Many of Missouri's Alfalfa Fields Support High Numbers of Potato Leafhoppers

Published: August 1, 2011

The potato leafhopper is a pest of alfalfa during growth of second and third alfalfa harvests. At present, numerous potato leafhoppers remain in most alfalfa fields where they may cause severe damage to alfalfa plants. Both quality and quantity of alfalfa can be significantly reduced by this pest. The most common symptom observed is yellowing of established plants called hopperburn. This is caused when adult and nymphal (immature) leafhoppers use their piercing-sucking mouthparts to penetrate alfalfa leaflets and stems. They remove plant juices causing sugars produced during photosynthesis to be held in the upper plant leaflets and stems causing yellowing, stunting, and possible senescence of leaf tissue. Heavy damaged plants may die, especially seedling alfalfa. Alfalfa plants under drought stress are most at risk of plant mortality.

Potato leafhoppers are about 1/8-inch in length, wedge shaped, and lime green to greenish-yellow in color. They are very mobile and quickly move sideways, jump, or fly when disturbed. This insect migrates into Missouri each spring from more southern states and Mexico. Recent numerous spring and early summer storms moving into the state from more southern locations of the US have transported high numbers of adult potato leafhoppers into the state, especially into western and northern counties. In Missouri, potato leafhopper adults generally arrive about May 5th of each year. The arriving adults generally feed initially on several tree species before moving to alfalfa to feed and reproduce. Two to three generations of potato leafhopper are often produced with economic damage generally occurring on alfalfa following removal of first harvest. This year potato leafhopper adults were found in alfalfa about May 10th in central Missouri. Several flights of migrating leafhopper adults along with successful reproduction have resulted in economic infestations of this pest in most areas of Missouri

Scouting for this pest is best accomplished using a 15-inch diameter sweep net. Take 10 pendulum sweeps at five random locations in the field. If the average number of potato leafhopper adult and nymphs per sweep reach or exceed the threshold numbers listed below, treatment is justified. The economic threshold for potato leafhopper in alfalfa depends on the height of the alfalfa and whether the alfalfa is a potato leafhopper resistant variety or a traditional alfalfa variety. It is especially important to determine potato leafhopper numbers after removal of an alfalfa crop by harvesting. Remember that whether a field contains a traditional alfalfa variety or a PLH resistant variety, the economic threshold for alfalfa growth 3-inches or less in height is about 5 times more susceptible to potato leafhopper damage than 8 to 10-inch tall alfalfa. These data indicate that the economic threshold for newly harvested alfalfa plants is an average of 1.0 or more potato leafhoppers per 5 sweeps for a traditional variety or 1.8 leafhoppers or more for PLH resistant varieties per 5 sweeps.