Covering the Foothills & Mountains West of Denver

spider mite damage to underside of leaf

Spider Mites Suck Plants Dry

Spider mites aren’t insects. Surprised? They’re actually arachnids, similar to spiders but much, much smaller. In fact, they’re so small (up to 1/20”) that they’re often difficult to see without a magnifying glass. They come in a range of colors, from red to brown, yellow and green, depending on the species and time of year.

Spider mites are a common pest that attacks many plants and trees here in Colorado.

Types of Spider Mites in Colorado

spruce spider mite - image by USDA Forest Service - Region 4 - Intermountain , USDA Forest Service, Bugwood.org

Spruce spider mite – Image by USDA Forest Service – Region 4 – Intermountain , USDA Forest Service, Bugwood.org

In the garden, look for the twospotted spider mite (Tetranychus urticae) on vegetables (e.g., beans, eggplant), fruits (e.g., raspberries, currants, pear) and flowers.

Evergreen trees are more commonly infested by other mites, such as the spruce spider mite (Oligonychus ununguis) on spruce and juniper, Oligonychus subnudus on pines, and Platytetranychus libocedri on arborvitae and juniper. Deciduous trees are attacked by yet other spider mites, including the honeylocust spider mite (Platytetranychus multidigituli) on honeylocusts, and a range of other mites on shade trees, such as elm, mountain ash, and oak. We also see a lot of spider mites on aspen trees, especially those in drier locations.

Identifying Spider Mite Damage

The first signs of a spider mite infestation are usually small, light-colored dots or flecks on the leaves. This is due to damage caused by spider mite feeding; they suck the fluids out of the leaves, damaging it in the process.

You may also notice webbing in the plant. Keep in mind that spider mites are tiny so the webbing is also small – much smaller than a regular spider web.

spider mite damage under leaf

Typical signs of spider mite damage on the underside of a leaf

If a plant is severely infested with spider mites, leaves will start to look grey or light bronze and will eventually scorch around the edges and fall off. A bad enough infestation can kill or seriously damage a plant.

To confirm a suspected spider mite infestation, shake an affected leaf over a sheet of white paper. If you see tiny colored dots moving around on the paper, you have spider mites. And if you have a magnifying glass (or zoom on your camera), you’ll see that the adults look spider-like, with 8 legs (juveniles only have 6 legs).

Spider Mite Control Options

arborvitae spider mite - image Rayanne Lehman, Pennsylvania Department of Agriculture, Bugwood.org

Arborvitae spider mite – Image Rayanne Lehman, Pennsylvania Department of Agriculture, Bugwood.org

Homeowners often inadvertently make spider mite infestations worse by trying to control them with “bug control” sprays. Unfortunately, these insecticides have little effect on spider mites but kill off their natural predators, such as ladybugs, big-eyed bugs, minute pirate bugs, and predatory thrips or mites. Plus, spider mites can quickly become resistant to typical insecticides, rendering them ineffective.

The end result is an explosion in the number of spider mites, with ensuing damage to their preferred plant targets.

Using Water to Control Spider Mites

Spider mites thrive in hot, dry conditions – the same conditions that stress plants and make them more vulnerable to spider mite damage. One way to address that is to water plants regularly and spray the leaves with a strong jet of water (this knocks spider mites off and ruins their webbing).

Using Chemicals to Control Spider Mites

When hosing of your plants isn’t practical or effective, the next option is to use a miticide – a pesticide that’s specifically developed for spider mite control. These pesticides usually don’t affect the eggs so they’ll need to be reapplied every 10 to 14 days until the population is under control.

We use a special treatment that’s injected directly into the soil around infested trees and lasts 30 to 45 days (so it doesn’t need to be reapplied every 10-14 days). It also allows us to inject a lot of water into the root zone at the same time, helping keep trees hydrated so they’re better able to fend off spider mite attacks. Between the two, trees improve quickly.