As we move into the summer, keep in mind that hot and dry conditions can lead to some specific pest problems. Two-spotted spider mite (Figure 1) is a common pest of many vegetable crops during prolonged hot and dry periods. This pest rapidly builds up in numbers during these conditions, and some pesticides used to control insect pests may reduce natural enemies that help to keep populations below economic levels. Mites can injure tomatoes, beans, muskmelons, eggplant, watermelons, and sweet corn. Infestations usually first occur at the edge of a field, typically near rank weed growth or dusty roads.
Symptoms & Life Cycle
Generally, mites feed on the undersides of leaves. They use their sucking mouthparts to remove sap from plants, giving upper leaf surfaces a speckled or mottled appearance. Leaves of mite-infested plants may turn yellow and dry up, and plants may lose vigor and die when infestations are severe. The undersides of affected leaves appear tan or yellow and have a crusty texture. Heavy infestations of the two-spotted spider mite produce fine webbing, which may cover entire plants. Mites can be identified by shaking leaves onto a sheet of white paper or by observing leaf areas with a hand lens. In hot dry weather, mites can cause plants to drop leaves in a few weeks. Fruit from severely infected plants are often unmarketable because defoliated plants tend to yield small, poor quality fruit. Under optimum conditions of high temperature and low humidity, the life cycle may be completed in 7 days. Females can lay 200 eggs.
Miticides are available for some vegetable crops but should be used only where justified. As with aphids, mark infestations with flags, and check them again every 3 or 4 days. Mites can easily be moved to infested plants on clothing, so always examine infested areas last during inspections. If the infestation is not spreading, treatment will not be required. Because mite populations are often localized, spot spraying may be effective. If you spray only a portion of the field, treat a buffer zone of 100 to 200 feet beyond the mite infested area.
Resistance to pesticides has increased the difficulty of controlling these pests. Because mites primarily occur on the undersides of leaves, applications of contact miticides need to be directed at both lower and upper leaf surfaces. Mite eggs are resistant to some miticides, so repeated applications are often necessary to control infestations. Two applications spaced a week apart may be necessary with some miticides. See, 2016-2017 Vegetable Production Guide for Commercial Growers (ID-36), for a complete list of available miticides for vegetable crops and their restrictions.
Protect Natural Enemies
Natural enemies of mites are present in and around fields and usually can keep mite populations low. Many insecticides used for control of insect pests severely reduce numbers of beneficial insects that keep mite populations in check. Therefore, apply insecticides only as-needed, rather than at regularly scheduled intervals. When possible, select pesticides which will have the least impact on beneficial insects.
Management of weeds adjacent to and within fields should be done routinely and throughout the season. Grass should be mowed regularly. Spraying or mowing of weeds after growth has become rank may increase the movement of mites to cultivated plants.
By Ric Bessin, Extension Entomologist