Spider Mites

Susceptible Plants
Many trees and shrubs, especially honeylocust (Gleditsia), maple (Acer), oak (Quercus), crabapple (Malus), burning bush (Euonymus), potentilla and rose (Rosa); many perennials and annuals.

Description & Symptoms
Mites are tiny, wingless, almost microscopic pests that suck fluids from the leaves of a wide variety of plants. Leaves become pale and flecked or stippled and often have a bronzy sheen. There often is webbing along stems and leaf surfaces, but this cannot always be seen without a microscope.

Timing & Life Cycle
Under favorable conditions, spider mite populations can explode. Mites that attack deciduous plants, perennials, and annuals are active in warm weather, reproducing more quickly as the temperatures rise. Mite populations usually peak during the hot, dry months of July and August. There are many generations per year, so eggs and live mites often can be found on a plant at the same time. Evergreens are more frequently attacked by cool-season mites, which are active in the spring and fall. Most spider mites spend the winter as eggs in twigs, branches, or grass and weeds.

In severe cases, entire plants defoliate and are seriously weakened.

Treatment & Solutions
Mites have many natural enemies that help control their numbers, including lady beetles, lacewings and predatory mites. The population of insect predators tends to rise with the spider mite population, making predation an important control. Chemical controls should be used on valuable plants only as a last resort because they will kill predators as well as spider mites.

Spider mites thrive on plants that are under drought stress. Keeping plants well watered helps them withstand a moderate number of mites. Heavy rains or hard sprays of water from a hose will knock mites off plants.

To assess the severity of a mite infestation, hold a white piece of paper under a branch or plant and strike the plant sharply. If more than two dozen mites land on the paper each time you strike the plant, consider implementing controls. Mites will look like tiny moving specks on the paper.

During the winter, plants can be sprayed with dormant oil to smother overwintering eggs. During the growing season, insecticidal soap or summer weight oil may be used against mites and eggs.

For more information about spider mites, call the Plant Information hotline at (847) 835-0972.