Extension Educator, Horticulture
With any extreme weather there are winners and losers. Fungi love wet weather while many insects yearn for hot and dry. Leaf diseases caused by fungi are less severe this season. Despite the drought my tomato plants look great without the usual leaf spot diseases. However this year two-spotted spider mites are literally sucking the life out of many plants including my tomatoes.
University of Illinois Extension entomologist Dr. Phil Nixon reported in the Home, Yard and Garden Newsletter that two spotted spider mites are plaguing a variety of deciduous and evergreen broadleaf plants. They and their relatives, including oak mite and honey locust mite, thrive in the hot, dry conditions. 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 and completely overwhelm predators and other natural enemies that normally keep spider mite numbers low. In contrast spruce spider mites (common on spruce, juniper and pine) are cool-season mites so are typically numerous in spring and fall.
Euonymus, maple, oak, Kentucky coffeetree, honey locust, cotoneaster and rose are some of the more commonly attacked ornamentals. Burning bush shows early red fall color when attacked by spider mites. They also feed on many annuals, perennials and vegetables.
From a distance the leaves of spider mite damaged plants may appear brownish to bronze colored. Damage caused by two-spotted spider mites initially appears as small, whitish areas on the upper sides of the leaves. Spider mites suck out the contents of leaf cells. The injured cells die and turn brown. Severely damaged leaves will curl and eventually drop from the plant.
Spider mites are found primarily on the leaf undersides which will appear dirty if there is a heavy mite infestation. Adults are one-fiftieth of an inch long so typically most people will need a magnifier to see them. Two-spotted spider mites commonly have two large dark green areas that can be seen through the body wall, giving them their name. These dark areas are accumulations of body waste, so only older individuals have them.
With heavy infestations spherical, clear to yellow spider mite eggs are obvious on leaf undersides. Fine, silk webbing may be seen on the leaf underside or between the leaf petiole and stem, but this is usually blown away on outdoor trees and shrubs. I have seen more webbing than usual this year.
Scout for spider mites by shaking suspected branches over a white sheet of paper. Some of the mites will fall onto the paper where they can be seen as greenish to grayish, slow-moving dots that streak green when smashed. Faster-moving red or clear mites that streak red when smashed are predatory mites that feed on the spider mites. If predatory mites are common, they are likely to control the spider mites without the need for pesticides.
Spider mites are controlled with two or three miticide sprays applied at five to seven day intervals. Most miticides do not kill the eggs, so the repeated spray is important. In hot weather, the mites grow up quickly, so a delay of only a day or two beyond a seven day spray interval is likely to require an additional spray application. Insecticide sprays for mites include insecticidal soap (not dishwashing soap), clove oil, canola oil and summer spray oil. Be sure to read and follow all label directions. For vegetables be sure to use insecticides labeled specifically for vegetables and follow labeled days between application and harvest. Forceful sprays of water onto leaves may also lessen populations.