Spider mites are baffling. Gardeners often have no idea they've arrived until the damage suddenly becomes obvious, and by then, it can be too late for action. Thus expert advice on spider-mite control seems to boil down to this: Whatever you do, do it yesterday.
Texas A&M entomologist Bart Drees, Ph.D., spent more than a decade studying the effects of spider mites—from mild damage, characterized by yellow-stippled leaves, to severe outbreaks that left plants with brown, dying foliage encased in fine webbing. Spider mites attack more than 180 plant species, including strawberries, daylilies, hollyhocks, and roses, says Drees, but they seem drawn to tomatoes and other solanaceous crops. They thrive in hot, dry weather. Their life cycle—5 to 20 days from egg to adult--is alarmingly efficient, and their tiny eight-legged bodies are hard to detect without a hand lens.
Predatory mites and minute pirate bugs prey on spider mites, but large infestations often require intervention. Drees recommends drowning mites where they live, on the undersides of leaves. "Use a high-pressure spray, start at the bottom of the plant, and work your way up," he says. Examine susceptible plants often and intervene quickly. At the first sign of mites, clip off and destroy infested plant material—few compost piles are hot enough to eradicate spider-mite eggs. Then start blasting. "On a hot day, you can spray the dog, too," Drees suggests. "Have some fun with it." Do this weekly, beginning early in the season, and the probability of controlling a mite infestation improves.
If water's not enough, spray the undersides of leaves with organic insecticidal soap. Repeat once a week, pruning diseased foliage before spraying. When cleaning up around affected plants in the fall, remove as many overwintering sites as possible. "Think sanitation, sanitation, sanitation," Drees says. "Yank weeds, move rocks and lumber, and get rid of leaves."
Image: Frank Peairs, Colorado State University, Bugwood.org