BLUE SPRINGS, Mo. — Warm, dry weather may leave your flowers and garden vegetables vulnerable to damage from spider mites.
“This year we have noticed significant damage to tomato, green beans, cucumber, eggplant, marigold, burning bush and roses from spider mites,” says Lala Kumar, University of Missouri Extension horticulturist in Jackson County.
The most common spider mite in Missouri farms and gardens is the two-spotted spider mite (Tetranychus urticae), which damages plants by piercing the cells and sucking out the contents, Kumar said.
Two-spotted spider mites spend the winter as dormant females and may be present around buds or under bark flaps of trees and shrubs. They also shelter under the surface of soil debris. Generations are completed in a little as 10 days and populations peak during June-August.
An early sign of infestation is a very fine, light speckling or localized pale yellow spots on the upper surface of leaves, Kumar said. Careful examination of the undersides of affected leaves, preferable with a hand lens or magnifying glass, will reveal colonies of mites. A more generalized bronzing discoloration develops as infestation progresses. Exceptionally high populations may produce visible webbing.
Spider mites prefer dry conditions, so watering and water management are critical in controlling two-spotted spider mites, Kumar said. Providing adequate water for plant growth is also important because plants stressed by drought or fluctuating wet/dry soil conditions are vulnerable to infestation. High humidity can decrease feeding by two-spotted spider mites, he said.
“Spraying susceptible plants with a fine mist of plain water twice a day may reduce mite damage,” he said. “Hosing of plants with water dislodges mites and also increases the humidity, which helps in mite control.”
Spider mites are difficult to control with pesticides, and many commonly used insecticides aggravate the problem by destroying their natural enemies. Applying horticultural oil or insecticidal soap every seven to 10 days can provide good mite control, Kumar said. Read the labels before spraying and make sure that the spray covers undersides of leaves.
“Last but not least, don’t forget to remove all plant debris in the fall to reduce the population of dormant female mites,” Kumar said.
For more information, contact your local MU Extension center or the Master Gardeners program in your area. Information is also available in the MU Extension guide “Aphids, Scales and Mites On Home Garden and Landscape Plants” (G7274), available for free download at extension.missouri.edu/p/G7274.