Potato leafhoppers continue to infest alfalfa,
exceeding economic threshold levels
COLUMBIA, Mo. – Potato leafhopper adults and nymphs continue to infest alfalfa throughout the state with many infestations exceeding economic threshold levels, said a University of Missouri agronomist. When these levels are exceeded, spraying is advisable.
Potato leafhopper populations are extremely heavy this season with many fields above economic thresholds by three to four times, said Wayne Bailey.
“Most fields of alfalfa are exhibiting the wedge-shaped yellow coloration on foliage referred to as hopper burn,” said Bailey.
This type of damage indicates stunting of plants and reduction in forage quality, he said.
In Missouri, two or three generations of potato leafhopper often are produced each year with economic damage generally following removal of the first harvest and possibly on second and third cutting, he said.
Newly sown fields or those just beginning to regrow following harvest are most at risk. More established and taller alfalfa plants can better withstand attacks from the pest, he said.
Potato leafhopper resistant varieties also can withstand higher populations before threshold is reached.
Scouting fields is best accomplished taking 10 pendulum sweeps at five random locations using a 15-inch diameter sweep net in the field and then determining the average number of insects per sweep.
The economic threshold for non PLH-resistant varieties of alfalfa with 8- to 10-inch stem length is an average of one insect per sweep. That number becomes two when the stems reach 12 to 14 inches.
If a producer is growing a PLH-resistant variety, multiply the average number per sweep by three to determine an appropriate economic threshold level. If the average number of adult and nymphs per sweep reach or exceed these threshold numbers, treatment is justified.
Treatment of potato leafhoppers on alfalfa is best achieved with an insecticide application. Unlike alfalfa weevils, which respond to several different control options such as early cutting, grazing or beneficial insects, control options are very limited for potato leafhoppers, Bailey said.