Controlling Spider Mites in the Garden

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20170728A David Cappaert, MSU

Two-spotted spider mites and their web. Photo:  David Cappaert, Michigan State University

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.

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Two-spotted spider mites and one of their eggs. Credit: Gilles San Martin, Wikimedia Commons

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).

Damaged Caused

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Spider mite damage on a tomato leaf: it’s gone beyond stippling to bleaching. Credit: Scot Nelson, Flickr

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.

Nearly Invisible

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.

20170728H Jill O'Donnell, Michigan State University Extension

Hold a sheet of white paper under a branch and tap. If mites are present, you’ll see them moving. Credit: Jill O’Donnell, Michigan State University Extension

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

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Hot, dry weather makes spider mites very happy! Credit: Clipart Panda

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.

Simply Spray

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Just blast spider mites off plants with a strong spray of water. Photo: Pixabay

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.

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Horticultural oil

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.

Prevention

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.

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Avoid nitrogen-rich  fertilizers.

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.

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Predatory mites often show up all on their own: don’t kill them with pesticides! Photo: Anders Sandberg, Wikimedia Commons

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.20170728A David Cappaert, MSU

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Hose Down Your Houseplants

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20170503A Nick Harris, Flickr.jpg

Spray your plants with water to control spider mites. Photo: Nick Harris, Flickr

On a nice spring day, put houseplants sensitive to spider mites (Tetranychus urticae)—tiny little dustlike arthropodsoutdoors and hose them down. This includes such plants as bananas, brugmansias, impatiens, ivies, palms and scheffleras. It needn’t be a particularly warm day as long as it’s above freezing. No, none of those plants actually likes cold temperatures, but they won’t be outside long enough to suffer from the cold.

If the water comes from an outdoor hose, it’ll be cold too, but don’t worry about the water temperature either. The spraying will only last a minute at most, after all.

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Spider mites look like tiny moving dust particles and are often accompanied by webs. Photo: Rasbak, Wikimedia Commons

Spray leaves above and below, and hose down the stems too, knocking the little beasts off. Then bring the plants back indoors to warmer conditions. Job done!

How It Works

Fresh water raining down on spider mites knocks them off their host plant, reducing their population to innocuous levels. Hosing down plants simply replicates Mother Nature’s way of dealing with spider mites: she rains on them.

Yes, the population will start to build up again, but it won’t be long before you can put the same plants outdoors for the summer and let Mother Nature do her job by sending cleansing rain showers their way, keeping them spider mite free.

Into every life a little rain must fall… and thank goodness for that!20170503A Nick Harris, Flickr

Don’t Kill Those Red Mites!

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This predatory mite is your friend!

If you look closely at outdoor plants – and sometimes even indoor plants – you’ll occasionally see a tiny red arthropod moving fairly quickly on a leaf or stem. It’s red or orange, it has eight legs, it must be a red spider mite, right? Wrong!

Small as it may be, if you can see it fairly clearly (assuming you’re wearing your glasses) and it is all on its lonesome, it’s not a red spider mite (Tetranychus urticae), but rather a predatory mite. There are all sorts of species of these mites, in several genera (Amblyseius, Balaustrium, Phytoseiulus, etc.), many of them are red in color (but green, orange, and yellow are also possible), and they are all beneficial, since they feed on pest mites, notably, but also on small harmful insects, like fungus gnats and thrips. You don’t want to kill these mites, but instead to encourage them.

Red Spider Mites Are Rarely Red

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The red spider mite looks like moving dust and is rarely red!

The true pest mite you should worry about is the red spider mite, also and more appropriately called the two-spotted spider mite (Tetranychus urticae).

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Web created by spider mites.

It is much, much smaller than a predatory mite (without a magnifying glass, all you see are moving “dust particles”) and is, in fact, rarely red. It only turns red in the fall and even then, only outdoors. Most of the time, it is actually greenish with two darker spots. It never lives alone, but always in colonies. When numerous, it spins webs that link stems and leaves, webs it uses as a highway to go from plant part to plant part and as protection against rain (and pesticides). It’s this ability to build a spiderlike web that gives it the name “spider mite”, as it is not a spider at all.

Spider mites have a long list of hosts and will attack thousands of different plants, including beans, strawberries, eggplants, melons, roses, arborvitae, and spires in the outdoor garden and palms, hibiscus, English ivies, brugmansias, and scheffleras indoors.

The Good and the Bad

So let’s recapitulate: the good mites (the predatory ones) are red, about half the size of a pinhead, move fairly quickly, and live on their own. Their very presence usually indicates a healthy environment, so congrats! You don’t want to spray these or even bother them. They are your friends.

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A good shower will remove spider mites.

The bad mites are dust-sized, live in colonies, spin webs and are rarely red. They turn leaves yellowish and will kill plants if allowed to proliferate. These you don’t want on your plants. A strong spray of water will often blast their webs to smithereens and reduce their population to a harmless level. Or add a bit of insecticidal soap to the spray to kill them outright. Indoors, just sticking the plant in the shower and giving it a thorough rinse will do wonders.

Spider mites thrive under hot, dry conditions, so rinsing your plants’ foliage occasionally, keeping temperatures cool and increasing the air humidity will help discourage them indoors. Outdoors, you’ll notice the spider mites rarely cause problems on outdoor plants when summers are cool and rainy, but when they are hot and, especially, dry, spider mites really go to town. When summers are dry, hosing your plants down occasionally may be all you need to do to control spider mites.

Good bugs, bad bugs: to be a good gardener, you have to know the difference!

Longer Days Awaken Pests

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Whiteflies may be in diapause in mid-January, but by March they’ll be wide awake and hungry!

You may not have noticed, but if you live in the Northern Hemisphere, many houseplant pests have been less visible of late than usual. That’s because many of them enter into diapause, a kind of semi-dormancy, starting in the fall. This is due to the influence of shorter day lengths. In fact, they are often so inconspicuous in late fall and early winter that you may even have thought you’d gotten rid of them entirely.

Other insects continue to remain active even when the days are short, but develop at a much lower rate than in summer and likewise may go sight unseen for a while. But when the days start to get longer and the increasing sun heats up your home or greenhouse just a bit more, the two groups are re-energized and begin to reproduce abundantly.

Even as early as late January, although it still feels like the middle of winter outdoors, days are getting perceptibly longer and gradually, depending on the species, the enemies of your plants will start get back to work. By early March, they are all active… and hungry.

The Culprits

Insects that are quiescent during short days (or almost so), but awaken as days lengthen include:

Aphids, fungus gnats and thrips, however, don’t seem to slow down much in winter. They are just about as active in January as in July! You have to keep your an eye open for these insects throughout the year.

What to Do?

  • 20160128C.jpgInspect your plants at the end of January, looking especially at leaf axils and leaf undersides. A magnifying glass may be needed to see red spider mites, as they are very tiny. Subsequently, an inspection every two weeks is never a bad idea.
  • Set out yellow sticky traps: often flying pests like whiteflies, fungus gnats and winged aphids will be caught before the infestation even begins, nipping it in the bud.
  • Isolate infested plants so the pests can’t spread.
  • Treat infested plants.

Treatments

20160129BInsecticidal soaps and horticultural oils are almost universal treatments against these creatures. Just follow the application details explained on their packaging. You can also spray infested plants with solutions of dishwashing liquid or other soaps, but make sure you test a few leaves first: they can be toxic de certain plants.

Another possible treatment is to spray a solution of 1 cup (250 ml) of rubbing alcohol in 1 quart (1 liter) of water to control scale insects, mealybugs, aphids and whiteflies. Warning: for your own protection, ventilate the room when applying rubbing alcohol on anything more than a limited scale. Its fumes can be toxic.

Keep an eye open and your finger on the spray bottle’s trigger. That way you ought to be able to stop pests in their tracks… and allow your plants to take full advantage of the lengthening days!

 

Laidback Gardener Tip of the Day

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Learning to Live with Spider Mites

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Photo: Wikimedia Commons

The red spider mite (also called the two-spotted spider mite, Tetranychus urticae) is widespread in nature: in fact, it is found almost everywhere. Outdoors, it’s found in both in temperate and tropical climates and indoors, it’s common on houseplants. It is said to be polyphagous: that is, it attacks a wide range of plants, including most vegetables and herbs and many ornamental plants.

The red spider mite is a tiny little eight-legged pest (it’s a not an insect, but a mite, as insects have six legs). it is barely visible to the naked eye. On a plant, it will look like dust, but dust that moves. You need a magnifying glass to see it well. It can be red, as the name suggests, especially in cool weather, but will more likely be beige or pale green. It is marked with two dark spots, but they are only visible when it is a pale color. The red spider mite pierces the leaves and stems of plants and laps up the sap that leaks out. This causes, at first, a general yellowing, often a bit mottled. As the infestation advances, “spider” webs appear along and between leaves and stems. These will appear dusty due to the presence of moving spider mites.

Despite being such a common and prolific pest, red spider mites are surprisingly easy to control. In nature, a good heavy rain will almost completely wipe out a colony. That’s why is is mostly a problem when rain is absent. In your garden, simply spray the affected plants with a strong stream of water and the problem will be solved. Indoors, place infested houseplants in the shower and rinse both sides of the leaves. It also responds well to insecticidal soap and pyrethrins… but why bring out the big guns when water will do?

On the down side, as long as the conditions that contribute to its development remain in force (hot, dry air and no rain), you’ll need to repeat the treatment occasionally, because it is very difficult to eliminate spider mites completely. A few always seem to survive to start the infestation again. So, aim for the control of spider mites, but forget the idea of ​​completely eliminating them: you simply will not succeed.

This is one of those cases where the “15 pace rule”, dear to laidback gardeners everywhere, applies. If, at 15 paces, the plant looks green and healthy, there is no reason to react, even if you’ve see a few spider mites (probably under a magnifying glass, otherwise they’re pretty much invisible). But if you see the symptoms of its presence at that distance (yellow mottling is the first clue, plus spider webbing if the infestation is more advanced), it is time to intervene.