Ag Sense: Spider mites threat to more than corn

By Mark Marsalis

If the dry fall and winter are any indication of what to expect this summer, corn producers should be aware of the potential for damage from spider mite infestations.

Mite outbreaks are most common in hot, dry, windy years and in drought-stressed corn, even in irrigated fields. In areas where spider mites are common, outbreaks are likely almost every year, even if just in isolated instances.

Spider mites pierce leaf cells and extract plant juices, causing yellow spotting or stippling on the upper surface of leaves, usually the first evidence of infestation. Mite damage can kill leaves and reduce productivity of corn, whether grown for grain or silage.

Look for mites, eggs and grayish webbing on the underside of leaves. A hand lens helps in identification. Infestations usually begin on lower leaves and move up the plant.

Spider mites have four pairs of legs and are about the size of pencil dots. Common in this area are the Banks and two-spotted grass mites.

Although both affect corn, they differ in the amount of damage they cause and their susceptibility to chemical (miticide) control. The Banks grass mite usually appears early in the growing season (mid-whorl), often remains on lower leaves, and is controlled by several miticides. In contrast, the two-spotted grass mite tends to proliferate after flowering, infests the entire plant and is harder to control with chemicals.

Spider mites can cause up to 40 percent of commercial yield losses in corn grown for silage. High populations can severely inhibit ear development, and will harm the whole plant by drying and killing leaves. Watch for mites just after corn sprouts. Scout fields often, particularly on the edges where mites enter and irrigation deficits are more likely.

Other plants can host mites, and corn fields close to ripening wheat, grass or alfalfa should be monitored closely as populations can spread from these hosts by wind. Mites often move from drying wheat down to look for another live host, which often is a young corn plant. Double cropping and nearby rotations of wheat and corn can perpetuate this problem.

Keep fields, field margins and ditches clean of weeds to help control mites. The two-spotted mite overwinters in plant residue and tree bark, and will feed on broadleaf plants such as apple trees and roses.

Broad spectrum insecticides used to control other pests (e.g., corn borer) could boost mite populations as these products often kill beneficial, predatory insects (e.g., lady beetles, lacewing, banded thrips) and predatory mites that feed on spider mites early in the season. Maintaining beneficial insects is key to the prevention of high spider mite populations. However, beneficial insects alone will not be able to keep up with escalating mite populations.

Several chemicals help control spider mites in corn. Effectiveness is greater when plants are less than 4 feet tall. Treatments likely will not work after the corn has reached dent stage. Treat mites chemically depending on severity of infestation and the crop’s value.

These products include: dimethoate (Dimate brand), propargite (Comite brand), spiromesifen (Oberon brand) and bifenthrin (Capture brand). Dimethoate is least expensive and effective on Banks grass mites, but won’t stop the two-spotted type and is more of a temporary control.

Comite propargite and Capture bifenthrin are normally more expensive, but they will control both types and last longer. Comite propargite and Oberon spiromesifen also kill eggs and limit re-infestation. Comite propargite has been effective in New Mexico when applied early (banded; plant 1 1/2 to 2 feet tall) to borders and before layby.

Mites not only impact corn. They can infest grain and forage sorghums. Infestations can develop rapidly, especially during corn pollination and if the producer gets behind on watering.

Mark Marsalis is an extension agronomist at New Mexico State University’s Agricultural Science Center at Clovis. Contact him at 985-2292 or by e-mail: