Webspinning Spider Mites

Adult female mites are larger and more elongated than European red mites and are green to yellow. Feeding mites have a dark spot on either side of the body that may enlarge to cover most of the body. 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. The tiny, spherical, colorless to straw-colored eggs are distributed over the infested area.

Mites feed upon leaves, removing the cell contents and gradually giving leaves a finely stippled appearance. Heavy infestations result in severe bronzing of foliage and premature defoliation. Fruits on heavily infested trees fail to color and size properly, and fruit production for the following year may be lowered.

Webspinning spider mites are typically most abundant during the hot summer months, especially in dusty and water-stressed areas of the orchard. Natural enemies keep spider mites below damaging levels in many orchards. Monitor for mites and their natural enemies from June through August to determine the need for treatment.

Varieties such as Red Delicious, Golden Delicious, Rome Beauty, and Jonathan are more susceptible to mite injury, while Gravenstein and Yellow Newtown show less evidence of leaf damage from moderate populations.

The western predatory mite, Galendromus (=Metaseiulus) occidentalis, is an excellent predator of webspinning mites. Predator mites have been developed that are resistant to carbaryl and most organophosphates used in apples. One predator to 10 webspinning mites is necessary for the predators to keep control of the pest mites. Use lower rates of miticides to minimize destruction of predators and allow some spider mites to survive. The apple rust mite is an alternative food source for the predator mite. Avoid sprays containing lime sulfur, which will kill rust mites. When alternative food sources are allowed to survive, the predaceous mites can build up large enough numbers to control webspinning mite populations.

Grass cover crops and sprinkler irrigation help to minimize dust in orchards. Provide adequate irrigation to avoid water stress. Do not mow the cover crop too short or let it dry, or the mites may move up into the trees.

Biological and cultural control and the use of resistant varieties are organically acceptable methods.

From July to August, or if you encounter high mite populations earlier in the season, collect five spur leaves at spaced intervals from one lateral branch located at eye level from each of 20 marked trees that have been established as representative trees in a block. Brush leaves in a mite-brushing machine and count mites but not eggs. Sample every 1 to 3 weeks. Treat when mite populations reach an average of 10 mites per leaf. Count predatory mites along with webspinning mites. If you find one predatory mite per 10 mites, you may not need a miticide application but continue sampling to be sure pest mite populations do not increase. If treatment is required choose materials least disruptive of biological control.