Though very tiny, spider mites can do a load of damage and they are so widely distributed in the environment that they are found practically everywhere. Fortunately they’re surprisingly easy to control when you recognize the symptoms and start your treatments early.
Let’s look at them in detail so you’ll better know how to handle them.
Mites, Not Spiders
First, spider mites are not spiders, nor are they insects, but mites, a type of arthropod more closely related to ticks. Though the adults do have 8 legs like a spider, that’s not where their common name comes from. They’re called spider mites because, when they’re very numerous, most species produce whitish webs.
There are actually several mites that are commonly called spider mites. The two-spotted spider mite (Tetranychus urticae), also called red spider mite (although it is rarely red), is the most ubiquitous and commonest, found all over the world. As well as being abundant outdoors, this is also the species that affects houseplants.
Other common species, all with similar life cycles, include spruce spider mites (Oligonychus ununguis), which prefers spruces, pines and other needled conifers, European red spider mites (Panonychus ulmi), more common on fruit trees and grape vines (and which really are red) and birch spider mites (Eotetranychus uncatus), a problem on Betula species, but there are many others.
In our gardens, the two-spotted mite is the most common culprit, affecting both vegetables (tomatoes, squash, etc.) and ornamentals (roses, arborvitae, etc.), although European red spider mites are also quite common (and not limited to Europe, by the way).
Spider mites harm plants by piercing tiny holes in the leaves and stem and sucking up the sap that flows out. In small quantities, they cause little visible damage, but as their numbers increase, the leaves start to take on a stippled appearance (green with yellow spots), then look bleached as the infestation progresses. Eventually, for some plants, damaged leaves curl under and/or turn bronze.
As numbers increase, webs start to appear that may come to envelop the foliage. By this point, leaves begin to die and fall off and the whole plant goes into decline. In very severe cases, spider mites will kill their host plant.
These tiny creatures are almost invisible to the naked eye. They are often only noticed when webbing begins to appear. The webs serve to protect them from rain and predators and act as a highway, allowing them to move from one point to the other on the infested plant. Only at this point do you really see them, as the web will seem dotted with tiny moving dust particles. These are, of course, mites. Even then, you’ll need a magnifying lens to get a good look at them.
If you’re observant, you can detect them before that, though. Hold a sheet of white paper under a branch or leaf with suspicious yellow stippling and tap. If the dust that falls on it begins to move, you’ll know the problem is spider mites.
Spider mites have no wings and can’t fly, but they are so light that the wind can transport them from one plant to another. They can also hitch rides on clothes, tools or pets (that’s often how they get indoors onto houseplants that have never been outdoors).
Linked to Heat and Drought
Spider mites are present throughout the growing season, but rarely cause problems under normal gardening conditions, as they are easily knocked off plants by rain, their worst enemy. In rainy weather, their populations are so reduced they cause few noticeable problems.
It’s during periods of drought and heat that spider mites really proliferate, multiplying at high speed. The two-spotted spider mite, for example, takes 36 days to mature, from egg to adult, in cool weather, but only 7 days in hot, dry weather. Since each female can lay up to 100 eggs, the population can pass from a few dozen individuals causing no notable damage to millions, a number capable of killing a whole plant—even a small tree!—in just a few weeks.
Spruce spider mites are the exception to the rule that spider mites like things hot and dry: they’re more numerous when it’s cool, so are most often a problem in early spring and late fall.
The easiest way to treat spider mites is simply spray them with a strong blast of water. Use the water’s force to destroy their webs and make sure you spray both the top and bottom of leaves, plus the stems. Repeat weekly if no rain comes to help you out.
You can also spray with insecticidal soap, widely available in garden centers and hardware stores. Note that despite a persistent popular belief that a homemade solution of dishwashing liquid is just as good as insecticidal soap, in fact, most modern dishwashing liquids contain no soap whatsoever but rather detergents and detergents aren’t very effective against mites and insects. Often, in fact, they’re no more effect on spider mites than a simple spray of water.
Horticultural oil sprays will also work on spider mites, but read the label carefully. They can’t always be applied in hot weather.
It may also be useful to remove heavily damaged stems or leaves. Not only will this reduce the spider mite population and allow your treatment to reach deeper into the plant, but such plant parts are so severely damaged they likely won’t recover anyway.
You can also purchase and use insecticides. Check and make sure they are also useful as miticides (remember, spider mites are mites, not insects). Indeed, most are effective against both mites and insects..
Neem (no longer sold in Canada) is a good organic insecticide/miticide, and there are plenty of chemical ones. It’s best to start by spraying plants with water and use chemical miticides as a last resort, as they are often harmful to the environment … and to the sprayer.
It’s essentially impossible to completely prevent spider mites: they are simply everywhere in the wild. You may not see them, but they’re around. But you can help keep the population low enough that it causes no damage.
In spring, spray plants that had problems in the past, like arborvitae hedges, with dormant oil or horticultural oil. This will help eliminate any females (T. urticae) and eggs (other species) that may overwinter in bark cracks and bud scales.
Avoid using nitrogen-rich fertilizers, such as cedar and hedge fertilizer, that is, ones with where the first number is highest (example 30-10-10), on plants prone to spider mites, as they boost the production of tender but weak young shoots that are very prone to attack.
Mulch the soil to keep it more evenly moist. Well-watered plants better resist spider mite damage.
Regularly spray the foliage of susceptible plants with water during hot, dry weather. Not only will this knock many spider mites off the plant, but it also increases the humidity level within the plant’s foliage and humid air impedes the development of spider mites.
You can release predatory mites (available from several mail order sources) to control harmful ones. In fact, they often show up all on their own. If so, though, don’t use any miticides, not even soapy water, as this can kill the predatory mites. You can spray with water, though.
The same goes for ladybugs and hoverfly larvae. They are also valuable allies in the battle against spider mites. Again, spraying with water won’t hurt them, but other pesticides are a no-no.
For information on controlling spider mites indoors, read When Spider Mites Invade Houseplants.