Protecting Alfalfa's Yield Potential from Pests and Weeds

Experts share tips on scouting, thresholds, and management for key alfalfa insect pests and weeds

Mar. 19, 2026 at 8:20am

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Alfalfa's high genetic yield potential can be reduced by a variety of insect pests and weeds if not managed properly. Experts recommend regular scouting, understanding economic thresholds, and using an integrated approach including cultural controls, resistant varieties, and targeted use of insecticides and herbicides to protect alfalfa stands and maximize forage quality and yield.

Why it matters

Alfalfa is a critical forage crop, but its yield potential is vulnerable to damage from insects like alfalfa weevil, potato leafhopper, aphids, and clover root curculio, as well as problematic weeds. Effective, timely management of these pests is essential for alfalfa growers to achieve the full genetic potential of their crop and maintain profitable, high-quality forage production.

The details

Key insect pests include alfalfa weevil, which can cause severe leaf damage, as well as potato leafhoppers, aphids, and clover root curculio, which impact plant health and yield. Weeds also reduce forage quality and harbor pests. Experts recommend regular scouting, using economic thresholds to guide treatment decisions, and an integrated approach combining cultural controls, resistant varieties, and targeted, rotated use of insecticides and herbicides.

  • Alfalfa weevil activity begins when temperatures reach about 48°F, with damage concentrated on the first cutting.
  • Potato leafhoppers migrate north from the Gulf Coast each spring, arriving in late spring or early summer.
  • Aphids can be problematic throughout the growing season, but natural predators often provide good biological control.

The players

Debby Samac

Supervisory research geneticist with the USDA Agricultural Research Service Plant Science Research Unit in St. Paul, Minnesota.

Marian Viney

Managing Editor at Progressive Forage, covering forage topics and serving as a trusted resource for hay, silage and pasture production.

Earl Creech

Researcher at Utah State University who provided key principles for controlling weeds during alfalfa establishment.

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What they’re saying

“The first phase is to go out in the field and do some sweeps, look in the net to see if there are any weevils or other insects, and if you find some, go to the second phase to determine if spraying is required.”

— Debby Samac, Supervisory research geneticist

“Even if the first cutting survives, toxins injected during feeding suppress regrowth and reduce yield in subsequent cuttings.”

— Debby Samac, Supervisory research geneticist

“Producers should avoid reacting too quickly. Insecticide treatment is rarely needed, unless populations exceed roughly 100 aphids per sweep – resistant varieties are available for several species, which add another layer of protection.”

— Debby Samac, Supervisory research geneticist

What’s next

Researchers continue to investigate reasons for the decline in effectiveness of parasitoid wasps that historically helped control alfalfa weevil populations in the eastern U.S.

The takeaway

Protecting alfalfa's full yield potential requires a proactive, integrated pest management approach that combines regular scouting, understanding economic thresholds, using resistant varieties, and judiciously applying insecticides and herbicides when necessary. This helps growers avoid unnecessary losses from key insect pests and weeds.