Spider mites are common pests in most landscapes, and tend to feed on just about any plant imaginable. They aren’t insects, but are actually closely related to ticks and spiders; however, they feed on plant chlorophyll (the green stuff in leaves). Spider mites are very small, usually only about the size of a period at the end of a sentence. They can multiply very quickly and can overwhelm a plant in just a few days!
An adult female spider mite can live several weeks and lay dozens of eggs with each egg hatching and reaching adulthood in just a week! This can allow for overlapping generations in one season which means a spider mite problem can get real bad real fast. Spider mites have needle like mouth parts that suck chlorophyll from leaves leaving the plant looking dry and speckled or bronzy looking. Most people look at a plant infested with mites and think its just dry so they water it. Two days later the plant is dropping leaves. By then the mites have spun webs over the leaves and are preparing to parachute in the wind to a new plant. Soon an entire garden can become infested with mites. By the time most people realize their plants are in trouble, it’s usually too late.
Spider mites can cause plants to drop leaves prematurely similar to what they would do during a drought. Severely infested plants become stunted and can die. Once spider mites have become a problem in a landscape they will periodically cause you problems for many years.
There are several spider mite species that cause landscape plants problems however the two spotted spider mite is by far the most destructive in Kentucky. If you suspect that a plant has spider mites, hold a white piece of paper under some of the leaves and shake them. If you have spider mites you will see small specks about the size of a period on the paper many of which will be moving. If you have very good eyes, you can even see the two spots on their backs.
To control spider mites there are a few things that you can do to help. First, if you spray a fine spray of water under the leaves three times for three days you will knock many of the mites off. This will disrupt their mating and egg lying thus reducing their numbers. This won’t get rid of the mites but will disrupt them. Insecticidal soap or horticultural oil will also help, however follow the label directions and don’t use on drought stressed plants or when temperatures are over 90 degrees. These products will suffocate the mites again helping lower their numbers; however it generally won’t kill the eggs so they keep hatching out. Spider mites get immune to pesticides relatively quickly because most people keep applying the same products to control them. I have battled spider mites for years and I can tell you first hand most of our common pesticides we buy over the counter just don’t work!
Here is a list of proven pesticides I have used or that the University has tested, most will require more than one application: Avid (don’t use on conifers), Hexygon, Joust, Kelthane, Morestan, Horticultural Oil, and Insecticidal Soaps. Avid, Hexygon, and Joust are the most effective but also the most expensive and usually won’t be cost effective for a small landscape. Kelthane and the oils and soaps are the cheapest and can be found at local garden centers and nurseries but will need to be applied several times. Be sure to switch them up; don’t apply the same thing over and over.
One last note, many of our pesticides now are pyrethroid based. This substance kills insects on contact but not their eggs. When sprayed on spider mites, the adult may be killed but the eggs are still there to hatch out. The pyrethroid stresses the mites and actually causes them to lay more eggs which will in turn cause your mite problem to get worse! Also don’t spray spider mite infested plants with Sevin, it can cause mite problems to increase as well! If you have any questions about spider mite control stop by the Washington County Extension Office for a free publication or call me at (859) 336-7741