Mites can be a problem in cotton, and is usually more severe during very hot and dry weather. Mites damage in both the adult and immature stages. They produce fine silken webbing which can sometimes be seen on infested leaves. At full size, spider mites are around 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. They do not have wings, move by crawling, can be blown by the wind, and transferred by people, equipment or animals moving through the field. Usually mites hide on the underside of the cotton leaf to help protect them. Rain can reduce mite populations quickly sometimes because it triggers a fungus (Neozygites) that will build up and attack the mites and can decimate a population fast, may be worthwhile to see if rain is in the forecast prior to treating.
Their piercing-sucking mouth parts allow them to pierce individual cells of a cotton plant and remove all liquid content from the cells, thus killing every cell they feed on. Their feeding results in a speckled pattern of feeding on the leaves, which will have a yellowish or reddish appearance. Mites reduce the plants ability to produce photosynthate, and under severe infestations, may cause defoliation.
Threshold according to Missouri Pest Management Guide: Treat when leaves appear to have mite injury on 30% to 50% of the plants and mites are present. Initial infestations are often localized, but they spread across the field rapidly.