Spider mites on outdoor plants: dust mites in hot dry summers

Spider mites show up in my Long Island yard on cue — around the second week of July, when we hit the first real hot dry stretch.

Spider mites show up in my Long Island yard on cue — around the second week of July, when we hit the first real hot dry stretch. They always appear first on the same plants: the Japanese maple in the driveway bed, which sits in reflected heat between the house and the pavement, and the dwarf Alberta spruce in the back corner where the irrigation doesn't quite reach. The plants look dusty, then bronzed, then start dropping needles or leaves.

Spider mites are not insects. They are arachnids — related to spiders and ticks — and their management is different from most garden pest problems. The typical response of reaching for a broad-spectrum insecticide makes the problem worse, not better.

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Identification

Spider mites are tiny — per UC IPM, "actual size less than 1/20 inch." They are not visible as individuals to the naked eye under normal conditions. You diagnose their presence by the damage they cause and the webbing they leave.

**The twospotted spider mite (Tetranychus urticae)** is the most common species on outdoor ornamentals and vegetables. Per NC State Extension, this mite has two dark spots on its back (one on each side, giving it its name). Females are typically pale green to yellow; males are slightly smaller. Eggs are laid on the undersides of leaves.

How to confirm a spider mite infestation: Hold a piece of white paper under a suspect branch or leaf and tap it firmly. If spider mites are present, you will see tiny dots moving on the paper. Per UC IPM, mites are easily distinguished from dirt and debris because they move.

Damage symptoms:

• Fine stippling (tiny white or yellow dots) on the upper surface of leaves and needles
• Leaves and needles taking on a bronzed or bleached appearance as feeding becomes severe
• Webbing on the undersides of leaves, particularly at the junctions of veins — though webbing may not be visible in early infestations
• Premature leaf drop
• In conifers and junipers: needles turning dusty gray-green before browning; in severe infestations, complete needle loss from entire branches

Per University of Minnesota Extension, "these symptoms may be confused with drought stress." This is accurate and worth noting: a plant with bronzed, stippled leaves in July in full sun may simply be heat-stressed. Check the undersides of leaves and do the white paper test before assuming mites.