Effect of varying potato leafhopper (Homoptera: Cicadellidae) population densities on snap bean yield

Varying infestation levels of potato leafhopper, Empoasca fabae (Harris), were created on commercial snap bean (Phaseolus vulgaris (L.)) cultivars by application of insecticides at predetermined thresholds, and subsequent yield responses were determined. Low (close to zero), medium (one adult per sweep or one nymph per leaf), and unrestricted infestation levels were compared on 'Early Gallatin', 'Lake Superior', and 'Bush Blue Lake-94' at two Wisconsin locations (Arlington and Hancock) using two planting dates and infestation levels in 1980 and three in 1981. E. fabae populations in 1980 were moderate. Two to four applications of fenvalerate were necessary to maintain populations at the low threshold, and little benefit in increased yield was obtained compared with untreated plots. In 1981, populations were high. A systemic insecticide and two or three foliar applications were needed to maintain populations at the low density, which yielded higher than the untreated controls in each variety, Only one or two foliar sprays were required at the medium threshold. Although higher leafhopper populations occurred in these plots, yield differences between the low and medium infestation levels were small provided that fewer than one leafhopper per sweep was detected before the first trifoliate stage. Throughout these trials, higher E. fabae populations were found in 'Bush Blue Lake-94' than in 'Early Gallatin'. Yield reductions were greatest in 'Early Gallatin', however, indicating a tolerance for E. fabae injury in 'Bush Blue Lake-94'. 'Lake Superior' was intermediate in leafhopper populations and yield reduction. An action threshold of 0.5 adult per sweep before the first trifoliate and one adult per sweep or one nymph per leaf for the remainder of the season is proposed for snap beans. Intensity, duration, and time of E. fabae infestation are crucial factors in the determination of subsequent effects on crop yields. Crop variety and additional plant stress factors, including low temperature and incidence of plant disease, can also affect yield response to leafhopper damage and should be considered in management decisions