Populations of spider mites go up and down from year to year. In 2018, for example, an abnormally dry summer led to large numbers of the pests.
In both my home garden and in the many gardens I tended professionally, I saw countless infestations, mostly on plants that I didn’t even know the mites could eat. By the end of the summer, I’d identified spider mite damage on hemlocks, goat’s beards, zinnias, nasturtiums, creeping thyme, flame willow and Calamintha, as well as on two different kinds of beans. My bean plants took the brunt of the attack, and I watched their leaves slowly lose their color before turning brown and crispy and then falling off altogether.
The specific pest in question is the two-spotted spider mite, scientifically known as Tetranychus urticae. They aren’t actually spiders, though they are arachnids rather than insects. Under a lens, you can see their distinctive coloring; they have yellow bodies with two spots on the backs of their abdomens. Two-spotted spider mites are tiny: 1/50th of an inch in length! With that measurement in mind, the amount of destruction that they can do to a full-grown tree would be impressive if it wasn’t so infuriating!
Such a tiny pest can only cause life-threatening damage in great numbers. Each female lays up to 100 eggs over the course of a few weeks, and an entire generation of spider mites can be completed in less than a month, which means the hatching mites are soon laying their own eggs. In hot, dry weather, populations can quickly swell, and the damage they cause their host plants is often exacerbated by drought stress. Up to 200 species of plants are susceptible to spider mites, which are not specialized feeders. Victims range from spruce trees to rose bushes to lettuce plants.
Scouting the Mites
As with any pest, you can’t determine a successful management technique if you can’t identify the pest correctly. I think that the easiest way to identify spider mites is by looking for symptoms of their feeding. The two-spotted ones will use their piercing, sucking mouthparts to feed on the liquid within plant cells, specifically on a leaf’s underside. Their feeding injures the leaf and creates whitish discoloration in stippled, dotted patterns. The overall leaf might appear bleached or yellowed, with leaf curling and browning accompanying large mite populations.
If you suspect spider mite damage, inspect plants regularly and thoroughly every three to five days during times of heat and drought. Look for them on the bottom of the leaves. They are too small for you to see their signature coloration without magnification, but you might be able to see their protective webbing. I’ve noticed that spider mites look like a mixture of dust and pepper stuck to a leaf’s underside. They won’t fall off of the leaves unless you physically wipe them off.
When I attempt to control any pest, I start with an option that doesn’t involve chemicals, because I don’t want to kill any beneficial insects that might be already helping my cause.
Since drought stress contributes to spider mite damage, protect your plants with adequate watering practices, especially during the summer months. Once you have them, the most harmless control option is spraying them with water. According to the recommendations, you can use a strong blast of water to physically remove spider mites from your plants. I tried this several times, and I don’t think it’s effective for large mite populations, especially on plants with many small leaves or needles. There is simply too much surface area to cover, and it’s difficult to spray all of it.
Insecticidal soap or horticultural oil can be highly effective for controlling spider mites on ornamental plantss, and they won’t adversely affect non-targeted insects. These pesticides only work by directly contacting the mites, so you will need to spray your plants thoroughly, especially on the undersides of their leaves. Repeat applications are usually necessary.
For severe cases of two-spotted spider mites, use stronger pesticides. Neem oil can work well, as can products containing pyrethroids, such as bifenthrin and cyfluthrin. When using any chemicals, be sure to read the labels and use them properly, especially when spraying around bees or near water. To prevent spider mites from developing resistance to one specific chemical, you should switch between chemical classes after every third pesticide application.