Along with the warm, sunny days of summer comes a scourge of pests – spider mites. These little arachnids, related to spiders, look like tiny moving dots. The eight-legged, oval shaped adults live in colonies, primarily on the under-surfaces of leaves and a single colony may contain hundreds of individuals. They overwinter as mated females in protected areas – on weeds, ground litter, or fallen leaves. They cover leaves, shoots, and flowers with very fine silken webbing; these strands allow them to spin down from infested leaves and to be blown by wind currents.
Spider mites feast on fruit trees, vines, berries, vegetables, and ornamental plants particularly June through September. Also called web spinning mites, they are among the most common of all pests in the garden. They thrive in low rainfall areas of the West where warm summer temperatures above 70°F and dry plant foliage favor their development. Their populations can build up to tremendous numbers and can decimate a plant in a very short time if the conditions are right – hot, dry weather and dust and dirt on leaf surfaces.
You’ll usually see plant damage first – stippling or yellowing of leaves, before seeing the mites themselves. As feeding continues, leaves turn yellow and drop off. Check the undersides of leaves for mites, their eggs, and webbing. Shake a few off the leaf surface onto a white sheet of paper; if you dark specks moving across the paper, it’s most likely spider mites.
Low numbers may result in some cosmetic damage; prolonged heavy infestations slow plant growth and cause leaves and fruit to drop prematurely. Severe infestations can kill plants, especially if they are suffering from water stress. Regular use of insecticides in the garden can destroy natural enemies and encourage spider mite outbreaks.
Predators of spider mites include the six spotted thrips, the larvae and adults of the spider mite destroyer lady beetle, minute pirate bugs, big-eyed bugs, and lacewings. The best protection against an invasion of mites is maintaining good cultural practices. Remove fallen leaves, branches, or fruit, as well as weeds that may serve as a host for them. Keep plants well-watered and apply mulch to minimize plant stress. Periodic hosing of plants with a forceful jet of water, including the undersides of leaves, can physically dislodge and kill many mites as well as remove the dust that collects on foliage and interferes with mite predators.
If you have an infestation now, prune out damaged portions of plants or those with significant webbing. As the seasons change, temperatures drop and rains return, high mite populations generally decline rapidly and take a break until next summer. Implement preventive measures early next year and you can keep spider mites at a tolerable level.