Iowa State University researchers are investigating a low-cost method of controlling potato leafhoppers.

By leaving an uncut strip in an alfalfa field as a trap crop, growers can minimize the pests' damage, according to entomologist John Obrycki and graduate assistant Laura Weiser.

An uncut strip, left to grow between cuttings, attracts leafhoppers, says Obrycki. Once the pests are concentrated in the strip, it can be harvested or sprayed with an insecticide. Instead of spending the time and money to spray the entire field, growers would only need to treat the uncut strip.

Leafhoppers could also be destroyed by the buildup of beneficial organisms in the uncut alfalfa, says Obrycki. These natural enemies could include predatory bugs that feed on immature leafhoppers, or an insect-specific fungus that attacks and kills leafhoppers.

"Of course, the uncut strip is going to get damaged by the leafhoppers," points out Weiser. "That is hay the farmer has to be willing to sacrifice for the quality of the rest of the field."

The researchers began their three-year study, funded by the Leopold Center for Sustainable Agriculture in Ames, last year.

They intended to observe uncut strips in seven fields at various locations throughout Iowa this summer. But heavy rains in May and June reduced the number of locations to four, says Weiser.

Those four fields vary in size from one acre to five acres. Two of the uncut strips are in the center of fields. At each of the other locations, the uncut strip is on a field edge. Each strip is approximately 20' wide x 100' long.

After the first cutting was taken, Weiser placed yellow sticky traps at five locations in each field.

"Small insects that land on the traps are going to stick there," says Weiser.

The traps were removed and replaced with fresh traps once a week for five weeks, beginning June 1. Weiser then counted the leafhoppers on each trap.

"We found that, for the first two weeks after the first cutting, the number of leafhoppers in the regrowth was greatly reduced."

On June 1, an average of 16 leafhoppers was collected on the sticky trap in the uncut strip vs. two or less leafhoppers on traps at the other locations. In the weeks that followed, leafhopper numbers in alfalfa regrowth increased steadily. By the fourth week, numbers were similar in cut and uncut areas at all locations.

Similar results were anticipated between the second and third cuttings.

At press time, the researchers were gathering more information about the value of leafhopper trap crops.

"If a farmer has a five-acre field or a 50-acre field, how many uncut strips would a farmer need?" Obrycki asks. "Or, how far away from the uncut area can an effect be seen?"