Spider Mites

Mite infestations on corn frequently include a mixture of spider mite species, including twospotted spider mite, Banks grass mite, Pacific spider mite, and strawberry spider mite. Of these mite species, twospotted spider mite and Pacific spider mite are most common. Adult mites are about 0.06 inch in length, have four pairs of legs, are greenish to pink or cream colored, and have various sized black spots on the body. Under warm conditions spider mites move rapidly within the colony area. Spider mites have four stages of development: (1) the spherical, somewhat translucent egg; (2) a six-legged translucent larval stage; (3) an eight-legged nymphal stage; and (4) the eight-legged adult stage. A resting or quiescent stage occurs at the end of the larval and nymphal stages. A generation may pass in as few as 5 to 7 days in midsummer, or in a month during cool periods.

All active stages of spider mites damage corn by removing juices from infested leaves, causing premature drying that results in loss of leaf tissue, stalk breakage, and kernel shrinking. Damaged leaves become somewhat yellowish and stippled on the upper surface and grayish due to webbing on the undersurface. Spider mites can be a serious problem on corn, particularly silage and sweet corn.

Keep spider mite populations in check by reducing dust and weed hosts and encouraging mite predators. If monitoring indicates a need, treatment may be necessary on mid-size corn.

Spider mite populations may be held at very low levels by a number of predatory insects and mites, particularly early in the season. Thrips are effective early season predators, feeding primarily on spider mite eggs. Spider mites provide an important food source for predators such as minute pirate bugs and bigeyed bugs. Minimizing early season insecticide applications, which may reduce populations of beneficials, will reduce spider mite outbreaks. Naturally occurring predatory mites exert some level of control. In some areas, releases of predatory mites have been used to manage spider mites in field corn. If predatory mites are to be released, be sure to release the appropriate predatory mite species for the area and time of year. Also use the correct release rate and the correct timing. Definitive guidelines have not been developed, but make releases before significant spider mite outbreaks occur.

Reduce spider mite problems by keeping fields, field margins, and irrigation ditches clean of weed hosts. Spider mite populations may increase more rapidly in areas where dust deposits are heavy on corn leaves. Thus, reducing dust may reduce the spider mite problem.

Biological controls and cultural controls are acceptable to use in organically grown crops.

Infestations usually begin on the lower portions of the plants and move upward as mite numbers increase. Evaluating spider mite infestations is most efficient if randomly selected, older, lower leaves are picked and inspected for stippling on the upper surface and webbing, mites, and feeding scars on the lower surface. Spider mite infestations that reach the ear leaf are most damaging.

If small colonies of spider mites are found on the lower leaves of young plants throughout the field, control may be cost effective. Treat when corn is 2 to 4 feet tall; applications made after the plants exceed 4 feet in height usually result in poor control because good coverage is difficult to obtain. Just treating a couple of swaths around the field can keep spider mites from spreading into the remainder of the field.