CALIFORNIA
Field Crops A SPECIAL GROWERS’ REPORT OF AG ALERT ®
With one effective insecticide available to farmers to combat the Western alfalfa weevil, right, University of California researchers say winter sheep grazing of alfalfa may be a way to control the pest, which causes considerable crop damage.
Advisor studies alternatives to treat pests in alfalfa By Bob Johnson
the next couple of years.” Western alfalfa weevils were first detected in Utah in 1904 and have since spread to cause damage throughout the Western states. Growers started applying hydrocarbon insecticides to control weevils in 1946, but by 1962 resistant pest populations began to appear. Beginning in 1969, the pesticide Furadan was used to control weevils and aphids. Resistance was not a problem with Furadan, but in 2009 regulators canceled its use in alfalfa. Since then, in addition to Steward, growers have used various pyrethroids and organo- phosphates, but resistance to these materials has developed. “Where they have been applying these products year after year, they are down to 15% control at best. It’s not a good picture,” Rethwisch said. The material Steward could also lose its effectiveness over time, he said, if resistance to the chemistry develops. Resistant weevil populations mate with susceptible weevils, which spreads the resis- tance to more fields, he said. One remedy, Rethwisch said, is employing sheep for winter grazing, which has been See ALFALFA, Page 11
With fewer pest control treatments available and as materials lose their efficacy, grow- ers are teaming up with researchers at the University of California to identify solutions to control weevils and aphids that threaten the alfalfa crop. “Alfalfa weevils oversummer as adults and lay eggs in the spring or late fall, depending on the strain,” said Michael Rethwisch, a UC Cooperative Extension farm advisor in Riverside County. “The eggs hatch, and the larvae feed on the alfalfa. This reduces the quality and reduces the yield.” In a presentation about management of California alfalfa pests as part of a UC Integrated Pest Management lecture series, Rethwisch said only one insecticide is reliably effective against Western alfalfa weevils. And help is not on the way, he added. “Steward (insecticide treatment) is the only thing getting better than 80% control of re- sistant weevil populations,” Rethwisch said. He noted the active ingredient in the product is an oxadiazine pesticide called Indoxacarb. “That basically leaves us with one product that can be used,” he said. “That’s dis- concerting because there’s nothing on the near horizon that’s going to be there in
10 Ag Alert June 5, 2024
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