Farm Progress

Time to sweep fields for pests that damage the quality and quantity of alfalfa.

April 26, 2017

2 Min Read
CLEAN SWEEP: Researchers at Cornell University sweep alfalfa trials for leafhopper. A University of Missouri agronomy specialist says farmers should take the same approach.

By Max Glover

Alfalfa stands are most vulnerable to potato leafhopper immediately after the first cutting, and when re-growth is slowed by excess rainfall. Early May is when potato leafhopper adults generally migrate into Missouri, though it may occur earlier depending on weather. In most years, these adults and their offspring will build in number and may severely damage alfalfa plants.

Potato leafhopper adults are very small, about an eighth of an inch in length, wedge-shaped, and greenish-yellow in color. They are very mobile and quickly move sideways, jump or fly when disturbed. Even if a field has severe infestation of potato leafhopper, do not expect to see the insects while just walking through the field, since they are very small and move fast. A fine mesh insect net is the only practical way to check for the presence of potato leafhopper.

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LOOK TO THE LEAF: Potato leafhoppers will appear after the first cutting of alfalfa stands. Farmers should look for a bright yellow pest on leaves in early May. (Photo by Cornell University)

This is a native insect, which migrates into Missouri each spring from Southern states and Mexico. The potato leafhopper is often transported into the state by late-winter and 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 (jets) which approach 100 mph in speed. Arrival of leafhoppers is 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. The arriving adults may feed initially on several tree species before moving to alfalfa to feed and reproduce. Immature leafhoppers, called nymphs, look very similar to the adult stage, except they possess wing pads instead of functional wings. Two to three generations of potato leafhopper are often produced each year, 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, stunting of plant growth, and possible mortality of seedling alfalfa.

Both forage quality and quantity are reduced by this alfalfa pest. Alfalfa plants damaged by potato leafhopper feeding will often turn yellow; sugars from photosynthesis are trapped in the plant foliage and cause the change in color to yellow, commonly referred to as "hopper burn."

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 leafhoppers per sweep (adult plus nymphs) reaches or exceeds the economic threshold numbers listed below, treatment is justified.

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Glover is an agronomy specialist with the University of Missouri Extension.

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