The hop vine borer is a stem-feeding caterpillar that can cause severe stand loss in young corn plants. It occurs primarily in northeastern Iowa, but there are a few fields infested with this insect in central Iowa (see map).
If corn has not yet emerged when the larvae hatch, they will feed in the stems of quackgrass, foxtail, and woolly cupgrass. Eventually they will move to corn. Their injury is unique and should not be confused with that caused by black cutworms (which cut plants) or stalk borers (which tunnel in the above ground stalk).
Hop vine borers attack the underground portion of corn and tunnel up from underneath the plant to hollow out the base of the stalk. Above-ground damage is detectable as wilting or death of the central whorl leaves. Usually 90-95 percent of all seedlings with two to eight leaves are killed if attacked. Larger plants with nine to twelve leaves become stunted and exhibit yellow or wilted outer leaves.
Where yearly infestations occur along fence rows, waterways, or field margins, apply one of the pyrethroids at plant emergence. Research conducted by University of Minnesota entomologists has determined that Asana XL sprayed at corn emergence to seven days post emergence gives excellent stand protection against hop vine borers. This fact is illustrated in the photograph where plants in the foreground were not sprayed while plants in the background were sprayed at plant emergence.
Timing of the insecticide is critical and an application made two weeks after plant emergence is not effective in preventing standloss. Once the borer is inside the corn plant, control with an insecticide can not be achieved. Corn rootworm insecticides applied at planting have not been effective in preventing stand loss. For long-term elimination of in-field trouble spots, crop rotation and control of grasses will be required.