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Spider Mites

Spider mites have been found on Clavey's dwarf honeysuckle at The Morton Arboretum. Look for spider mites on these plants as well as on other ornamental plants. Warm-weather spider mites include the two-spotted spider mite, honeylocust spider mite, oak spider mite, and other related species. These spider mites build up under dry summer conditions and are less numerous when it is cooler and rainy. Conversely, mites that cause severe damage to needled evergreens are more common in the spring and fall when it is cooler.

Spider mites are tiny, eight-legged, sap-sucking pests more closely related to spiders than to insects. When temperatures get into the upper 80s and 90s (degrees F), their life cycle speeds up to where they hatch, mature, mate, and lay eggs in about five days. This allows their populations to explode in numbers and completely overwhelm predators and other natural enemies that normally keep spider mite numbers low.

Damage by spider mites appears as fine stippling or tiny white dots most easily seen on the upper sides of leaves. This stippling quickly turns brown and is so small that the individual dots are usually not visible except upon close examination. From a distance, damaged leaves appear bronzish to brownish. On the leaf underside, stippling is not as obvious, but the leaf looks dirty from cast mite skins and the mites themselves. Euonymus, maple, oak, Kentucky coffeetree, honey locust, cotoneaster, and pyracantha are some of the more commonly attacked ornamentals. Winged euonymus shows early red fall color when attacked.

The mites are difficult to see on the leaf. Striking or shaking infested leaves over a sheet of white paper causes the mites to fall onto the paper, where they can be more easily seen against the white background. Rub your hand across the paper to squash the mites, which will produce streaks. Greenish streaks are indicative of plant-feeding spider mites. Reddish or brown streaks are probably made by predatory mites that are likely to keep the spider mites under control. Predaceous mites also run faster across the paper than do spider mites. Numerous spider mites on the paper means numerous mites on the leaves--and that treatment is warranted. Damaging levels of several mites per large leaf will result in a couple of dozen on the sheet of paper.

Control mites with miticides such as summer spray oil, insecticidal soap, dicofol (Kelthane), fenbutatin-oxide (Vendex), abamectin (Avid), or bifenthrin (Talstar). Two or three treatments at weekly intervals (five-day intervals under hot conditions) are needed to obtain control because some mite eggs survive sprays of these insecticides, allowing populations to rebound.

Author: Phil Nixon


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