Two weevil varieties can give growers double headaches
Alfalfa growers in the central part of Nebraska should keep in mind that they may see two different varieties of weevils in their crop, said University of Nebraska–Lincoln (UNL) Butler County Extension Educator Mike Rethwisch.
The eastern strain usually invades the crop in time for the first cutting, Rethwisch said. The western strain, prevalent in the western two-thirds of Nebraska, peaks one to three weeks later. So growers may treat for one strain, then may have to treat again for the other.
To check for weevils, Rethwisch advised producers to use a sweep net. They will catch the caterpillar stage of the weevils in the net. Some growers look at the alfalfa plants, where small holes eaten in the leaves at the growing tip indicate weevil damage.
"If you see lacy leaves, there’s a good chance the crop is already damaged," Rethwisch said. "The importance of that damage depends on the proximity of harvest. With alfalfa pushing $100 a ton or more, it doesn’t take much to add up to a substantial loss."
"I have collected some clover leaf weevil larvae already this past week. Alfalfa weevil larvae should also be found if people begin to look," he said. "If the crop is less than 10 inches tall and the producer can find one weevil per stem, it’s probably time to treat."
UNL has some publications with charts that will help determine the economic threshold for applying insecticides at http://entomology.unl.edu/fldcrops/pestipm.shtml. Click on one of the choices under Alfalfa Weevil.
Larvae generally damage the first crop, Rethwisch said, while the adults damage regrowth by feeding on the developing crown buds.
Rethwisch also mentioned potato leafhoppers as an intermittent pest for Nebraska alfalfa. Although eastern alfalfa sustained huge populations of potato leafhoppers last year, weather patterns and the way the wind blows will determine whether it will return in 2008, he said. Since the insect doesn’t overwinter in Nebraska, it depends on winds to bring it back. Every year is different.
Grasshoppers may damage alfalfa sometimes and cutworms will attack it as well. Cutworms feed at night, so they’re not visible during the day. The alfalfa doesn’t green up after cutting.
"We’ll see some variegated cutworms somewhere in Nebraska this year; they’re just not as widespread as the weevils," Rethwisch said. — University of Nebraska-Lincoln Extension