Buffalo County Extension Educator
Reports of spider mites in corn and soybeans have begun to come in.
Current and projected weather conditions favor an increase in spider mite populations, so farmers and field scouts should keep a close eye on fields.
Two species of spider mites, the Banks grass mite and twospotted spider mite, commonly feed on Nebraska corn. Banks grass mites feed almost exclusively on grasses, including corn and sorghum. Twospotted spider mites not only feed on many species of grasses, but also on soybeans, fruit trees and a variety of vegetables and ornamental plants. Although these two species are somewhat similar in appearance, they differ in several biological characteristics and in their susceptibility to pesticides.
Banks grass mites often over winter here, so they usually appear earlier in the season, feed mostly on the lower leaves of the corn plant and are moderately susceptible to many of the commonly used miticides. On the other hand, twospotted spider mites tend to fly in from the south and appear in mid to late season, increase rapidly, feed over the entire plant and often are not consistently controlled by available pesticides.
The most useful characteristics for identification are the overall shape of the body and the pattern of pigmentation spots on the back. In Banks grass mites, the pigments accumulate along both edges of the body, near the rear and along the sides of the body. Since they overwinter here, they usually are found next to grassy field borders, often on the south side of the field.
In twospotted spider mites, the pigments accumulate along the sides of the body, in two distinct spots, and do not extend back more than halfway on the body. The Banks grass mite also is slightly less robust than the twospotted spider mite and is slightly flatter from top to bottom. Since they blow in by the wind, they may be found anywhere within the field. Mites damage crops by piercing plant cells with their mouthparts and sucking the plant juices.
The first evidence of mite feeding, which can usually be seen on the top of the leaf, is a yellow or whitish spotting of the leaf tissues, in areas where the mites are feeding on the lower leaf surface. Leaf discoloration caused by mite feeding can be easily identified by checking the undersurface of leaves for the presence of mites, eggs and webbing. Banks grass mites and twospotted spider mites produce webbing, and a fine network of silken webs likely will be associated with mite colonies. A magnifying glass of 10X hand lens is helpful in examining plants for mites.
The economic injury level should be based on the value of the crop and the amount of infestation. In general, irrigated corn that will cost $20 per acre to treat reaches economic level when 25 to 30% of the leaves are infested and 15% of the total leaf area is damaged. In dryland corn, economic threshold would be when 50 to 60% on the leaves are infested and 25 to 30% of the total leaf area is damaged.
Labeled products for spider mite control on corn include dimethoate (several formulations), Comite 6.55EC and Capture 2EC. Dimethoate has performed reasonably well in Nebraska against Banks grass mites, but not twospotted spider mites. If twospotted spider mites are present, Comite or Capture would provide better control.
However, knowing whether you have potential spider mites is much more important than treatment levels, if you are considering treatment for rootworm beetles, corn borer or western bean cutworms. Low levels of spider mite infestations can suddenly increase to highly damaging levels, if we spray for other insects and kill all of the beneficial insects that hold spider mite populations down.
Since products differ in their effects on the two species, it is important to determine the mite species present in the field before making an application. Products that have sometimes been associated with Banks grass mite and twospotted spider mite problems after their use include permethrin (Pounce, Ambush) and to a lesser extent, carbaryl (Sevin), which under some circumstances may even reduce Banks grass mites. Other products, including parathion, are most likely to be associated with mite population increases, only when twospotted spider mites are present. Parathion seems to suppress Banks grass mites, but not twospotted spider mites. Still other chemicals only have a slight effect on spider mites or tend to suppress them to some extent, such as Lorsban.
So, what are some strategies to use if you find some spider mites present?
1. Three chemicals are available for mite control, in corn. These are dimethoate, Comite II and Capture 2EC. One of these products can be used to treat for the other insects.
2. Fields may be spot treated, if the infestation is localized, then other chemicals can be used on the rest of the field. But check other areas for mites (especially downwind of infestation) and extend treatments into these areas, if large numbers of mites are found.
3. Localized or borders where infestation of spider mites is occurring may be skipped when the rest of the field is treated and all insects allowed to thrive in those areas.