September 5, 2017 at 3:32 p.m.
Bruce Potter, from the University of Minnesota Extension, presented "Alfalfa Weevils and Other Troublesome Pests in Alfalfa" during the seminar in Rochester. There were also day-long Forage Day workshops in Avon, Cromwell, Detroit Lakes and Lamberton.
Potter first focused on the alfalfa weevil, which are not native to this area. There are two types of alfalfa weevils - the eastern strain and the western strain, which is becoming more prevalent in the area.
Alfalfa weevils damage leaves by chewing large holes, usually seen at the end of the first cutting and the beginning of the second, Potter said. They are also more problematic during a dry season.
"Another way to tell if you have it is if it looks like your alfalfa has a frost on it," Potter said. "If you get to that point, it's too late. If you have a bad alfalfa weevil injury out there it might reduce a healthier stand and translate into poor survival over the winter."
Potter suggests using a sweep net - using the 180-degree sweeping style - in different parts of fields to determine if alfalfa weevils are a problem. About 25 larvae per sweep is threshold. Light feeding is around 301 to 438 degree-days and major feeding happens between 439 to 595 degree-days before the feeding stops.
When treating for alfalfa weevils, Potter said there are a few aspects to keep in mind. Don't treat blooming alfalfa. At that point, the only option is to harvest, he said. Avoid insecticide before you have a problem in order to keep the natural enemies around as long as possible.
Potato leaf hoppers are another prominent pest in alfalfa. They were brought to the Midwest because of the wind patterns.
"It's just the way the wind blows," Potter said.
New seedings are the most vulnerable to potato leaf hoppers. They damage the leaves, leaving producers with yield losses and quality losses. Potato leaf hoppers begin their damage in June and continue through the rest of the season.
Sweeping fields using a pendulum swing is the best way to find whether fields are affected by the pest. Threshold is 0.1 leaf hopper per sweep per inch of alfalfa.
"If you're close to cutting, don't reschedule harvest just to control leaf hoppers," Potter said.