Alfalfa weevil provides challenges in Big Horn Basin for forage producers

With growing challenges frequently coming in the form of fertility, moisture and weeds for forage producers in northwest Wyoming, alfalfa weevil and cutworms can also present a problem, as the endemic bugs can severely decrease yields in alfalfa fields.

“Weevil is a rough bug to deal with,” comments Jeremiah Vardiman, University of Wyoming (UW) Extension specialist in Park County. “We’ve had it here in Wyoming since the 1950s, so it’s nothing new.”

Changes

However, Vardiman notes the presence of weevil has changed in fields over time. 

“Recently, producers have told us there’s been a change in the timing as to when they see weevils,” he explains. “In recent years, the weevil has appeared in first-year stands, where producers might not have seen large populations until the third or fourth year of a stand in the past.”

“In the past, they only sprayed weevils once the year they found it, but now they have to spray once – if not twice – every year,” Vardiman continues. “We’re no longer breaking the pest cycle, which is a concern.” 

Looking at weevil

Alfalfa weevil is a snouted beetle that is slightly bigger than a grain of rice. During the summer months, it can be found in alfalfa fields, and it spends winters in the crown of the alfalfa or in the grass at the edges of fields. 

“Now, we also see weevils moving up the dry canyons in Utah, for example,” Vardiman says. “They come back to the fields in the spring and lay their eggs.”

Each female lays between 500 and 800 eggs in the stalk of the alfalfa plant. 

“The eggs hatch and release larvae that are grub-like,” Vardiman explains. “The grubs cause the damage to the plant.” 

The larvae move to the tips of alfalfa, eating the tips of the leaves during the third and fourth instar stages. By the fourth instar stage, they are visible to the naked eye and cause the majority of damage to the plan. 

“Recent studies say we are now seeing multiple hatches per year, but we don’t know why,” he says. “We see two and even up to four hatches of weevils each year.” 

Taking action

For producers, addressing alfalfa weevil means diligent monitoring and spraying of insecticide for control.

“We can only control the weevils that get hit by insecticide,” Vardiman says. “If they’re inside the whorl of the leaves, they’re protected and can survive.” 

Additionally, insecticides have no impact on adult weevils. 

“With multiple hatches coming, it is hard to spray just once and see weevil control,” he comments. “If we’re selling hay, it’s hard to be out multiple times in a sprayer and make it worth our money, so producers should be careful with their management.” 

Working with Extension, producers may be able to determine the optimal time to spray using a growing degree day calculator. 

Anecdotal information

“We have seen anecdotal information recently about a gentleman in Fremont County who raises alfalfa but doesn’t see any problems with weevils,” Vardiman comments. “He grazes his pastures with horses in the winter and early spring.” 

Vardiman says it is possible that the large number of horses from a pack string depletes the weevil population through crushing and habitat removal. 

He adds, however, “I’m not encouraging everyone to graze all their alfalfa fields with large numbers of horses though.”

Cutworms

While alfalfa weevil is a challenge for many producers, the emergence of cutworms resulted in partial control of weevil. 

“A producer in Laramie County sprayed the border of their field for the purpose of controlling weeds and Army cutworms,” Vardiman says. “They found the strip that was sprayed had really good control of cutworms and weevil both.” 

He continues, “However, the middle of the field was riddled with weevils.”

“We need to talk about weevil control and how we can achieve control into the future,” Vardiman adds.

Vardiman was one of a series of speakers who looked at forage production issues at 2019’s WESTI Ag Days, held in mid-February in Worland. 

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to roundup@wylr.net