Webspinning Spider Mites

Pacific spider mite and twospotted spider mite are the webspinning mites commonly found in cherry. These two spider mites have similar life histories and are controlled in the same manner. Overwintering female mites are red- or orange-colored and are found under rough bark, in ground litter, and on winter weeds. Adult males do not overwinter and are smaller than females. During the growing season their color ranges from yellow to green to black depending on age and host food. Both species have dark spots on their bodies.

Eggs are laid on the foliage. Immature mites molt three times. Early in the season mites are found in lower to central areas of the tree. The mites reproduce rapidly during warm weather between June and September. During favorable conditions, mites develop within 7 days with eight to ten generations per season.

Mites damage foliage by sucking cell contents from leaves. The damage begins with leaf stippling. Leaves can turn yellow and drop off. High populations cover tree terminals with webbing. Crop reduction shows up the year after damage occurs.

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. Keeping cherry orchards well irrigated during summer will help reduce the likelihood of mite outbreaks that are severe enough to warrant treatment.

Several species play a large role in mite control, including the western predatory mite (Galendromus [=Metaseiulus] occidentalis), sixspotted thrips, spider mite destroyer, brown lacewing, and green lacewing. The western predatory mite is the most reliable mite predator. It is the same size as spider mites, but lacks spots and ranges in color from cream to amber red. This predator maintains good control unless the proportion of leaves with spider mites is higher than the proportion of leaves with predatory mites.

Reduce dusty conditions in orchards by oiling or watering roadways and maintaining a ground cover. Prevent water stress, as this condition results in higher mite densities and intensified damage.

Biological and cultural controls and oil sprays are acceptable for use on an organically certified crop.

Regular monitoring will help determine if biological control is keeping spider mites under control or if treatments are needed. Begin monitoring leaves for mites in March. From March through May, monitor every other week; monitor weekly from May through August. Check trees at random throughout the orchard, and separately sample trees along dusty roads, areas of the orchard that are stressed, and areas that have had mite problems in the past. Sample at least 15 leaves from each tree, taking leaves from inside the canopy on the lower part of the tree. Record the number of leaves with pest mites and the number of leaves with predators. If the number of leaves with predators is nearly the same as the number with pest mites, no treatment is needed. Return in a week or so and sample again. If the number of leaves with pest mites is increasing and the number with predators is not, then treat. Presence of a large number of mite eggs is an indication that the population is increasing.

Spot treatments may be sufficient because heavy infestations usually start in dusty or stressed areas of the orchard. Use materials that are least harmful to natural enemies. Spray oils can be used as long as trees are not stressed, but oils are not effective on mite infestations that have developed heavy webbing. Oil sprays reduce mite populations about 50% in two weeks; so monitor mite numbers again two weeks after treatment to see if an additional spray is needed.

If treatment is needed early in the season and predators are present, you can use below-label rates of a miticide to reduce the pest population and help preserve predators. Treatments are not needed after the first of September; mite populations decline naturally at this time.

• Photos of natural enemies of mites