By Philip Kaatz
Increased numbers of the annual pest potato leafhopper can cause havoc in alfalfa, especially in dry weather.
When asked, “What is the No. 1 insect pest for alfalfa?” the answer is, hands-down, potato leafhopper. Hot, dry weather the first part of June has increased the risk of a high population of this small, 0.125-inch insect pest.
Southern weather fronts that originate in the Gulf of Mexico carry the insects with them into Michigan where they are deposited onto fields of susceptible crops such as dry beans and alfalfa. New alfalfa seedings are especially at a high risk of damage due to potato leafhoppers and should be scouted.
Alfalfa growers should begin scouting alfalfa with a sweep net. Michigan State University Extension suggests taking five sets of 20 sweeps in several parts of the field. Count the adults and nymphs. The thresholds, according to the MSU Entomology Department bulletin, “
Alfalfa Insect Pests,” vary by plant height in inches:
- Under 3 inches = 20 adult potato leafhoppers per 100 sweeps
- 3 – 8 inches = 50 adult potato leafhoppers per 100 sweeps
- 8 – 12 inches = 100 adults and/or nymphs per 100 sweeps
- Over 12 inches = 200 adults and/or nymphs per 100 sweeps
If the management decision is to culturally control potato leafhopper with cutting, no insecticide spraying is necessary. However, continue scouting the susceptible new growth and further cuttings since multiple generations of potato leafhopper are possible during summer. This is especially critical during dry periods since damage from leafhopper feeding is worse under drought stress.
If the management decision is to chemically control potato leafhopper, most insecticides are effective. Before spraying, check insecticide labels for pre-harvest intervals prior to spraying, as well as for new honey bee warnings that were added with the recent concern for pollinators.
Most insecticides are rated highly- to moderately-toxic, meaning they kill bees on contact during application and for several days after treatment. The simple way to avoid complications with pollinators is don’t spray a crop in bloom. An infested alfalfa stand that is advanced enough to be flowering should be cut for potato leafhopper management and the new growth monitored.