Brian Jarvis Master Gardener
Spider mites can cause problems on a variety of plants, both indoors and outdoors. But sometimes, spider mites seem to have a particular penchant for tomatoes. Usually, you first notice that you might have a spider mite problem when very small, light-colored spots that look like they've been sandblasted start to appear on the leaves of your plants. This type of damage is usually caused by spider mites.
Spider mites have what are called piercing-sucking mouthparts. Aphids, which we are more familiar with, also have the same type of mouthparts. These mouthparts are used to pierce plant leaves and suck out the juices from them. Aphids are insects, but spider mites are spiders, so they belong to the arachnid family.
The big difference between the two is that aphids are easily visible on the plant, while spider mites are hard to see because they are so tiny. To check if your plant has spider mites, place a piece of white paper under the branch and shake the branch a bit; the mites will fall off and you will start to see small black spots on the paper. Another sign besides the damage spider mites cause are spider webs. If you see spider webs in association with spotted leaves on your plant, you may have a significant spider mite infestation.
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Damage to leaves reduces their ability to provide nutrients to the plant through photosynthesis. These leaves will eventually turn brown and fall to the ground. If enough leaves are in this state, the plant will die.
Spider mites thrive in hot, dry environments, so dry, stressed plants are an easy target for them. Since you can't control the weather, your first defense against spider mites is to make sure your plants are well watered.
Excessive use of pesticides also promotes the proliferation of spider mites, which in turn reduces overall insect populations, including those that help protect plants from spider mites.
One way to deal with spider mites on plants is to spray them liberally with a strong stream of water. Not all plants can tolerate this method, so there are some organic insecticides that work well. One of them is insecticidal soap.
Insecticidal soap is exactly what it sounds like: soap (potassium salts of fatty acids) mixed with water. There are currently many recipes on the Internet for making your own insecticidal soap using dish soap, but soap and dish soap are different things. Does dish soap work? Yes. Can dish soap harm plants? Yes. If you want to make your own insecticidal soap instead of buying ready-made insecticidal soap, at least use pure soap, which is often sold in health food stores, rather than dish soap. However, real insecticidal soap is not that expensive.
Neem oil also works well against spider mites, but spraying your plants with oil in the middle of summer is not a good idea. If you use neem oil, test it on a small section of your plant first to see how it interacts with the Oklahoma summer.
Whether you use water, insecticidal soap, or neem oil, you'll need to water your plants thoroughly, making sure to spray under the leaves where spider mites like to congregate. Good luck!
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To get answers to all your gardening questions, call the Tulsa Master Gardeners Helpline at 918-746-3701, stop by the Diagnostic Center at 4116 E. 15th St., or email mg@tulsamastergardeners.org.
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