Spider mites · Pests · GreenerThumbAI

Two-spotted spider mite (red form) with silk threads on a bean leaf Photo: Gilles San Martin · CC BY-SA 2.0

Two-spotted spider mite (red form) with silk threads on a bean leaf Photo: Gilles San Martin · CC BY-SA 2.0

In short

Spider mites are tiny eight-legged relatives of spiders that feed on leaf cells, mostly from the undersides. They thrive in hot, dry, still air, and the first sign is usually a fine pale speckling on the upper leaf surface, followed in heavy infestations by silk webbing.

How it looks

Adults are under 1 mm long, so they look like moving specks of dust; a hand lens shows an oval body with eight legs. The two-spotted spider mite is yellowish-green with a dark blotch on each side during the growing season, and turns orange-red when it prepares to overwinter, which is why it is also called red spider mite. Eggs are tiny, round and translucent. Colonies of hundreds live on leaf undersides under fine silk, among white cast skins and empty eggshells. Tapping a shoot over white paper shows them as dust that starts to walk.

Signs

Fine pale dots or mottling on the upper leaf surface, which can spread until leaves look dull, bronzed, yellow or reddish and then drop. In heavy infestations fine webbing covers leaves and shoot tips, and a badly infested plant can die.

Life cycle

Development speeds up sharply with heat: the RHS gives about 55 days from egg to adult at 10 °C but only about 12 days at 21 °C, and in warm conditions a generation can take less than a week. In cold climates mated females shelter in cracks and debris over winter and resume feeding and laying when it warms.

What favours it

Hot, dry, dusty conditions, water-stressed plants, low humidity and poor ventilation, and crowded plants under glass. Broad-spectrum insecticides such as carbaryl, organophosphates and pyrethroids can trigger outbreaks by killing their natural enemies.

Prevention

Water plants properly so they are not drought-stressed.
Improve air movement and raise humidity by misting or grouping plants; this reduces the risk but will not control an established infestation on its own.
Check leaf undersides with a hand lens, especially in hot, dry spells, and inspect new plants before they join your collection.
Clear away plant debris, old stakes and ties where mites can shelter.
Avoid broad-spectrum insecticides, which kill predatory mites and other natural enemies.

Treatment

Start with the first step and move down only if the problem persists.

Rinse or spray leaves, especially the undersides, with water regularly to knock mites off and break up webbing.
Cut off and bin badly infested leaves, or discard a heavily infested plant.
Release the predatory mite Phytoseiulus persimilis, which eats all stages; Neoseiulus (Amblyseius) californicus and the predatory midge Feltiella acarisuga are alternatives, and lacewing larvae and minute pirate bugs also feed on them.
Spray insecticidal soap or horticultural oil with thorough coverage of leaf undersides, repeating as directed; follow the product label.
Use a product labelled as a miticide (acaricide) only if other steps fail, and follow the product label; general insecticides can make spider mites worse.

Pros and cons

Cons

Heavy feeding causes leaf drop and can kill plants.
Populations build very fast in hot weather.

Look-alikes

False spider mites (flat mites): Make no webbing, are flatter and very slow, and cause brown, scabby patches along the midrib.
Thrips damage: Silvery patches are dotted with dark, varnish-like droppings and there is no webbing.
Leafhopper damage: Similar white speckling, but active jumping insects are present and there is no webbing.
Predatory mites: Beneficial: Phytoseiulus persimilis is orange-red and pear-shaped, without the two dark side blotches.

Affected plants

A very wide range of indoor, greenhouse and garden plants, including vegetables and fruit trees.

Sources

Written in our own words from these sources. Check a product label before using any treatment.