The first potato leafhopper adults arrived in Wisconsin from the southern U.S. in late May, reports Eileen Cullen, University of Wisconsin Extension entomologist.
“Scout alfalfa fields beginning with second-crop regrowth as arrival of adults brings potential of sharp increases in populations in June,” says Cullen.
To get an accurate population estimate, she says to sample for leafhoppers when alfalfa is dry, and avoid field edges. Cold, wet or windy conditions may temporarily knock adults and nymphs from plants, resulting in inaccurate sweep counts. Use a standard 15”-diameter insect sweep net. A total of 100 sweeps should be taken throughout the field in an approximate M shape to obtain 20 consecutive sweeps in each of five randomly selected areas. Click here to view an instructional UW Extension video on scouting for potato leafhoppers.
Economic thresholds are based on the average number of potato leafhoppers/sweep. Keep a running total of the number of leafhoppers caught and divide by 100. Nymphs are not generally recovered in the bottom of the sweep net with adults, but found along the collar of the net (along the wire- hoop rim). Nymphs move quickly, typically sideways, when disturbed.
Spraying is warranted if the average number of leafhoppers per sweep reaches 0.2 in 3”-tall alfalfa, 0.5 in 3-6” alfalfa, 1.0 in 8-11” alfalfa and 2.0 in fields 12” or taller. But cut the crop instead of spraying, if possible, if you’re within seven days of your normal cutting schedule.
Potato leafhoppers have three or four generations per year in the Midwest, so continue monitoring for the remainder of this season, Cullen advises.