Q: Several of my shrubs turn yellow about midsummer. It appears as speckles on tops of the leaves and happens quickly. I think they are getting adequate water. What is it?
-Jon in Denver
A: Spider-mite populations peak during the warm temperatures of summer. They can complete their development from an egg to an adult in about a week. Because populations can increase exponentially very quickly, extensive injury sometimes appears in a short amount of time.
Although spider mites feed on the underside of the leaf, the damage is most evident on top. They use needlelike mouthparts to pierce individual plant cells and suck out the contents. This is what produces the speckled appearance. Spider mites may kill plants if population levels are high over an extended period of time.
The mites are so small, a hand lens is needed to see them. They have eight legs (the larval stage has six legs) and an oval-shaped body. Spider mites do not have wings or antennae. They can be yellow, green, red, gray or brown, and produce a webbing that is visible when populations are high. There are many types of mites, but most common is the two-spotted spider mite.
One common way to determine if your plants have mites is to hold a white sheet of paper under the suspect plant and give the foliage a shake. Mites will appear as small, moving dark specks on the paper.
The use of insecticides actually helps increase spider-mite populations by killing natural predators. One such chemical is Carbaryl, sold under the trade name Sevin. Malathion is another chemical to use with caution in your yard if you have had past problems with spider mites.
Proper irrigation can help reduce susceptibility to spider-mite damage. A brisk spray from a hose once a week can knock off the mites and help reduce infestations during the summer months. Chemical controls include horticultural oil, insecticidal soaps, and miticides. Most miticides do not kill eggs, so it is important to respray every one to two weeks until the problem is under control.
Commercial applicators have products available to them that target the eggs, interfere with the development of the mites, and kill the adults. You may need to consult with a professional if your mite problem is severe or if many shrubs that are infested with mites.
Robert Brudenell is a registered consulting arborist and owner of The Natural Way Inc. Send questions to robert@thenaturalwayinc.net.
