The Big Sandy Mountaineer -

Insecticide-Resistant Alfalfa Weevils Found in Bighorn County

Kevin Wanner (MSU Extension Entomologist) informed Extension that the alfalfa weevil (Hypera postica) has developed resistance to pyrethroid insecticides in Bighorn County. He also put together an article on the subject. Below are a few segments taken from the article. After years of exposure, insect populations develop resistance to insecticide toxins, making them ineffective. Changing the genetic makeup of insect populations can produce alarming and costly consequences when materials are no longer effective.

Pyrethrum, the “precursor” to pyrethroids, was derived naturally from chrysanthemum flowers and used as a natural insecticide as early as the first century A.D. Beginning in 1968, synthetic versions were produced by agricultural chemical companies, yielding active ingredients like bifenthrin, cyfluthrin, cyhalothrin, cypermethrin, deltamethrin and permethrin.

Many products with trade names such as Baythroid, Grizzly Too, Mustang-Maxx, Paradigm, Pounce, Proaxis, Silencer and Warrior II have active ingredients in the pyrethroid class. With growing issues of pyrethroid resistance, these products will no longer effectively control alfalfa weevils. Larger alfalfa weevil populations cause greater forage loss, and their management is becoming more difficult and expensive.

If a producer is in a pocket of high resistance, there is only one viable and effective alternative insecticide called Steward. However, Seward can also develop resistance if used repeatedly. At some point, pyrethroids may become effective again, but a rotation would need to rely on them minimally.

If a producer is in an area with no or moderate resistance, reduce the use of pyrethroid products. Survey fields and apply insecticide only when economic thresholds have been met. Avoid routine use of pyrethroids including early applications mixed with herbicide. Wait to know whether threatening populations are present before spraying. Rotate Steward and a pyrethroid with non-insecticide options like swathing. Do not apply Steward or a pyrethroid more than once every three years. Apply higher label rates, optimize timing and use adjuvants that improve coverage and persistence.

Montana State University, U.S. Department of Agriculture and Montana C