Omar Koester, Extension unit assistant in crop systems in Monroe County, found a fairly severe infestation of spider mites in a cornfield near Waterloo. Leaves were "fired" from the ground to the ear, but symptoms of injury (yellowing of leaves) were visible all the way to the top of the plants. Spider mites and eggs were found on all leaves sampled. One cutting of orchardgrass hay had been taken from the field across the road.|
Corn leaves "fired" as a result of spider mite injury. (Photo courtesy of Omar Koester, Extension Unit Assistant, Crop Systems, Monroe County.)
We are more familiar with spider mite problems in soybeans, but spider mites can injure corn if the weather is hot and dry, allowing numbers of mites to build up to injurious levels. Two species of spider mites may be found in corn in Illinois--Banks grass mite, Oligonychus pratensis, and twospotted spider mite, Tetranychus urticae. They are difficult to tell apart. Both are extremely small (adult females are about 0.45 millimeters long) with either six legs (larvae) or eight legs (protonymphs, deutonymphs, adults). Protonymphs and deutonymphs are pale to bright green. Characteristics used to distinguish between the two species in the field include the copious webbing produced by twospotted spider mites and location of the food balls of the adult females. In Banks grass mites, green food balls accumulate around the periphery of the abdomen; in twospotted spider mites, the food balls concentrate in two spots on either side of the abdomen. Eggs are pearly white and spherical.
Close-up of spider mites and eggs on a corn leaf. (Photo courtesy of Omar Koester, Extension Unit Assistant, Crop Systems, Monroe County.)
Feeding by spider mites on corn causes localized cell damage and eventually a loss of water and chlorophyll. Banks grass mite injury begins as concentrated chlorotic areas along the midrib and folded areas of the leaf. Chlorosis can spread, particularly on the basal half of the leaf. Chlorotic areas resulting from feeding by twospotted spider mites are less concentrated and more evenly distributed across the leaf.
I write this article simply to alert you to the possibility of spider mites in corn. Thus far, I have received only the one report. However, if the weather remains hot and dry, spider mites may become troublesome in both corn and soybeans. Watch the edges of fields first; symptoms of spider mite injury usually show up first at field margins.
If control of spider mites in corn becomes necessary, the best choice for control is Capture 2EC (restricted-use product) applied at 5.12 to 6.4 ounces per acre. Although dimethoate is labeled for control of spider mites in corn, it cannot be applied during the pollen shed period. Please follow all label directions and precautions.--Kevin Steffey