Dakota Farmer

Stop pests from bugging alfalfa

Scout and manage alfalfa fields from pests to save quality and yields.

Sarah McNaughton, Editor, Dakota Farmer

May 28, 2024

4 Min Read
alfalfa
PEST PARTY: A myriad of sucking and chewing insects can make themselves at home in alfalfa fields, robbing both quality and yields. FMC technical service manager Eric Rebek says a good management plan hinges on scouting early and often during the growing season.GoodLifeStudio/Getty Images

Pests in any crop can lead to significant yield loss and a drop in crop quality. Alfalfa fields have more than one pest that can bug with high yields.

Eric Rebek, FMC technical service manager in Minnesota and parts of the Dakotas, says that pests like alfalfa weevils, caterpillars, grasshoppers, potato leafhoppers and aphids are all found in fields across the Northern Plains. Scout thoroughly often, and have a management plan in place for control.

Scout for thresholds

“There’s a wide range of pests affecting alfalfa crops: leaf-feeding insects and caterpillars, sucking pests such as potato leafhopper, the green clover worm that is also doing some damage, and grasshoppers are always an issue for multiple crops later in the season,” Rebek says.

With more than just alfalfa weevils to keep track of this year, he recommends growers start scouting early and continue scouting all season long. “We’ve had some reports of alfalfa weevil adults coming out of hibernation the past few weeks, and in southeast South Dakota, they’re coming out of dormancy.”

Sweeping with a net is a good method to determine thresholds for pests, along with checking for predatory bugs in fields. “Predators and parasitic wasps in particular really keep that population of pests low and maintained below economic thresholds,” he says. “Over-reliance of insecticides like pyrethroids can do some damage to those natural enemy populations. It really does pay to scout, using a sweeping net to look for those pests and also potential activity of predators and parasites.”

“In addition to using that sweep net, we can go out and start looking for signs of activity and feeding damage that’s going on in that crop,” Rebek says, referring to another way to scout effectively. “[These include] notching of the leaves, pinholes in leaves that alfalfa weevil larvae will leave behind, [and] discoloration can be seen in the leaves as a result of sucking pests like potato leafhoppers and aphids.”

While some might not like to rely on economic thresholds, Rebek says these thresholds are heavily researched and should be used as a marker of when to treat fields. “The cost of control is a factor that goes into determining economic thresholds or that action threshold when we need to treat with an insecticide to get on top of those insects,” he says. “Some growers might not have a lot of faith in economic thresholds, and they can get antsy when they see pests build up.”

But he explains the data that goes into creating economic thresholds, noting, “These thresholds are built on years and years of studies and refinement of those thresholds through further studies as time goes on, with management practices improving and as the economics of the crop change.”

For growers, these economic thresholds serve as a trigger point for making management decisions.

Manage effectively

Knowing which pests are afflicting fields can give growers a different gameplan to manage populations. “With alfalfa weevil, for instance, just the act of cutting can get rid of a lot of those weevils in the field,” Rebek says. “Cultural management is one strategy, along with insecticides playing a big role in managing, and again, that’s where those economic thresholds are so important.”

Although many broad-spectrum insecticides are available, Rebek urges growers to not rely on these too much. “That’s going to help us with resistance issues first, and also going to help maintain those natural enemies and get that biological control from predators and parasites,” he says. “Softer chemistries, like our product Steward EC insecticide, it has a favorable profile in terms of not harming predators.”

Conserving predators and parasites and reducing the number of sprays based on economic threshold, Rebek says, is an ideal way to maintain a natural level of control in the field. “Rotation is going to be very important in that spray management, just to make sure we don’t hammer those bugs with the same thing every time, cause that’s how we build up a resistance to any particular product.”

Growers can use products such as chlorpyrifos, which is back on the market, or pyrophosphate. If there are issues securing these chemicals, Rebek says, there is an alternative.

“We have another product called Dimethoate 400 EC insecticide that belongs to the same chemical family as chlorpyrifos," he says. "It’s an organophosphate that works great on both alfalfa weevil larvae and aphids, and we’ve seen success using it with a product like Mustang Maxx insecticide."

For more management methods to knock out alfalfa pests, contact your local FMC dealer or visit fmc.com/en.

About the Author(s)

Sarah McNaughton

Editor, Dakota Farmer, Farm Progress

Sarah McNaughton of Bismarck, N.D., has been editor of Dakota Farmer since 2021. Before working at Farm Progress, she was an NDSU 4-H Extension agent in Cass County, N.D. Prior to that, she was a farm and ranch reporter at KFGO Radio in Fargo.

McNaughton is a graduate of North Dakota State University, with a bachelor’s degree in ag communications and a master’s in Extension education and youth development.

She is involved in agriculture in both her professional and personal life, as a member of North Dakota Agri-Women, Agriculture Communicators Network Sigma Alpha Professional Agriculture Sorority Alumni and Professional Women in Agri-business. As a life-long 4-H’er, she is a regular volunteer for North Dakota 4-H programs and events.

In her free time, she is an avid backpacker and hiker, and can be found most summer weekends at rodeos around the Midwest.

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