Reports of Potato Leafhopper Injury Are Numerous

June 18, 1999
Duane Frederking, field sales agronomist with Pioneer Hi-Bred, has observed very impressive densities of potato leafhoppers in many alfalfa fields across central Illinois. One particular field in Sangamon County had as many as six to eight leafhoppers per sweep! During the critical regrowing stage, alfalfa plants are extremely susceptible to feeding injury by potato leafhoppers. In fact, it takes only 0.2 leafhopper per sweep on 3-inch stubble and 0.5 leafhopper per sweep on 3- to 6-inch plants to cause economic problems.

Because potato leafhoppers will remain for the duration of the growing season, they have a longer period of opportunity to create challenges for alfalfa producers. Potato leafhoppers not only reduce yields but also may have a significant negative effect on the nutritional quality of hay and impair the vigor of a stand. Growers should begin scouting alfalfa fields now and at least on a weekly basis 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 about 1/8 of an inch in length (Figure 2). 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 backward ("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 they arrive in the Midwest, the females begin to lay eggs in stems and larger leaf veins. The eggs hatch into nymphs in 6 to 9 days. 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 piercing­sucking 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 sweep net. The purchase of a 15-inch (diameter) sweep net is 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.

As reported last year, entomologists with Iowa State University suggested 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 stands (12 inches or more).

Insecticides suggested for control of potato leafhoppers include the following: *Ambush 2E (3.2 to 12.8 ounces of product per acre), *Baythroid 2 (0.8 to 1.6 ounces of product per acre), dimethoate (see product label), Imidan 70-W (1-1/3 pounds of product per acre), Lorsban 4E (1/2 to 1 pint of product per acre), *Penncap-M (2 to 3 pints of product per acre), *Pounce 3.2EC (4 to 8 ounces of product per acre), Sevin XLR Plus (2 pints of product per acre), and *Warrior T or 1E (1.92 to 3.2 ounces of product per acre). Those products preceded by an asterisk may be applied only by a certified applicator. Please follow all label directions and precautions.--Mike Gray

Author: Mike Gray