If you look closely at outdoor plants – and sometimes even indoor plants – you’ll occasionally see a tiny red arthropod moving fairly quickly on a leaf or stem. It’s red or orange, it has eight legs, it must be a red spider mite, right? Wrong!
Small as it may be, if you can see it fairly clearly (assuming you’re wearing your glasses) and it is all on its lonesome, it’s not a red spider mite (Tetranychus urticae), but rather a predatory mite. There are all sorts of species of these mites, in several genera (Amblyseius, Balaustrium, Phytoseiulus, etc.), many of them are red in color (but green, orange, and yellow are also possible), and they are all beneficial, since they feed on pest mites, notably, but also on small harmful insects, like fungus gnats and thrips. You don’t want to kill these mites, but instead to encourage them.
Red Spider Mites Are Rarely Red
The true pest mite you should worry about is the red spider mite, also and more appropriately called the two-spotted spider mite (Tetranychus urticae).
It is much, much smaller than a predatory mite (without a magnifying glass, all you see are moving “dust particles”) and is, in fact, rarely red. It only turns red in the fall and even then, only outdoors. Most of the time, it is actually greenish with two darker spots. It never lives alone, but always in colonies. When numerous, it spins webs that link stems and leaves, webs it uses as a highway to go from plant part to plant part and as protection against rain (and pesticides). It’s this ability to build a spiderlike web that gives it the name “spider mite”, as it is not a spider at all.
Spider mites have a long list of hosts and will attack thousands of different plants, including beans, strawberries, eggplants, melons, roses, arborvitae, and spires in the outdoor garden and palms, hibiscus, English ivies, brugmansias, and scheffleras indoors.
The Good and the Bad
So let’s recapitulate: the good mites (the predatory ones) are red, about half the size of a pinhead, move fairly quickly, and live on their own. Their very presence usually indicates a healthy environment, so congrats! You don’t want to spray these or even bother them. They are your friends.
The bad mites are dust-sized, live in colonies, spin webs and are rarely red. They turn leaves yellowish and will kill plants if allowed to proliferate. These you don’t want on your plants. A strong spray of water will often blast their webs to smithereens and reduce their population to a harmless level. Or add a bit of insecticidal soap to the spray to kill them outright. Indoors, just sticking the plant in the shower and giving it a thorough rinse will do wonders.
Spider mites thrive under hot, dry conditions, so rinsing your plants’ foliage occasionally, keeping temperatures cool and increasing the air humidity will help discourage them indoors. Outdoors, you’ll notice the spider mites rarely cause problems on outdoor plants when summers are cool and rainy, but when they are hot and, especially, dry, spider mites really go to town. When summers are dry, hosing your plants down occasionally may be all you need to do to control spider mites.
Good bugs, bad bugs: to be a good gardener, you have to know the difference!