Spider mite damage, characterized by bronze-like stippled patterns on leaf surfaces and wispy webbing found at the top of affected plants, has become a mighty big problem this summer. Warm, dry weather in June helped to spur a surge in reproduction leading to serious garden damage.
Zack Snipes, the Clemson Extension commercial horticulture agent who works closely with area farmers, reports that this year was one of the worst ever for spider mites in commercial tomato fields. "I saw entire fields lost this year," he says.
Home gardeners have reported damage on a variety of ornamental and edible plants, including thyme, verbena, Black-eyed Susan vine, tomatoes and hydrangeas.
Spider mites are not insects but are members of the arachnid family related to ticks and spiders. They feed on sugary plant juices, extracting them through their sucking mouthparts from the undersides of leaves. Leaves that appear yellow, bronze, stippled and curled are signs of spider mite damage. Heavily damaged plants may die from the inability to conduct photosynthesis.
The two-spotted spider mite, (Tetranychus urticae), is the most one seen in summer. This extremely small yellow to orange mite has two black spots on each side of its body, hence the common name. Its eggs are amber-colored spheres found dotted on the undersides of leaves. As the mite matures and ingests chlorophyll, its color appears greener.
Use a hand lens to magnify the mites for a closer look or try a simple field test. Shake a leaf over a piece of white paper, if tiny specks of "dust" appear on the paper and begin moving around, you have spider mites.
Female spider mites start their reproductive cycle early in the spring after overwintering in soil and leaf litter. They begin depositing between 70-100 eggs, which hatch within 3-19 days, on the lower leaves of garden plants.
The newly hatched mites feed for up to three weeks and then begin laying new eggs as they move upward. As temperatures rise into the upper 90s, the eggs hatch and the young mature even quicker.
When humidity is low, these hungry mites grow and mature faster because their feeding becomes more efficient. The low humidity and high temperature allows waste products exuded from their bodies to evaporate and more plant juices can be sucked from the leaf tissues. This accelerates their maturation rate so they are able to lay eggs sooner, providing up to 10 new generations of spider mites a year that can feed on plants.
Controlling spider mites is not easy for home gardeners or farmers. Recent rainfalls, lower temperatures and higher humidity, however, may help keep spider mite populations in check.
Since spider mites are not insects, gardeners are strongly cautioned against using broad-spectrum pesticides such as carbaryl or imidacloprid. These pesticides are not labeled for mites and will leave them unscathed while killing all the other insects, including the beneficial insects that help control the spider mite populations. Remember to read all labels prior to using any pesticide.
Beneficial insects that consume spider mites include ladybugs, big-eyed bugs, damsel bugs, green and brown lacewings, minute pirate bugs and syrphid flies. Predatory mites that consume damaging spider mites also are available through online outlets specializing in biological pest controls. Timing is the key to releasing these beneficial mites, which are effective in controlling the pests.
Other control strategies include using a strong spray of water to knock the mites off the plants, and reducing the population manually by removing and disposing of heavily affected plant parts.
Chemical controls available to home gardeners are not very effective but insecticidal soap and highly refined horticultural oils can help. Pesticides containing the active ingredient acephate or bifenthrin applied according to label directions can help, but may also lead to more problems.
If you choose to use synthetic chemicals as your last defense, it is important to note that the first application will kill the spider mites as well as the beneficial insects that feed on them. Once you commit to a chemical control option, a second application, typically five days after the first one, is required. If this second application is not completed, spider mite populations can spiral out of control because the "good bugs" won't be around to do their jobs.
As Snipes tells the farmers, "Spraying just once can increase the spider mite problem. The best method for controlling spider mites is to provide a habitat for beneficial insects and allow them to do their job."
Amy L. Dabbs is a Clemson Extension Urban Horticulture Extension Agent. Send questions to firstname.lastname@example.org.