No. 7 Article 6/May 12, 2006

Potato Leafhoppers Have Arrived in Some Alfalfa Fields

Earlier this week, Matt Montgomery, crop systems extension educator, Sangamon-Menard Extension Unit, reported that potato leafhoppers could be found in alfalfa. We anticipate increasing reports in the coming weeks with respect to this migrant insect. Producers in the southern third of the state (where the first cutting has already occurred) need to pay close attention to potato leafhopper numbers and the potential for injury to regrowth over the next several weeks. Potato leafhoppers not only reduce yields, but they also may have a significant negative effect on the nutritional quality of hay and may impair the vigor of a stand. Growers should begin scouting alfalfa fields now and continue at least weekly throughout the growing season. Potato leafhoppers will be with us through the first several hard frosts.

What do potato leafhoppers look like? Adult potato leafhoppers are lime-green, wedge-shaped insects approximately 1/8 inch in length. Adults have fully developed wings and are very active fliers. The nymphs, smaller versions of the adults, are yellowish green and lack wings. When disturbed, the nymphs move sideways or backwards (with "crablike" movements).

What kind of life cycle do leafhoppers have? 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 six to nine days into nymphs. Multiple generations are accomplished throughout the summer, with leafhoppers persisting until cooler fall temperatures return.

How do leafhoppers injure plants? 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.

What's the best way to scout for potato leafhoppers? Invest in a 15-inch (diameter) sweep net, a valuable insect sampling tool for alfalfa producers. 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 2 provides some economic thresholds for your consideration based on crop value, control costs, and leafhopper densities. In general, their thresholds are less conservative than those mentioned in the previous paragraph for shorter plants and are more conservative for taller (12 inches or more) stands.

Insecticides listed in the 2006 Illinois Agricultural Pest Management Handbook for control of potato leafhoppers in alfalfa include the following: *Ambush 25W, *Baythroid 2, dimethoate, Imidan 70W, *Lorsban 4E, *Mustang Max, *Pounce 3.2EC, *Proaxis, Sevin XLR Plus, and *Warrior. Products preceded by an asterisk may be applied only by a certified applicator. Please follow all label directions and precautions.

Potato leafhopper adult (courtesy of Matt Montgomery, Menard-Sangamon Extension Unit).

Potato leafhopper adult close-up (courtesy of Matt Montgomery, Menard-Sangamon Extension Unit).

--Mike Gray

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