Spider mites (or red spider mites), Tetranychus urticae, are everywhere: on our outdoor plants in the summer and on our houseplants in the winter. Despite their common name, they aren’t really spiders, but rather type of mite. They are tiny pests you can barely see with the naked eye. In fact, they look like specks of dust, but specks of dust that move. You’ll need a magnifying glass to see them clearly. When you can see them up close, they may be red, as the name red spider mite suggests, but are more likely to be beige or green and often (but not always) marked two dark spots.
Spider mites damage plants by piercing their leaves and stems and sucking the sap that flows from the wound. This causes a slight yellowing of the plant that increases over time. At first, you might not be able see the pests themselves. Try placing a sheet of white paper under discolored leaves. Tap the leaves, then watch for tiny moving creatures on the paper. Those are spider mites.
Most people don’t notice spider mites at that stage, though. They realize they have a problem when they start to see webbing much like the kind a spider produces forming between the leaves and along the stems. Think of these webs as highways allowing spider mites to travel rapidly from one part of the plant to another. They also serve as protection against their enemies, including humans. And when you look at the webbing, there are plenty of tiny moving critters. Oops! This means your plant is not just lightly infested, but seriously so and in fact, in danger of dying.
Spider mites are present on houseplants throughout the year, but are rarely noticed as long as they are few in number. But the population explodes under certain conditions, especially when the air is hot and dry. That’s why spider mites appear out of nowhere on our houseplants in late autumn and winter: when you start heating a house, its atmospheric humidity drops precipitously. In many homes, the air in winter is drier that that of the Sahara Desert! And spider mites just love dry air!
What houseplants are attacked? Probably most can be, but in fact a lot of plants will support a small population of mites without any noticeable damage. It is mostly plants with thin leaves that suffer visibly from spider mites: bananas, brugmansias, calatheas, crotons, hibiscus, impatiens, English ivies, palms, scheffleras, etc.
To control spider mites, first give them a thorough washing, either in the shower or with a damp soapy cloth. You have to remove the webs first or your other treatments won’t be effective: spider mites will be out of range of pesticides behind their webs. When the webs are gone, then you can spray with insecticidal soap, neem, pyrethrin, etc. if you want, but… my experience is that a thorough shower or washing, regularly repeated, is usually all you need to get rid of spider mites… temporarily. If the air remains hot and dry, though, they will be back!
So, to really solve the problem, you need to change the conditions in the room where you grow your houseplants. First, if you can, lower the temperature in the room, especially at night (the cooler the air, the less quickly spider mites reproduce). Even more importantly, do whatever you can to increase the atmospheric humidity. Moist air alone will not cure an infestation (once established, spiders continue their attack in spite of the improved conditions), but it will prevent their return. Therefore, a room humidifier is often the best prevention possible when it comes to spider mites!
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