Controlling Spider Mites on Your Houseplants

Spider mites are so small they can be mistaken for dust particles. Photo: gardeningheavn.com

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 see spider mites up close, under a magnifying glass, for example, they may be red, as the name red spider mite suggests, but are more likely to be beige or green and are often (but not always) marked two dark spots.

Spider mites cause damage to plants by piercing leaves and stems and sucking the sap that flows from the wound. This causes a general yellowing of the plant.

At first, you might not be able to see the pests themselves: yes, they’re that small! 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 a filmy web, much like that of a spider, forming between the leaves and along the stems. Think of these webs as roadways 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.

Largely A Winter Problem

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 than that of the Sahara Desert! And spider mites just love dry air!

Which houseplants are attacked? Probably most can be, but in fact a lot of plants easily support a small population of mites without any noticeable damage. It’s mostly plants with thin leaves that suffer visibly from spider mites: bananas, brugmansias, calatheas, crotons, hibiscus, impatiens, English ivies, palms, scheffleras, etc.

Making Life Uncomfortable for Mites

To control spider mites, first wash them thoroughly, 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 are out of range of pesticides when they hide 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 all you need to get rid of spider mites … temporarily. If the air remains hot and dry, though, they will be back! 

Keep the Air Cool and Moist

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 they reproduce). 

Even more importantly, do whatever you can to increase the atmospheric humidity. Moist air alone will not cure an infestation (once established, spider mites will 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!

Derived from an article originally published December 7, 2014.