First generation European corn borer and spider mites are to be monitored
Extension Specialist, Colorado State University Extension
The two major insect pests in corn that need to be monitored at the moment are the European corn borer and banks grass mites. The current hot and dry weather conditions are expected enhance early development and increase of banks grass mite in corn.
European corn borer: historic European corn borer moth emergence and duration of infestation data are found on our pest alert web site: http://northernipm.colostate.edu/ which contains pheromone trap counts from different locations and years in Colorado.
First generation moths prefer taller and early planted fields for laying eggs; your non-Bt hybrid cornfields should be scouted the next two to three weeks. Some hybrids have useful resistance to the first brood of European corn borer, which feeds in the whorls and later enters the stalk. Control can be expected with Bt corn hybrids, except those containing only events that target corn rootworms.
European corn borer usually goes through two generations each year. The young larvae feed first on the leaf near where they hatched. As the larvae grow, they move to the whorl or leaf sheath area, and feed. When leaves emerge, the “shot hole” feeding signs in the leaves can be seen. Most of the mature larvae will bore into the stalks, feed, and finish development there. Second generation larvae cause ear damage, tunneling in the shank and feeding on silks, kernels and cobs. Signs of infestation include: dropped ears, broken shanks, stalk breakage, sawdust-like castings on leaves, and holes in the stalks.
To determine infestation levels of first generation and make management decisions, 25 plants in four locations in a cornfield should be checked for leaf infestations. Larval damage is noticed as feeding scars and shot holes in plant leaves. Chemical control of first generation corn borer is justified when 25 percent of the plants in your sample show feeding damage and show presence of larvae. Chemical control of the pest must be applied before the feeding larvae bore into the stalks. More detailed management information including effective products, rates of application and others can be checked in the High Plains IPM Guide: https://wiki.bugwood.org/HPIPM.
Banks grass mites (BGM) are minute, 0.45 millimeter (0.017 inch), greenish colored arthropods with eight legs and a rounded body. BGM has dark pigmentation along both edges of the body near the rear and along the sides. Fertilized female BGM move into winter wheat in the fall as their summer hosts, especially field corn but also other grasses, begin to dry down. These over-wintering forms are bright orange in color. They return to corn by walking short distances or by being windborne on silk threads over longer distances. In the spring small pearly white eggs are laid which eventually give rise to pale to bright green male and female adults.
Webbing on leaves and discoloration are often the first signs of an infestation. Initially, BGM are most abundant on the lower third of the plant and density declines as the infestation moves up in the plant. Mites damage corn and small grains by piercing plant cells with their mouthparts and sucking the plant juices.
Banks grass mite builds up on the plant from the bottom up. Treat when there is visible damage in the lower third of the plant and small colonies are present in the middle third of the plant before hard dough stage. An alternative method for determining the need for control of BGM in grain corn, developed by T. Archer and E. Bynum at Texas A&M University at Lubbock, is based on total infested leaves and total percentage of leaf area damaged (https://wiki.bugwood.org/HPIPM).
If the mites build up early and hot, dry weather persists, a second application may prove necessary. Areas of stressed plants are often the main source of infestation for the rest of the field. All registered materials have failed to control mite outbreaks at one time or another. In the areas where both BGM and two-spotted mite species occur, select insecticides that are effective for both species of mites (https://wiki.bugwood.org/HPIPM).