Twospotted spider mite

What is twospotted spider mite?

The twospotted spider mite is not an insect but an arachnid, related to spiders and ticks. It feeds on more than 200 kinds of plants, from beans, tomatoes, cucumbers and strawberries to roses, azaleas, arborvitae and spruce. Penn State calls it a general feeder on shade trees, shrubs and flowers.

The mites feed on the undersides of leaves, sucking out cell contents. That leaves tiny pale specks on the leaf surface. Heavy feeding turns leaves bronze, gray or bleached, and they may drop. Colonies spin fine webbing that protects mites and eggs. Because a generation can be finished in about a week in hot weather and each female lays a dozen or so eggs a day, numbers can explode in a hot, dry spell. Damage is often mistaken for drought stress.

Hot, dry, dusty conditions favor mites and work against their natural enemies. Some common insecticides, such as carbaryl, kill the predators that keep mites in check and can make outbreaks worse. Good watering, a hard spray of water, and soaps or oils are the first steps.

Plants it attacks: beans, tomato, eggplant, cucumber, squash and melons, strawberry, raspberry and blackberry, blueberry, roses, marigold, arborvitae, azalea, spruce, Houseplants (Colorado State).

Signs and damage

• Leaves covered in tiny pale or yellow specks (stippling), giving a dusty, mottled look.
• Leaves that turn bronze, gray, yellow or bleached, then drop early.
• Fine silk webbing on leaf undersides, twigs and fruit, sometimes like a cobweb.
• Moving specks that fall onto white paper when you tap a branch.
• Damage that shows first on dusty plants, plants along dusty roads, and water-stressed plants in hot, dry weather.

Twospotted spider mite identification: what to look for

• Adult: Females 0.4 to 0.6 mm, oval, pale yellow to greenish, with two dark spots (food showing through the body wall). Eight legs. Overwintering females turn yellowish-orange.
• Larva: Newly hatched larvae have six legs; later stages have eight and look like small adults.
• Eggs: Tiny, round, clear to white, turning cream before hatch; laid on leaf undersides, 2 to 6 a day and up to about 100 per female. They hatch in 2 to 6 days. Empty shells left behind help confirm mites.
• Damage: Stippled, mottled leaves that bronze, yellow or bleach and drop; webbing; plant death in severe cases.
• Timing: Builds up fast in hot, dry weather, often peaking June through September. Generation time is 7 to 20 days depending on weather.

Life cycle

Mature females spend the winter in sheltered spots such as bark cracks, under ground cover or garden debris at the base of host plants, turning yellowish-orange. In spring they move back onto plants and lay eggs on leaf undersides, 2 to 6 a day and up to about 100 each. Eggs hatch in 2 to 6 days, and young mites molt three times before maturing. A generation takes 7 to 20 days, and in hot weather less than a week, so Pennsylvania sees 10 to 15 overlapping generations a year. Populations often crash in late summer when predators catch up or weather turns cool and rainy (Penn State, UC IPM, Colorado State).

When to look: Check plants every 3 to 5 days during hot, dry weather, especially in midsummer (UMN). Look under leaves on plants near dusty areas and on water-stressed plants first.

Twospotted spider mite look-alikes

A few things get mixed up with it:

• Spruce spider mite (Oligonychus ununguis): A cool-season mite of spruce and juniper that Colorado State notes is active in cooler weather, unlike the summer-loving twospotted mite.
• Aphids and thrips: Also cause speckled or curled leaves, but they don't spin webbing. Webbing is the easiest way to tell spider mites apart (UC IPM).
• Drought stress: Can look similar from a distance; check leaf undersides with a hand lens for mites, eggs and webbing.

How to control twospotted spider mite

Cultural control

• Keep plants watered, about an inch a week, and mulch to hold moisture; water-stressed plants suffer most (UMN, UC IPM, Colorado State).
• Hose plants with a strong spray, especially leaf undersides, to knock off mites, wash away webbing and clear dust that hinders predators.
• Control dust by watering paths, using mulch or ground covers, and rinsing plants mid-season (UC IPM).
• Don't fertilize plants during drought, which adds stress (UMN).
• Choose drought-tolerant plants for hot, dry sites (UMN).
• On houseplants, trim and bag badly infested leaves and rinse the plant every week or two while mites persist (Colorado State).

Biological control

• Predatory mites (such as Phytoseiulus persimilis and Galendromus occidentalis), spider mite destroyer lady beetles (Stethorus), sixspotted thrips, minute pirate bugs, big-eyed bugs and lacewing larvae all eat spider mites.
• UC IPM says purchased predatory mites work best in large plantings, released at about one predator per 10 spider mites; Colorado State notes outdoor results had not been shown in Colorado's dry climate.
• Avoid long-lasting, broad-spectrum insecticides that kill these predators.

Chemical control

Insecticidal soap and horticultural oils (petroleum or plant-based such as neem or cottonseed oil) are the preferred home options; they only kill mites they touch, so cover leaf undersides and repeat. Don't use them on water-stressed plants or above 90 degrees F, and test a few leaves first (UC IPM, UMN). Colorado State calls summer-rate horticultural oil perhaps the most effective home miticide, and Penn State suggests oil when mites first appear in late June or early July. Most products do not kill eggs, so repeat treatments about 7 to 14 days apart are usually needed, and mites can become resistant, so Penn State advises switching miticide classes after every third application. Carbaryl, some organophosphates such as malathion, some pyrethroids, and soil-applied imidacloprid can trigger outbreaks by killing predators. Sulfur controls mites on some plants but can burn cucurbits and should not be used within 30 days of an oil spray. Use only products labeled for this use; the label is the law.

Use only products labeled for this pest and this site; the label is the law. For trees, work with an ISA Certified Arborist. If you hire someone to apply pesticides, make sure they hold your state’s pesticide applicator license; the rules for each state are in our pesticide applicator license guide.

Prevention

• Water deeply during hot, dry spells and mulch beds.
• Keep dust down around gardens and plantings.
• Skip unnecessary broad-spectrum insecticide sprays that wipe out mite predators.

When to call a pro

If mites keep returning on valued shrubs or evergreens such as arborvitae and spruce, or on large trees, call an ISA Certified Arborist. Restricted miticides must be applied by a licensed pesticide applicator (see our pesticide applicator license guide).

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