Pest Alert: Spider Mites

Hot, dry summers stress plants out, and some pests take that as an opportunity to strike. One common pest in the height of the summer is the spider mite, a tiny, spider-like mite that sucks water and nutrients from the undersides of leaves.

Yellow stippling is a symptom of spider mite damage.  By Rasbak - Own work, CC BY-SA 3.0

Yellow stippling is a symptom of spider mite damage.
By Rasbak - Own work, CC BY-SA 3.0

How do I tell if I have spider mites?

There are a handful of different species of spider mites, some of which specialize on just a few plants while others attack a broad range, but the symptoms and treatments are all similar, so we’ll group them together. The first thing you’ll notice in a spider mite infestation is that the leaves of your plants will be covered in tiny yellow dots. If the infestation is bad enough, you may eventually have more yellow or bronze than green on the leaves. When spruces and other needle-bearing plants have a mite infestation, they look kind of dusty, like there’s a film of brown over the needles.

If you flip a leaf over, you’ll see the mites themselves as little orange specks. To see if they’re still alive and active, hold a white sheet of paper under the plant and shake it vigorously to knock some mites off. They will crawl around on the paper after being knocked off if they are still alive and eating your plants. In especially bad infestations, you’ll also see some webbing on the leaves.

What do they do to my plants?

Spider mites suck the moisture from your plants leaving an unattractive appearance and stressing the plants out. Since they tend to be a worse problem when plants are already experiencing drought stress, serious infestations can cause leaf loss, stunting, or even death in smaller plants, and they can leave larger plants more vulnerable to additional problems. In a healthy plant, a mild to moderate infestation is no serious problem, but if it spirals out of control in a stressed landscape you’ll need to start to worry.

What can I do to get rid of them?

In serious infestations, leaves become stunted and the mites start to create webs.  By Rasbak - Own work, CC BY-SA 3.0

In serious infestations, leaves become stunted and the mites start to create webs.
By Rasbak - Own work, CC BY-SA 3.0

The best treatment is prevention. Spider mites have a whole host of natural enemies, so the best thing you can do to control them is to avoid using pesticides on your plants unless your are actively controlling another harmful pest problem. Excessive or inappropriate pesticide use actually creates more problems than it solves. Mites aren’t killed by many common insecticides, so they will survive while their predators are killed off if you use chemicals without having an actual need for them.

Since drought increases the threat of spider mites, keeping your landscape watered during serious dry spells can help protect them, and it can help reduce problems if mites have already invaded. For tips on keeping your landscape happily watered, check out last week’s blog post.

But if you already have an infestation, there are some steps you can take. Mites don’t move very fast or very far, so spraying plants with a strong setting on your hose can knock them off as long as you’re targeting the undersides of the leaves. Start with lower pressure on more delicate leaves and check to see if the pests are knocked off so that you don’t damage the leaves with the water. Since there may be eggs left behind or some mites may hold on tighter, you might need to spray the plants down weekly or twice a week while hot, dry weather persists.

In widespread, intense cases, chemicals may be necessary. Insecticidal soaps and horticultural oils work on spider mites during the growing season. Dormant oils can be used in the winter to smother the adults that overwinter on the branches. Miticides designed specifically to kill mites do exist, but they must be applied by a state licensed professional. If you think chemical action may be warranted, it’s not a bad idea to contact a professional or submit a sample to the Purdue Plant and Pest Diagnostic Lab first to get customized instructions for your situation. We are happy to help! If you want more information or to see more pictures, Purdue and Ohio State have excellent resources that go into more detail.

Recommended Posts