dusty conditions, so keep orchards well irrigated, and treat orchard roads, if necessary, to keep dust to a minimum. Proper pruning and adequate amounts of fertilizer to maintain tree vigor will also discourage twospotted and Pacific mites.

Biological controls, including predator releases, and cultural controls and various types of oil sprays are organically acceptable management tools.

From May through August, monitor for mites at least weekly. If the orchard has problem areas such as trees along roads or water-stressed trees, monitor every few days. Before July 1, focus monitoring on hot spots—i.e., areas that develop mites first; these are often dusty or water stressed areas of the orchard. Once the treatment threshold has been reached in these areas, sample the remainder of the orchard to determine if a spot treatment is sufficient or the entire orchard requires treatment. After July 1, monitor the whole orchard, dividing it into sampling areas that could be treated separately. Populations begin to decline after August 15, and treatments are generally not needed after this point.

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 (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 predator per 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

• Mite sampling using times searches (PDF)