Pests and Diseases: Spider Mites


Since I touched on spider mites in my organic vs. conventional gardening article, I thought it would be wise to expand on them a bit more. Many people believe that spider mites are insects but they aren’t. At the fear of offending an entomologist, I’ll just say that insects have 6 legs whereas arachnids have 8 legs. Guess how many legs a spider mite has…if you guessed 8 you would be correct.

When viewed up close, as in the picture to the left, the spider mite is a ferocious beast to behold. I have observed them under a microscope and you can just see them ripping and tearing into the plant tissue to obtain their dinner. They use their needlelike mouthparts to literally suck the life out of your plants. They are very small and can build to damaging populations very quickly; spider mites can develop from an egg to an adult in a week’s time. They are only about as big as the period at the end of this sentence so they often go unnoticed until their damage is evident.

Their damage is usually seen as stippling on the upperside of the leaves. With a heavy or prolonged infestation, the leaves will turn brown and drop off. The damage may be apparent on the tops of the leaves but the villains reside on the undersides of the leaves where they can be protected from weather extremes and predators…and pesticides. While I don’t advocate the use of conventional pesticides, anything that kills (-cide) pests is a pesticide in my eyes. That includes some of the control methods that we’ll discuss later.


Spider mites are usually at their worst when it is hot and dry. When I say hot and dry, I’m not only talking about when the temperatures are high and the humidity is low. You can have an average summer where it rains periodically but if you site a plant that is susceptible in an area that is hot and dry, such as by an asphalt drive or against a brick wall, that microclimate is hot and dry even though the surrounding area isn’t.

Plant stress also triggers spider mite development. Plants produce all kinds of hormones in reaction to stress and plant pests are wise enough to be able to detect them. If a plant is located in an area that is not conducive to growth, such as large shade trees planted in small parking lot islands, spider mites and all types of plant pests will move right in. Think of it as the plant posting on its Facebook wall that dinner is served.


The best investment that you can make in terms of combating pest and disease issues is a good quality hand lens. The 10X magnification is sufficient to cover 90% of the pests you will encounter. Equipped with a hand lens, you can easily see the spider mites on the undersides of the leaves.

Another trick you can use is to hold a white piece of paper under the plant and then tap on the leaves…the spider mites should show up on the paper as little crawling specks.

The most obvious ID tactic is to look for webbing…now it won’t look a Halloween display but it should be readily visible when the critters are present.


  • If your infestation is particularly bad, you may have to count your losses and remove the plant. Dwarf Alberta Spruce are notorious for spider mites and if your plant looks like this, you may be better off pitching it. There is a saying in the nursery industry: “if in doubt, throw it out”.
  • You can keep the plants showered with water and that will usually be enough to send the mites running. Now I’m not talking about a continuous shower…I mean spraying them 1-3 times per day for about a week with a garden hose. This could open you up to other issues such as fungal diseases so be sure to allow enough time for the plant to dry before the sun sets for the evening.
  • Mix up a potion of soap and water and spray the little monsters. You have to make sure that you are spraying them on the undersides of the leaves, not just the top of the leaves. That’s what makes controlling them and many other pests difficult…they know exactly where to hide.


The long and short of spider mites is that they are a pest whose population can quickly explode into damaging populations. But if you consider where you site your plants before installing them, it can dramatically lower your chances of having an infestation of epic proportions. And if you do see them wreaking havoc on your plants, give your plants a cool shower a couple times of day…chances are the spider mites will decide that your landscape is inhospitable to uninvited guests and move on. If you’ve had any issues with the eight-legged critters we discussed today, post your experience in the comments below. And remember that I am here to help you, so don’t hesistate to e-mail me with any questions or concerns you may have at Happy gardening!

December 13, 2011Permalink 1 Comment

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  1. Pingback: Pests and Diseases: AphidsMid-Atlantic Gardening

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