Webspinning Spider Mites

Adult female mites are yellow in color. Feeding mites have dark spots on either side of the body. The tiny, spherical, colorless to light-straw-colored eggs are distributed over the infested area. Overwintering females are orange, and hibernate under bark scales on the tree and in trash on the ground. They move up the tree in late March and April, feeding on leaves. Rapid reproduction occurs in hot, dry weather and the infestation peaks in July and August.

Webspinning mites produce a characteristic blackening of pear leaves when they feed. Pear trees can tolerate fewer webspinning mites than European red mites. Usually two to three mites feeding near the midrib of a leaf produce black areas from the midrib to the margin. This blackening may appear even after mites have been controlled, especially if a period of hot weather follows the spray application. High mite populations may cause defoliation. Severe defoliation can stunt fruit and may cause the trees to bloom in fall, thus reducing next year's crop. However, if defoliation is limited to water sprouts in the top or interior of the tree, it will not adversely affect the crop or tree.

Webspinning spider mites are typically most abundant during the hot summer months, especially in dusty and water-stressed areas of the orchard. Orchards with high predator-to-pest-mite ratios and good dust and water-stress management may not need treatment, especially orchards using codling moth mating disruption. Monitor regularly. When treatment is needed, choose products least disruptive to biological control.

The western predatory mite, Galendromus (=Metaseiulus) occidentalis, is an excellent predator of webspinning mites. A ratio of one predator to 10 twospotted mites is necessary for the predators to keep control of the leaf-feeding mites. Use lower rates of miticides to minimize destruction of predators and allow some spider mites to survive. Biological control by predatory mites can also be encouraged by suppressing spider mites with oil added to one or more codling moth sprays to improve predator-prey ratios. Western predatory mite is most effective in sprayed orchards and may not compete as well in organic orchards where there are many other natural enemies of mites.

Orchards with cover crops and sprinkler irrigation have been most suitable for an IPM mite program because these practices minimize dust. Do not allow the cover crop to become dry as this will cause webspinning mites to disperse to trees. Do not allow cover crops to grow into trees forming bridges for the mites to move from the cover crop to the trees. Low-growing grasses host fewer webspinning spider mites than broadleaf weeds such as morningglory, tall grasses such as johnsongrass, and broadleaf cover crops such as clover. Mow or apply herbicides at a time during the year that will not trigger migration of webspinning mites into the trees.

Biological and cultural controls and sprays of certain oil products are organically acceptable.

Begin sampling for webspinning spider mites when pears turn down. Collect 5 spur leaves at eye height from one scaffold branch from each of 20 marked trees that have been established as representative trees in a block. Examine 5 leaves from each shoot with a hand lens (10 to 14X) and count both European red mites (eggs, nymphs, and adults) and webspinning mites (nymphs and adults only). As a general guideline, if no mites are found in the sample, you can wait 3 weeks to resample. If there is less than one mite per leaf, resample in 2 weeks, and if there is one mite per leaf, take your next sample in 1 week.

Once a week during the summer months, also check 5 leaves on 20 top shoots for presence of webspinning mites. For more information regarding sampling, see SAMPLING DURING FRUIT DEVELOPMENT.

Following harvest, check 5 leaves on 20 top shoots for webspinning spider mites. For more information about monitoring at this time, see POSTHARVEST SURVEY.

The following mite thresholds are for Bartlett pears and include all stages of European red mite and webspinning mite nymphs and adults. Bosc pears are more susceptible to spider mites and should be treated at lower thresholds. Asian pear and other varieties usually tolerate more mites than French pear varieties, making biological and cultural controls easier to implement.