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Corn pests need to be monitored PDF Print E-mail
Written by Assefa Gebre-Amlak, CSU Extension specialist   
Among the major corn pest problems, western bean cutworm and banks grass mites need to be monitored the next three to four weeks in Colorado.

Extension has seen increasing number of western bean cutworm moths in pheromone traps in northeastern Colorado recently. For current as well as the historic data on western bean cutworm moth flight period, check www.nocopestalert.org.

Western bean cutworm eggs are deposited in clusters of 4-200 on the top surface of upper leaves. When first laid, the eggs are white with a thin red ring around the top. As they age, they change to brown, then immediately prior to hatching, they are purple to black in color. The eggs hatch in five to seven days.

Following hatch, young western bean cutworms move to one of two places on the corn plant, depending on the stage of development of the corn. If corn has not tasseled, larvae feed on pollen in the developing tassel. If corn has tasseled, larvae feed on silk in the ear. This type of silk feeding may cause pollination to be poor.

Once the ear has formed, larvae feed on developing kernels. Destruction of the kernels may reduce corn yields by as much as 30-40 percent.

Fields should be scouted closely, as once the larvae move into the ear, good control will be difficult to obtain. Chemical control should prove economical if 8 percent or more of the plants have egg masses or small larvae in the tassels and the crop is at least 95 percent tasseled. If tasseling is much less than this, the percentage of infested plants should be raised as fewer larvae are likely to reach the ears.

Many of the insecticides registered for western bean cutworm control have been associated with spider mite outbreaks, so fields should be monitored for mites after a treatment is made. For effective products, check the High Plains IPM Guide at http://wiki.bugwood.org/HPIPM:Crops.

Western bean cutworm is also a pest of dry beans. Egg masses and larvae are easier to scout in corn than in dry beans. If corn is infested, adjacent beans are likely to be infested also. For management information, check the High Plains IPM Guide.

Low to medium infestation of banks grass mite infestations are commonly seen in cornfields. Continue monitoring corn fields for this pest in the area. Banks grass mites 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. Effective products for BGM management in corn can be found the High Plains IPM Guide at http://wiki.bugwood.org/HPIPM:Crops.

 

Holyoke Enterprise July 25, 2013