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Controlling Weeds, Biological options exist for invasive weed control

by Wyoming Livestock Roundup

Many ranchers and rangeland managers are trying to find ways to halt prolific spread of cheatgrass, medusahead, leafy spurge, yellow star thistle and other invasive weeds and reduce or eliminate them in areas that are already dominated by them.

Jeremey Varley, Idaho Department of Agriculture Manager for noxious weeds, says the first step in any control program is knowing which weed is taking over pastures, fields or rangelands. 

“We also need to determine if the weed problem is beyond the threshold where other control measures might be better,” Varley says. “For instance, if it’s a small patch it might be easiest to just pull those plants. Or, maybe we can spray for three years in a row and it might be gone.”

  He adds, “We need to look at all options, and sometimes a combination of strategies might be best.” 


For instance, Varley recommends producers try mowing or grazing and biocontrol with insects. 

Intensive grazing with animals that eat a particular weed might provide an option for producers to control invasive species. Sheep and goats are often used for that purpose. Producers just need to make sure the weed is not poisonous to the animals that would be eating it. 

Some times of year, producers can graze invasive plants – especially when they are still young and palatable – and set them back. 

“With leafy spurge, many people have utilized goat grazing,” Varley comments. “Biocontrol agents have more effectiveness after grazing. The plants have new growth following grazing, and the new growth is more susceptible to the biocontrol agents.” 


“A producer might choose biologic control if weeds are widespread and beyond the point where they can economically cover the area with conventional control method,” Varley comments. “In that situation, biologic control may be the best tool. Just make sure to use the correct biologic control agent for the target weed.”

“For instance, the root weevil for spotted knapweed doesn’t do well in riparian zones or shaded, cool areas,” Varley notes. “These weevils prefer hot, dry conditions. If we release them next to a stream or where there are a lot of trees, where they don’t want to be, they won’t establish very well.”

For producers who need help in determining the best strategy for using biologic control, talk to a professional. Most states have a biocontrol specialist, federal contact or county weed superintendents.


Biocontrol research is ongoing, to find the best way to control certain invasive weeds. These agents are species-specific and affected by climate and habitat conditions. 

“An organization called the Center for Agriculture and Bioscience International (CABI) from Switzerland is doing a lot of research on biocontrol agents and always looking for species that will work on certain weeds,” Varley says. “They go to the native land of a plant we consider a noxious weed here to find the native range of that plant and its natural predators.”

CABI then determines the life cycle of the predator agent and makes sure it only feeds on that particular plant and can’t complete its life cycle on any other plant, says Varley. Then we know we can safely release these agents in a noxious weed patch in this country, and they won’t damage any other plants.

Various people fund this research. 


“For instance, a group concerned with rush skeleton weed is looking for a new biocontrol,” Varley says, noting that after CABI finds a potential biocontrol agent, they send it to the U.S. for more testing. “After all the studies are done, we can be sure that a certain biocontrol agent can be approved for release.” 

Each agent must go through this process to be approved for use in the U.S. It takes several years and a lot of money to bring biocontrol agents to this country, Varley comments. In some cases, these agents work well, but in other situations, they may not be as effective as hoped. 

“We keep studying these agents and following up to make sure they are working how we want them to. In many cases they do, by reducing seed count or plant vigor in the target weed,” says Varley.


However, Varley says producers may not see immediate results from releasing a population of insects because it takes between three and five years before the insect populations become establish. 

“It’s not like herbicide where we spray the weeds and go out the next day and find them dead. With some biocontrol agents, it may be five to 10 years to get good control of those weeds,” he says.

Monitoring the site with photos can show the reduction of a target species, over time. 

“These agents are living organisms, and it does take time for them to multiply and have a beneficial effect. They have to get their numbers up, but then they will do a good job – if they are in the right place, targeting the right species,” he explains.

Every region is unique in climate, and there are weeds in some parts of the U.S. that we don’t see in other areas. 

“Ranchers in southern parts of the country have to deal with weeds we will never see in Idaho,” Varley explains. “There are biocontrol agents available, however, for most noxious or invasive weeds and some that we haven’t discovered yet.” 

“Anyone facing a noxious weed problem, losing production or seeing a drop in land value because of invasive weeds should look at all tools available,” Varley says. “Biocontrol agents may provide the best benefits because they provide a long-term solution.”

  Heather Smith Thomas is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to

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