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Award of Merit Article

Spider Mites

by Nanette Londeree, Consulting Rosarian

Spider mites are small arachnids related to spiders and are common in low rainfall areas of the West where warm summer temperatures (above 70įF) and dry plant foliage favor their development. The two-spotted spider mite, Tetranychus urticae, is orange, green, or yellow, with two dark spots on the sides of the body. Like spiders, mites have two body segments and eight legs. They have a rasping mouthpart with which it pierces the epidermis of the leave and apparently injects some of its saliva in the process. After feeding, two minute chlorotic spots appear as the leaf tissue collapses. During severe infestations, the entire plant may turn yellow and die. They tend to feed on the underside of rose leaves, especially older leaves.

Mites are less than 1/16 inch but are visible to the unaided eye, and a magnifying glass or hand lens makes them easy to see. The most common symptom on the rose is foliage that is bronzed color and stippled. From a distance, leaves look yellow and dusty.

Bronzing of Leaves
You also may be able to see very fine webs between the leaves. Heavily infested rose leaves turn brown, curl, and fall off. If you shake suspicious looking leaves over a piece of white paper you can see what looks like pepper on the paper.

Spider Mite Webbing
Mites over winter as mated females in protected areas such as on weeds, ground litter, or fallen leaves. They do not over winter on roses, which means dormant oils are not effective. Females emerge in the spring and begin laying eggs on plant foliage. Eggs hatch in about a week, depending on temperature. A complete life cycle requires one to three weeks while in very hot weather it is often around seven days.

Spider mite populations can build up to tremendous numbers and can decimate roses in a very short time if the conditions are right. Hot, dry weather, plants that are very close together with little air circulation, and little water offer a haven for these little creatures. Dust and dirt on leaf surfaces reduce the cooling effect of transpiration and favor mite buildup. Miniature roses seem to be particularly susceptible to spider mite attack and can completely defoliate in a matter of days. Roses experiencing drought stress are also more susceptible to spider mites.

The best protection against an invasion of mites is good cultural conditions and a little overhead shower. Remove any fallen leaves or branches, as well as weeds, which may serve as a host for spider mites. Keep plants well watered and apply mulch to rose plants during drought conditions to minimize stress. Periodically washing plants reduces numbers of mites while preserving the predator complex. Many other insects feed on mites, such as minute pirate bugs, big-eyed bugs, and thrips. Use of broad-spectrum insecticides such as carbaryl (Sevin) may eliminate mite predators, allowing a rapid increase of mite numbers. Selective control of mites is available with miticides such as abamectin (Avid), bifenthrin (Talstar), dicofol (Kelthane), or Neem oil, insecticidal soap or a high purity horticultural oil. If you use a miticide, frequent chemical applications may be necessary and not very effective if plants are not also washed with water.

When hosing off the plant, make sure to rinse the entire plant, top to bottom, as well as the undersides of the leaves using a hard spray of water. Once spider mites are knocked off of the plant, they canít crawl back. The watering should be done in the morning or early evening so that the leaves can dry before nightfall. You donít want to trade one scourge for another!

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Last Modified: 08/06/2013