Although we haven't said much about potato leafhoppers this year, people who are scouting in alfalfa fields need to focus on this very important pest in June and July. Although the previous cool, wet weather probably kept leafhopper numbers from increasing, the more recent warm weather will accelerate their development. Some people have begun to report larger numbers during this past week. Following is information that should help you scout for and potentially manage potato leafhoppers in alfalfa.|
Description. Adult potato leafhoppers are lime green, wedge-shaped insects approximately 1/8 of an inch in length. Adults have fully developed wings and are very active fliers. The nymphs, smaller versions of the adults, are yellow-green and lack wings. When disturbed, the nymphs move sideways or backwards ("crablike" movements).
Potato leafhopper adult.
Life history. Potato leafhoppers do not overwinter in Illinois. Instead, they migrate northward from southern states assisted by wind currents. Soon after their arrival in the Midwest, the females begin to lay eggs in stems and larger leaf veins. The eggs hatch in 6 to 9 days into nymphs. Multiple generations are accomplished throughout the summer with leafhoppers persisting until cooler fall temperatures return.
Injury. Potato leafhoppers suck fluids from alfalfa plants with their piercing-sucking mouthparts. They inject saliva that contains a compound toxic to plants. In addition, their feeding clogs the conductive tissue of plants, resulting in an accumulation of starches. This accumulation causes a relative nitrogen deficiency, resulting in yellowing or injured leaves. Most people notice the first symptom of potato leafhopper injury as a V-shaped yellowing at the tips of the leaflets, commonly referred to as "hopperburn" or "tipburn." As injury progresses, the leaves may turn purple or brown and then die. Severely injured plants also are stunted and bushy in appearance because the internodes stop growing normally.
Potato leafhoppers and "hopperburn" on alfalfa.
Scouting and thresholds. A 15-inch diameter sweep net is the required sampling tool for potato leafhoppers in alfalfa. Static treatment thresholds are based on the number of leafhoppers per sweep of the net. The threshold increases as alfalfa grows and becomes more tolerant to leafhopper feeding: 0.2 per sweep on stubble (up to 3 inches), 0.5 per sweep on 3- to 6-inch alfalfa, 1.0 per sweep on 6- to 12-inch alfalfa, and 2.0 or more per sweep on plants 12 inches or taller. The sweep net should be swung like a broom through the top 4 to 6 inches of growth. Each thrust with the net is a sweep. Take 20 sweeps per location in 5 to 10 sites within the field if the alfalfa is taller than 4 inches. If the alfalfa is shorter than 4 inches, more sweeps are required for a precise sample estimate. Count the number of leafhoppers caught in the net and divide by the number of sweeps taken. If the number exceeds suggested treatment thresholds, an insecticide application may be warranted.
Entomologists with Iowa State University suggest that economic thresholds need not depend on plant height. Table 1 provides some economic thresholds for your consideration, based on crop value, control costs, and leafhopper densities. In general, these thresholds are less conservative than those (based on plant height) mentioned previously for shorter plants and more conservative for taller (12 inches or more) stands.
Insecticides. Insecticides suggested for control of potato leafhoppers in alfalfa include the following: *Ambush (3.2 to 12.8 ounces per acre), *Baythroid 2 (0.8 to 1.6 ounces per acre), dime-thoate (see product label), Imidan 70W (1 to 1 1/3 pounds per acre), *Lorsban 4E (1/2 to 1 pint per acre), *Mustang (2.4 to 4.3 ounces per acre), *Pounce 3.2EC (4 to 8 ounces per acre), Sevin XLR Plus (1 quart per acre), and *Warrior (1.92 to 3.2 ounces per acre). Use of products preceded by an asterisk is restricted to certified applicators. Please follow all label directions and precautions.--Kevin Steffey and Mike Gray