Potato Leafhopper Numbers Increasing In Alfalfa Fields

Published: May 30, 2008

Although alfalfa weevil larvae are still causing problems in Missouri, potato leafhopper adults have arrived and are increasing in numbers in alfalfa. Potato leafhoppers are about 1/8-inch in length, wedge shaped, and 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. The potato leafhopper is often transported into the state by early spring storms which move in a northeast direction. The leafhoppers are thought to actively fly into the storms and be 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 the storms. In Missouri, the potato leafhopper adults generally arrive about 5 May of each year. However in 2008 they were first found about a week later on May 14 in central Missouri. The arriving adults may 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 nymph (immature) leafhoppers use their piercing-sucking mouthparts to penetrate alfalfa leaflets and stems. They remove plant juices and often cause yellowing of established plants, stunted plant growth, and mortality of seedling alfalfa. Both forage quality and quantity are reduced by this alfalfa pest. Scouting for this alfalfa 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. Recently cut alfalfa is at highest risk from potato leafhoppers as alfalfa plants just recovering from harvest can only tolerate 1 leafhopper in 5 sweeps if a traditional, non-PLH resistant alfalfa variety is grown. If a PLH resistant variety of alfalfa is used, then the threshold increase by a multiple of 3, so the economic threshold level increase to 1 leafhopper in 15 sweeps for alfalfa less than 3-inches in height. Similarly, a traditional alfalfa variety 8-10 inches in height has a threshold of an average of one or more leafhoppers per sweep, whereas a PLH-resistant variety can withstand 3 potato leafhoppers before treatment is justified.