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Posted on Sun. Aug. 24, 2014 - 12:01 am EDT

Spider mites are harmful

Q. My bean plants are not doing very well. The foliage is turning yellow and then brown. Do you know what is wrong?

A. I looked at the sample you dropped off at the Extension office. Under my microscope, I observed numerous spider mites. Now that the weather is turning hotter and drier, spider mites are having a field day on various vegetables flowers and trees.

These spider-like creatures spin tiny webs on plants, feed on plant leaves or needles and can quickly turn a healthy plant into a very sick specimen. Spruce trees, junipers, arbor-vitae, roses and honey locust are a few favorite trees and shrubs species preferred by mites. However, mites really like beans.

Since mites are so tiny, one has to look closely for other clues that mites might be present. Often one observes abnormal bronzing, curling and browning of the needles or foliage. Spider mite damage onspruce usually begins from the bottom and inside of the plant and works upward.

To check to see if you may have mites, place a white sheet of paper under the affected needles or foliage and tap the branch or leaf sharply. If you see what appears to be grains of pepper (sometimes orange in color) scurrying around on the paper surface, then you have a mite problem. One can also look at the leaf or needles with a magnifying glass or hand lens. One can observe the tiny webs the mites spin, and sometimes observe the little devils crawling around on the leaf. It is best to look on the underside of the leaf first.

To control mite infestations one can begin by using high pressure water spray on the foliage from time to time to wash the mites off the plant. Organic controls for mites include insecticidal soap or diatomaceous earth. I like to use D-Earth (as I call it) on the beans. Just dust the foliage with a small amount. I use a flour sifter that I tap lightly to distribute small amounts of this material on the foliage. A little bit of D-Earth goes a long way. It does need to be re-applied after a rain - and it does lose effectiveness when it absorbs moisture from the air after a few days. More garden centers now carry garden variety diatomaceous earth than in previous years – which is a good thing. Products containing pyrethrum (organic), resmethrin, or permethrin may also be used. The insecticide Eight is a form of pyrethrum created in a laboratory. Always read and follow label directions – for instance, Eight can damage foliage if it is too hot and sunny. The organic pesticide Neem also has some activity against mites.

Research conducted at Purdue University has shown that many insecticides used for mite control may just make the problem worse because mite populations tend to rebound to higher levels after their use than if nothing was done at all. Often, the best we can accomplish is limit their damage and keep mites under control.


The Plant Medic, written by Ricky Kemery, appears every other Sunday. Kemery is the extension educator for horticulture at the Allen County branch of the Purdue Extension Service.

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