Potato leafhopper, a small bright green insect which migrates annually from the gulf coast to Kentucky on wind currents (Figure 1), can cause significant injury to alfalfa if left unmanaged. It does not normally impact the first cutting of established alfalfa stands. Infestations in later cuttings can stunt growth and reduce yields. The most common symptom of leafhopper damage is yellowing of the leaves, commonly referred to as “hopper burn” (Figure 1). Although nutrient deficiencies can also cause yellowing of the leaves, damage from leafhoppers can be distinguished by a characteristic inverted yellow “V” at leaf tip (Figure 1).
Scouting and Economic Thresholds
Determining leafhopper populations requires the use of a sweep net with a 16-inch opening and a 3-foot handle. Sampling is accomplished by quickly walking through the stand as 10 sweeps are collected in a back and form motion that grazes the top of the alfalfa canopy. Adult and nymph leafhoppers should be immediately counted. Sampling should be conducted in three to five areas of the field and then averaged. Economic thresholds for the application of insecticides can be found in Table 1. In general, if the number of leafhoppers collected in 10 sweeps equals the height of the alfalfa plant in inches, the economic threshold has been reached. For stands that are under high levels of stress, the economic threshold levels are lower. Conversely, stands that are vigorously growing and under lower levels of stress possess higher economic thresholds. If potato leafhopper resistant cultivars were used, economic thresholds are three times higher than those outlined in Table 1. If the alfalfa stand is 7 to 10 days from harvest, treatment is NOT recommended. Following harvest, regrowth should be closely monitored for reinfestion. Insecticide recommendations can be found in Table 2. For a more information on the management of insects in alfalfa, contact your local extension office or visit the entomology website. ~ Chris Teutsch and Raul Villanueva