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Pests in Gardens and Landscapes: Quick Tips

Spider mite, actual size less than 1/20 inch. Photo by Jack Kelly Clark.

Spider mites cause leaf stippling or spotting and may leave webs when numbers are high. Photo by Jack Kelly Clark.

Spider mites are common pests of fruit trees, vegetables, berries, vines, houseplants, and ornamental plants. Mites are tiny and difficult to see. Although related to insects, mites are arachnids just like spiders and ticks. If leaves are stippled with white dots or have webbing, check the undersides to see if spider mites are present. Sprays of water, insecticidal oils, or insecticidal soaps can be used for management. Spider mites have many natural predators that often limit their numbers.

What to look for:

• To the naked eye, spider mites look like tiny, moving dots. Use a magnifying lens to see them.
• Adults are less than 1⁄20 inch long and have eight legs, an oval body, and two colored eyespots near the end of the head.
• Spider mites live in colonies, mostly on the underside of leaves; a single colony can contain hundreds of mites.
• When numbers are high, dense webbing can cover leaves, twigs, and fruit.

What damage do spider mites cause?

• Spider mites cause damage by sucking cell contents from leaves.
• A small number of mites isn’t usually a problem, but very high populations can be damaging, especially to annual plants.
• Often, damage first appears as a stippling of light dots on the leaves; sometimes leaves turn a bronze color. Heavily infested leaves can turn yellow and drop off.
• Damage is usually most severe in hot, dusty conditions and on water-stressed plants.

Protect predators of spider mites.

• Spider mites have many predators or “natural enemies”, which prevent them from causing plant damage.
• Key natural enemies include predatory thrips, lacewings, and minute pirate bugs.
• Predatory mites, which are about the same size as plant-feeding mites but have longer legs and are more active, are also effective in reducing spider mite numbers.

How do I control spider mites?

• Water plants enough so they are not drought stressed, which increases mites and mite damage.
• Keep dust down. Plant ground covers, use mulches, and irrigate regularly.
• Most woody plants can tolerate low to moderate mite populations, and natural enemies are often abundant.
• If plants are infested, apply a water spray or mist to the undersides of leaves at least once a day.

What about pesticides?

• If an insecticide is needed, use an insecticidal oil or insecticidal soap (or a combination of the two). Completely cover the undersides of leaves. Be sure mites are present before treating. Don’t spray when plants are water-stressed or if it is very hot.
• Avoid using insecticides that kill natural enemies.
• Spider mites frequently become a problem after applying certain insecticides such as some organophosphates or pyrethroids. These insecticides are not very effective against mites, often kill mite predators and can result in spikes of spider mite populations.

Minimize the use of pesticides that pollute our waterways. Use nonchemical alternatives or less toxic pesticide products whenever possible. Read product labels carefully and follow instructions on proper use, storage, and disposal.