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

Published: July 5, 2010

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 is a native insect which 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. Transport of this pest often occurs when leafhoppers actively fly into approaching storms and are carried great distances by low level winds which approach 100 mph in speed. Leafhoppers are usually associated with strong thunderstorms containing hail. After a storm passes, high numbers of leafhoppers can often be found in the trail of these storms. In Missouri, potato leafhopper adults generally arrive about 5 May 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. Damage is caused when both adult and nymphal (immature) leafhoppers use their piercing-sucking mouthparts to penetrate alfalfa leaflets and stems. They remove plant juices and often cause yellowing of established plants (hopper burn), stunted plant growth, and possible mortality of seedling alfalfa. Both forage quality and quantity are substantially reduced by this alfalfa pest.

This year potato leafhopper adults were found in alfalfa about May 10 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.