Poor Alfalfa Weevil Control Raises Concern Among Farmers

By Andi Anderson

Farmers in several regions are reporting poor control of alfalfa weevil after using pyrethroid insecticides. This has raised concerns about the development of resistance and the effectiveness of current pest control methods.

Resistance can develop when the same insecticides are used repeatedly over many years. Alfalfa weevil populations often remain in the same area because they feed mainly on alfalfa and clover. Continuous spraying of pyrethroids before the first crop cutting can create resistant populations within specific farms or regions.

Another important factor is the loss of natural pest control. In the past, beneficial insects such as parasitoid wasps helped reduce weevil populations. These natural enemies were introduced through long-term programs and helped maintain balance in the ecosystem. However, overuse of insecticides has reduced these helpful insects, making pest problems worse over time.

In recent years, many farmers have added insecticides to sprays as a preventive measure. While this approach saves time and cost in the short term, it can harm beneficial insects and increase pest resistance. This cycle of repeated spraying and rising pest populations is known as the pesticide treadmill.

Incorrect timing of spraying is another major issue. If insecticides are applied too early, they may miss peak larval activity. Weather conditions, such as cool temperatures, can delay larval development and reduce the effectiveness of earlier sprays. In such cases, the pesticide residue may not last long enough to control later emerging larvae.

When resistance develops, control options become limited. One alternative is indoxacarb, which works differently from pyrethroids and can be effective against resistant weevil populations. Older insecticides such as carbaryl and methomyl can also be used, but they have disadvantages such as harming bees or having shorter protection periods.

Overall, poor weevil control is often caused by multiple factors, including resistance, improper timing, and reduced biological control.

Recommendations:

• Regularly scout fields and spray only when pest levels exceed economic thresholds.
• Rotate insecticides with different modes of action to reduce resistance risks.
• Protect beneficial insects by avoiding unnecessary or early pesticide applications.

Photo Credit: gettyimages-gabrielabertolini