Basil (Ocimum basilicum) leaves fresh from the garden are a classic summertime treat, and the annual herb obligingly moves indoors to a sunny windowsill to keep the harvest coming in winter. But sometimes colonies of tiny spider mites move indoors along with the basil plant. Spider mites flourish in hot, dry conditions, and either sun- or furnace-generated heat will do. Once they strike, only a quick response prevents mite numbers from exploding.
Detecting an Infestation
Weather that raises dust whenever you set foot in the garden is exactly what spider mites love. Spotting the leaf-sucking pests is nearly impossible without a 10X hand lens, but a basil plant mottled with light speckling may have an early infestation. To test it, place a piece of white paper or cloth beneath the discolored leaves. Shake the plant lightly and look for scurrying, dark dots against the white background. The mites also spin fine webs on the backs of leaves, where they're protected from predators.
Basil plants already under stress from lack of water are most likely to suffer serious damage from spider mites. As the pests drain their nutritious fluids, damage goes from speckling to yellowing or bronzing. In severe cases, the plans weaken and drop the leaves. Healthy plants seldom suffer severe damage, and light infestations are rarely lethal. Rain or the return of cool weather end a garden spider mite infestation.
The Water Treatment
Attaching a spray nozzle to the garden hose and spraying the basil with cold water dislodges and drowns the spider mites while washing the dust from the plants and the surrounding area. It also waters the basil so it's better able to tolerate an infestation. This method also works indoors. After wrapping the basil's container to keep the medium from washing out, set it in a sink or shower and wash it down with a sprayer attachment. Always spray the plants thoroughly, paying particular attention to the backs of the leaves where the mites feed.
Organic Insecticidal Soap
A stubborn mite infestation may require spraying with ready-to-use insecticidal soap. Coat the basil until the soap drips from all its surfaces. Spray an outdoor plant in the early morning or after dark, so the wet soap won't harm foraging honeybees. Set indoor basil in the sink or cover nearby surfaces. For continuing control while conditions favor mite attacks, repeat the treatment every one to two weeks or as often as the label suggests. Wear long pants, a long-sleeved shirt and protective eyewear and follow the manufacturer's application and safety recommendations when using the soap. The sprayed leaves are safe to use for cooking after rinsing.
Preventing Future Infestations
To discourage spider mites on indoor basil, pour gravel into a saucer. Barely cover the gravel with water and set the plant on top of it. As the water evaporates, it boosts the humidity around the plant. Regularly monitor all nearby plants. Isolate and treat mite-infested ones. If they're heavily infested, dispose of them in sealed plastic bags.