Spider Mites on Tomatoes: Identification, Damage, and Control

Spider mites are not insects. They are arachnids -- eight-legged relatives of spiders -- and this biological distinction matters because most insecticides are ineffective against them. Using the wrong product on a spider mite infestation does nothing useful and may make the problem worse by killing.

Spider mites are not insects. They are arachnids — eight-legged relatives of spiders — and this biological distinction matters because most insecticides are ineffective against them. Using the wrong product on a spider mite infestation does nothing useful and may make the problem worse by killing predatory insects that would otherwise control mite populations.

Spider mites on tomatoes are most severe during hot, dry periods. A moderate infestation in June can explode into a plant-threatening infestation by August if conditions are right and no intervention is taken.

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Identification

The two-spotted spider mite (Tetranychus urticae) is the primary spider mite on tomatoes. Per UC IPM:

• Adults are 0.3–0.5mm long — barely visible to the naked eye as moving specks on leaf undersurfaces
• Two dark spots visible on either side of the abdomen (a magnifying glass is needed to see this)
• Pale green to yellowish-green in summer; reddish-orange in overwintering stage
• Produce fine silk webbing on infested leaf surfaces and between leaves

A simple diagnostic test: hold a white sheet of paper beneath a suspect leaf and tap the leaf sharply. If mites are present, tiny specks will fall onto the paper and move. Per Penn State Extension, this is the most reliable way to detect low-level infestations before damage becomes visible.