Economic Injury Levels and Economic Thresholds for Potato Leafhopper (Homoptera: Cicadellidae) on Alfalfa in Minnesota1

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Abstract
Experiments were conducted at Rosemount, Minn., to establish economic injury levels and economic thresholds for the potato leafhopper, Empoasca fabae (Harris), on established of alfalfa, Medicago sativa L. Gradients of potato leafhopper numbers were established in plots of cv. ‘Vernal’ alfalfa by using insecticidal sprays and bordering uncut strips of alfalfa. In 1981, damaging potato leafhopper numbers occurred on third regrowth and showed negative linear relationships with dry-matter yield and U.S. dollar value of yield. In 1981, damaging potato leafhopper numbers occurred on both second and third regrowths. On second regrowth, nymphal numbers were high and had significant negative effects on percent crude protein, plant height, dry-matter yield, and dollar yield as computed from the cost of an equivalent substitute ration of corn and soybean meal. On third regrowth, significant negative relationships were found between potato leafhopper numbers, primarily adults, at 5 and 17 cm of regrowth and dry-matter yield, height at harvest, and dollar yield. Calculated economic injury levels for vacuum and sweep-net sampling were 30.3 potato leafhoppers per 0.48 m2 per sample week and 37.1 potato leafhoppers per 25 180° sweeps per sample week, respectively. The calculated economic threshold in 1979 for 12 cm of regrowth alfalfa was 0.4 potato leafhopper per pendulum sweep. Values in 1981 for 5 and 17 cm of alfalfa were 0.32 and 0.5 potato leafhoppers per pendulum sweep, respectively.