Alfalfa Continued from Page 10
shown to reduce alfalfa weevil larvae while also managing winter weeds. “In areas of Southern California, we can ‘sheep off’ the fields in February,” Rethwisch said, referring to the widely used sheep grazing practice. “The trouble is there are not enough sheep to go around, and there can be hoof damage after the rains,” Rethwisch said. Another management tool available to growers is to adjust their cutting schedule to deprive weevils of the best areas in the canopy to lay eggs. “Some people will do an early harvest to remove stems large enough for oviposi- tion,” Rethwisch said. Weevils tend to be an issue in the low desert in June and July, and in the inter- mountain region from April to June, or even as late as September. “What you’re going to see depends on where you are,” Rethwisch said. “Alfalfa weevils live up to three years; this is not a one and done.” The problem often multiplies, he add- ed, because the presence of weevils often means there is another destructive pest in the field. “Weevils are often accompanied by aphid infestations, and dealing with aphids is much more complex than weevils,” Rethwisch said. “We can have multiple
University of California researchers say weevil damage in alfalfa is usually accompanied by aphid infestations, which reduce yields.
aphid species that respond to insecticides differently in a field at the same time, and we can have multiple generations during a cutting cycle.” The most destructive aphid may be the blue alfalfa aphid, which injects a toxin that can kill smaller plants and reduce the yield of larger ones. “We are seeing multiple biotypes of blue alfalfa aphids, and some of them have in- secticide resistance,” Rethwisch said. This pest is, unfortunately, more mobile than weevils. When populations are very
high or when the alfalfa plant weakens, the aphids develop wings and take flight. “It can do some pretty big damage for us real quickly when they fly in,” Rethwisch said. Crop losses from weevils and the blue alfalfa aphids that often accompany them can be as much as 1,000 pounds an acre in the low desert and up to 2,400 pounds an acre in the intermountain region near the Oregon border. “It’s not unusual for us to see over a half-ton yield decrease from blue alfalfa
aphids,” Rethwisch said. Some widely used materials are of little help against this pest because they knock down populations of the large seven-spot- ted ladybug that provides biological con- trol, he added. “The pyrethroids did not give us much better control than the untreated plots because they killed off some of the benefi- cials,” Rethwisch said. (Bob Johnson is a reporter in Monterey County. He may be contacted at bjohn11135@gmail.com.)
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June 5, 2024 Ag Alert 11
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