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Spider Mites
Classification and Description: — Mites (Order Acari) are not insects, but rather, are more closely related to spiders. The two spotted spider mite (Tetranychus urticae) is the most common mite found infesting cotton. At full size, spider mites are only 0.3-0.4 mm long. They are greenish-yellow to orange, and under magnification, a dark spot can be seen on either side of the body. The adult and nymphal stages have eight legs, but the larval stage that emerges from the egg has six legs. Both immature and adult stages of spider mites cause injury to cotton with piercing-sucking like mouthparts. As their name suggests, fine silken webbing is produced by the mites and may be observed on infested leaves.
Hosts and Distribution: — The two spotted spider mite has a worldwide distribution and an extremely wide host range that includes cotton, corn, soybean, orchard, greenhouse and ornamental plantings. A number of wild hosts, including cutleaf evening primrose, may be important in helping to establish mite populations in cotton fields.
Life History: — Spider mites overwinter as adults but may remain partially active throughout the year. They have no wings, so mites disperse by crawling or wind transport. Eggs are usually deposited on the underside of leaves. Spider mites can complete a generation in as little as 4-5 days, giving them a very high reproductive potential. Many generations are possible in cotton.
Pest Status and Injury: Two spotted spider mites are only occasional pests in Tennessee cotton. Infestations are often most severe during hot and dry weather, in part because a fungus (Neozygites floridana) that attacks this species is not effective in these conditions. Spider mites injure cotton by feeding on the contents of individual cells. They may feed on all plant structures but are most commonly observed on the undersides of leaves. The top surface of infested leaves may have white or yellow stippling. Older damage often appears as a reddening or purpling of leaves. Injury is often concentrated near the petiole, around leaf veins and at leaf folds. Mites reduce the plant's ability to produce photosynthate, and under severe infestations, cause premature defoliation.
Management Considerations: — Spider mite infestations often start and end suddenly. Population crashes following a rain are sometimes observed because this triggers a Neozygites epizootic. Spider mites can be difficult and expensive to control. Two successive pesticide applications, at a 4-5 day interval, may be needed to control mites and those that subsequently hatch from eggs. Miticides are listed in the Insect Control Recommendations for Field Crops (PB1768). The current treatment threshold is when 50 percent of plants are infested.
Print-Ready PDF Publication (Spider Mites W035)