Two-spotted spider mite on tomatoes — symptoms and treatment

Severity: medium

What is Two-spotted spider mite

Two-spotted spider mites (Tetranychus urticae) are tiny arachnids — relatives of spiders, not insects and not a fungus. This is a pest, not an infection, and that changes how you deal with it: there is no pathogen to protect against, only a colony to suppress. The mites live on the leaf undersides and feed by puncturing leaf cells and emptying them, which is what causes the stippling and bronzing you see from above.

Symptoms

The first sign is easy to miss: a fine pale stippling across the leaf, as though it had been touched repeatedly with a needle point. Each dot is a cluster of cells drained by a feeding mite. As numbers build, the stippling runs together and the leaf takes on a dull bronzed cast — a sickly sheen rather than the clean yellow of a nutrient problem. Turn the leaf over and you find the colony: fine webbing across the underside, and moving specks within it. In severe cases leaves dry and drop.

• Early: fine pale stippling on the leaves, most visible held up to the light.
• Progressing: yellowing and bronzing as the stippling merges; fine webbing on leaf undersides.
• Advanced: leaf drop; webbing over shoot tips; plants weakened and unproductive.

Use a hand lens on a bronzed leaf's underside. Moving mites and webbing confirm it — mite damage makes no defined spots, lesions or mould.

Causes and conditions

Mite populations build fastest in hot, dry weather, and reproduce quickly enough that a small colony becomes an outbreak in a short stretch of favourable conditions. They spread by crawling leaf to leaf, drifting on their own silk, and hitching a ride on hands, clothing, tools and newly bought plants. Two conditions matter most. Water stress is one: water-stressed plants are more susceptible. Broad-spectrum insecticide use is the other — sprays that kill predatory mites and other natural enemies leave the pest to rebound into a worse infestation than you started with.

Treatment

Water management — cultural

Timing: throughout the season. Maintain adequate irrigation, as water-stressed plants are more susceptible — proper watering is genuine control, not just housekeeping. Spray plants with water: a forceful spray at the leaf undersides knocks mites and webbing off.

Phytoseiulus persimilis — biological

Timing: early infestation. Release these predatory mites as biological control. They hunt two-spotted spider mites and are most effective in greenhouse conditions, where they stay on the crop. Release early, while the infestation is light — predators suppress a young colony but cannot rescue a plant already smothered in webbing.

Abamectin — chemical

Timing: at first detection. Apply the miticide when mite populations are first detected. Target leaf undersides, where the mites live — a spray that only wets the top of the leaf misses them. Pre-harvest interval: 7 days. If you plan to release predatory mites, check the product guidance first, as residues may affect them.

Pesticide registrations vary by country — check local approval before use.

Prevention

• Keep plants adequately and consistently watered; do not let them run dry between waterings.
• Inspect leaf undersides regularly with a hand lens, especially in hot, dry spells.
• Quarantine and check new plants before they join a greenhouse or bed.
• Avoid unnecessary broad-spectrum insecticides, which remove mites' natural enemies.

Frequently asked questions

Is this a disease? Can it spread to my other plants? It is not a disease — it is an arachnid pest, so nothing is infecting your tomato. But it spreads easily, and unlike a tomato-specific fungus it has a wide host range: beans, cucumbers, strawberries and ornamentals.

Can I eat the tomatoes? Yes — the mites feed on leaves and the fruit is fine to eat. If you have applied abamectin, respect the pre-harvest interval of 7 days.