The two-spotted spider mite (TSSM) is a common and destructive pest with an extremely wide host range that includes many trees, shrubs, flowers, weeds, fruits, and vegetable crops. Problems increase during hot, dry weather but early signs and symptoms are easy to overlook. Not only are these mites tiny but they live out of sight on the underside of leaves (Figure 1).
TSSM use needle-like mouthparts to remove the contents of individual cells as they feed. There are no holes in the leaves; just tiny pinprick spots caused by emptied cells (Figure 2). This causes infested leaves to gradually lose color and appear to be suffering from drought stress.
An infestation typically starts at the bottom of the plant. TSSM-damaged leaves lose water and photosynthetic capability. As mite numbers increase, infested areas of leaves change from gray-green, to yellow, to coppery brown and drop. As leaf quality drops, the mite infestation moves upward.
Weather and natural enemies usually keep TSSM populations at acceptable levels. Very dry conditions upset the balance and mite populations can explode. Drought stress triggers:
- TSSM to leave grass or other hosts where they had been living.
- Improved food quality of other hosts due to higher nutritional content of sap.
- Reduced effectiveness of fungal diseases that attack mites. These diseases are favored by cool temperatures and high humidity.
- TSSM development is more rapid at higher temperatures.
Homeowner options for TSSM control are limited to insecticidal soaps and horticultural oils. These require direct contact so thorough plant coverage is required, along with repeated applications.
Spider Mites on Vegetables
Reports of TSSM activity on vegetable crops has increased over the past couple of weeks, particularly on tomatoes and beans. As populations get larger, webbing may become apparent between leaves and other plant parts (Figure 3). The mites can be seen with a hand lens so producers should confirm active populations if they see foliar damage.
Monitoring and Management
Growers should be monitoring for spider mites and mite damage at least on a weekly basis, particularly during hot, dry weather. Greenhouses and high tunnels are more likely to have spider mites than the field since the hot, dry conditions within tunnels promote more rapid development of mite populations. While mites can attack a wide variety of vegetable plants, tomatoes, beans, watermelons, and eggplant are among those most commonly affected by spider mites.
Commercial growers have a number of miticides available to control mites; these are listed in Vegetable Production Guide for Commercial Growers (ID-36). Some of the materials are only used as miticides, while Brigade and Danitol are pyrethroids that are insecticides and they are miticides at high rates.
By Ric Bessin and Lee Townsend, Extension Entomologists