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Weevils may slow alfalfa regrowth
Kansas
A scattering of grayish-silver alfalfa fields appeared across Kansas this spring. As a result, Kansas State University entomologists are hoping alfalfa growers and crop consultants will pay special attention to regrowth--particularly any slow regrowth--during the two weeks following the cutting that's already started in northern and central Kansas.
"Most early alfalfa weevil treatments were well-timed in infested areas. Unfortunately, though, a few growers with heavy alfalfa weevil populations didn't recognize the threat. The color change brought by weevil damage to their fields soon became obvious, even when seen from a distance," said Randy Higgins, field crops specialist with K-State Research and Extension.
Given the ample rain many areas have received recently, new alfalfa growth should be rapid.
"If it's not, you should check for alfalfa weevil adults. We're already seeing significant numbers of them in some fields," said Phil Sloderbeck, who is based in Garden City at K-State's Southwest Area Research and Extension Center. "While you're at it, you also could check for variegated cutworms and possibly for alfalfa weevil larvae in the leaf litter."
Heavy populations of weevil adults and surviving populations of larvae can lower production - in part, just by delaying new growth. In effect, the delay shortens this year's growing season.
"Growers with swathed and raked hay sitting in windrows may find the damage is much worse under those windrows, where pests are protected from the sun," added K-State entomologist Jeff Whitworth.
K-State recommends growers consider treating fields that have been cut when four to eight weevil larvae per square foot are present and regrowth is being delayed.
"We have less information on the treatment thresholds and product options for controlling adult alfalfa weevils," Higgins said. "But if you find stems that are 'barked'--have lost their outer tissue--and that damage is widespread, I'd certainly suggest you consider treating with a product that can be effective against adults."
"Barking" or tissue removal at the base of stems forces the plants to initiate and sometimes re-initiate all new growth from buds arising from the crown. This places insect-damaged plants under additional stress, Higgins said.
"If you're planning stubble treatments, however, remember that you'll need to remove any swathed hay first," Whitworth said.
K-State's "Alfalfa Insect Management 2005" publication notes that carbofuran, methyl parathion, and phosmet have shown reasonable results as adult alfalfa weevil controls. That publication, which also includes trade names, is available at county K-State Research and Extension offices, as well as on the Web at www.oznet.ksu.edu/library/ENTML2/MF809.pdf.
Date: 5/26/05
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