No. 12/June 12, 1998
Numbers of Potato Leafhoppers Are Increasing
Entomologists in adjacent states and several observers in Illinois have reported that numbers of potato leafhoppers are increasing. One observer indicated that 8 of 10 alfalfa fields in northwestern Illinois have numbers of leafhoppers that exceed published economic thresholds. The increase in densities of leafhoppers in alfalfa is not unusual after the first cutting has been made, so alfalfa growers and their advisers need to monitor alfalfa fields carefully during the next few weeks to avoid serious losses of yield and hay quality.
The adult potato leafhopper is a small (about 1/8 inch), yellow-green to green, wedge-shaped insect with piercing-sucking mouthparts. It does not overwinter in Illinois. Rather, potato leafhopper adults are carried northward from southern states on wind currents in the spring. Shortly after they arrive in alfalfa fields in the Midwest, the females lay eggs in stems and larger leaf veins. The eggs hatch in 6 to 9 days into the immatures, or nymphs. Young nymphs range in color from bright yellow to yellow-green. They resemble the adults, except they lack wings and are more yellow. Characteristically, nymphs walk sideways rapidly when they are disturbed.
Both nymphs and adults cause injury by sucking sap from alfalfa plants and plugging up vascular tissue. The most typical symptom initially is a wedge-shaped yellowing at the tips of leaflets, often called "tipburn" or "hopperburn." However, growers should not wait until symptoms of injury appear before scouting for leafhoppers. When injury appears, some yield and quality have already been lost. Therefore, sampling for leafhoppers needs to be accomplished while the alfalfa still looks healthy.
Scout for leafhoppers by using a 15-inch-diameter sweep net. Economic thresholds are based upon numbers of leafhoppers per sweep of a sweep net, not upon the appearance of injury. Remember that the "static" economic thresholds for potato leafhoppers vary with the height of alfalfa. An insecticide application might be warranted if the following combinations of leafhopper numbers and alfalfa height are reached or exceeded: 0.2 per sweep in alfalfa 0 to 3 inches tall; 0.5 per sweep in alfalfa 3 to 6 inches tall; 1 per sweep in alfalfa 6 to 12 inches tall; and 2 per sweep in alfalfa 12 inches or taller.
Research results recently published by entomologists at Iowa State University indicate that economic thresholds need not depend upon plant height. They used regression analysis with economic injury levels to generate economic thresholds. The thresholds suggested in the previous paragraph were not based upon economic injury levels, so the more recent research is more robust. They developed a chart of economic thresholds of potato leafhopper adults per 10 sweeps, depending upon crop value ($ per ton) and control cost ($ per acre). That information is modified to reflect numbers of adult leafhoppers per sweep in Table 1. In general, these thresholds are less conservative than those listed in the previous paragraph for shorter alfalfa, and more conservative than those listed in the previous paragraph for taller (12 inches or taller) alfalfa.
Table 1. Economic thresholds for potato leafhoppers (numbers per sweep) at different crop values and control costs (from Iowa State University)
($ per ton)
|$6 per acre||$8 per acre||$10 per acre|
Insecticides suggested for control of potato leafhoppers in alfalfa arepresented in Table 2. Please follow all label directions and precautions.
Table 2. Suggested insecticides for control of potato leafhoppers in alfalfa
|Insecticide||Rate of application|
|*Ambush 2E||3.2 to 12.8 oz|
|*Baythroid 2||0.8 to 1.6 oz|
|Dimethoate 400||0.5 to 1 pt|
|Imidan 70WP||1.33 lb|
|Lorsban 4E|| 0.5 to 1 pt|
|*Penncap-M||2 to 3 pt|
|*Pounce 3.2EC||4 to 8 oz|
|Sevin XLR Plus||2 pt|
*Use restricted to certified applicators only.
Kevin Steffey (firstname.lastname@example.org), Extension Entomology, (217)333-6652