Growers need to stay on top of pests

Alfalfa is a shining star, but because it is considered a cool weather crop growers need to be vigilant in tracking pests.

Jeff Whitworth, associate professor of entomology, Kansas State University, said alfalfa is a cool weather crop and those fields are where scouting should start early and continue to the end of the growing season. Whitworth was a presenter at a virtual Alfalfa U.

He called alfalfa weevil potentially the No. 1 pest every year, followed by army cutworm.

Alfalfa weevils actively mate and lay eggs in alfalfa stems during the fall and spring. The eggs, he said, are hard to kill. In early spring eggs hatch and larvae become active after 48 degrees. And he has watched eggs hatch in early February and that makes it hard to treat with insecticide.

“Treatment timing is key,” he said. “If larvae are in the pupa stage it is too late.”

An adult can stay active into late spring and early summer if conditions are right and cause “barking” in the plant. Alfalfa weevil in some years an adult can stay active into spring or June in a wet year.

Spring temperature plays an important role in the treatment and because of that unpredictability alfalfa weevil are difficult to control, he said.

A cold spell in spring, while tough on other crops, can kill larvae. Insecticides can work but timing is more important and growers should stick to routines that work.

Army cutworm originates from a moth that migrates from Rocky Mountains in late summer. Eggs are laid in the soil and the larvae hatch in fall and feed until cold weather sets in. They resume feeding anytime the temperature exceeds 50 degrees Fahrenheit.

“Primarily damage is noticed in the spring but larvae is there in the fall but is not as noticeable because they are small,” Whitworth said.

A mature army cutworm consumes plant matter as it gets bigger and usually finishes feeding by late April. “Larvae will feed on grasses and hide in clods under plant residue and cracks.” The easiest way to find an infestation is when the grower observes a flock of birds feeding in wheat and alfalfa fields.

The moth is the army cutworm adult and is commonly called the miller moth. The moth usually dies in the fall and they are attracted to lights. Moths can lay many eggs and outbreak years are generally preceded by a dry July and a wet fall. They can cause 2 to 4 feet damage in plants in established stands and 1 to 2 feet in newly seeded stands.

To watch Whitworth’s full presentation, and the rest of the Alfalfa U sessions, visit The event was sponsored by John Deere, Alforex Seeds, Ward Labs, Staheli West, American Ag Credit, Harvest Tec and High Plains Journal.

Dave Bergmeier can be reached at 620-227-1822 or [email protected].