July 11, 2009 in Washington Voices

As it gets hotter, spider mites attack plants

Pat Munts
 

The aphids caught a lot of gardeners off guard this year. Fool me once, shame on the bug – fool me twice, shame on me. As we head into the hot, dry part of the summer, get ready for our annual spider mite attack.

Spider mites are not a true spider, though they are related. The reference to “spider” comes from the eight legs adult mites have. Two types of spotted spider mites are the most common of four mites in the Northwest. They are tiny, only about one-fiftieth of an inch long when mature. That’s smaller than a poppy seed. You are likely to see their damage before you see them.

They range in color from light yellow or green to dark green or brown. Both males and females have two characteristic dark spots visible on their abdomen. They are usually found on the underside of leaves or needles, which makes them even harder to spot. The mites create a network of fine, silky webs that covers the underside of the foliage and provides them a transportation network and a place for females to lay eggs.

Spider mites can infest more than 300 different plants, including fruit trees, small fruits, evergreens, shade trees, shrubs, vines, vegetables and flowers. Dense conifers like Alberta spruce are a favorite. They feed by puncturing individual plant cells on the underside of the leaf and sucking out the leaf cell contents.

Usually the first sign of a mite invasion is the appearance of stippled leaves or needles with a bronze cast. One mite can puncture upward of 100 cells an hour. Multiply that by a few thousand mites on one plant and it’s easy to see how much damage they can do very quickly. Damage increases when it gets hot and dry because the mites are looking for more moisture to replenish their bodies.

To identify an infestation, look on the undersides of leaves and needles for the telltale web or tiny colored dots. Hold a piece of white paper under a suspicious branch and try to shake some of the mites onto it. If small dots fall and start moving, you probably have spider mites.

Treating an average infestation is as easy as picking up your hose and spraying your plants with a hard stream of water to knock the mites off the leaves. This also washes dust off and adds humidity to the leaf surfaces, all of which reduce the effects of the heat and dryness that increase mite activity. Be sure to spray the underside of the leaves and into tight bundles of foliage.

There are several predatory mites and insects that feed on spider mites. Given half a chance, they usually do a pretty good job of keeping the mites in check. Let an infestation go for a few weeks so the predators can find it.

For a severe infestation, it may be necessary to apply an insecticidal soap. Be sure to get the undersides of the leaves. Keep in mind this will also kill predatory insects.


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