Jeremey Varley
Various biological weed control agents have been approved for pasture and range use — most of them insects. Here, a knapweed root weevil goes to work on its host plant. Insects used for biological weed control are host-specific; they only attack the plant they’re adapted to, leaving other, more desirable plants alone.

Got invasive weeds in your pasture? Get some bugs

Using insects and other biological agents to control invasive weeds is catching on in cattle country.

During human immigrations into North America, thousands of foreign plant and animal species came along, too. About 1 in 7 of those hitch-hiking plants have become invasive, pushing aside native species.

If you make your living from livestock, that’s a problem. And that’s why ranchers and rangeland managers are constantly trying to find ways to halt the prolific spread of knapweed, leafy spurge, yellow star thistle and other invasive weeds. Paul Brusven, with the Nez Perce Biocontrol Center, Lapwai, Idaho, says biological controls — using insects that feed on certain plant species as their natural host — are proving successful. 

“Many of our weed problems originated in Europe or Asia in climates similar to North America, but in their native land they are part of the natural landscape, along with their natural checks and balances,” says Brusven. 

Those natural checks and balances do not exist in North America, but we have imported some of them to combat their host weed. “Biological control puts the natural enemies — plant, animal or insect — to work against invasive pests. Release of carefully selected natural enemies [usually insects] that are host-specific to a targeted weed reduces the vigor and reproductive potential of the non-native weed,” he explains.

The term “host-specific” is key. “It means that the biological control agent will die without its host weed. It also means that other plants that are part of the landscape — agriculture crops, native plants — will not be harmed by a biological control. The most common biological control agents are insects, but mites, nematodes, and plant pathogens are also used,” says Brusven.

And patience is a virtue. “Biocontrol is a sleeper because you don’t see an initial dramatic impact, like with spraying. It’s a long-term solution.”

Better than a hoe

Using multiple agents against a weed results in quicker control, but it usually takes at least three years to get the agent numbers high enough to reduce a weed population. Biological controls will not completely eradicate the weeds, but can reduce them to a desirable level.

“In the Pacific Northwest, biologic agents have shown success in control of Mediterranean sage, St. Johns wort [Klamath weed], tansy ragwort, Dalmatian toadflax and purple loosestrife. They are helpful in areas where other control methods [herbicides, mowing, hand-pulling, replanting competitive vegetation] are not practical. Biological control agents can also be used in conjunction with other control methods as part of an integrated pest management strategy,” he says.

There are five different biocontrol agents currently used for yellow star thistle, and they are all seed feeders. “We have 11 different biocontrol agents for spotted knapweed. Three are root feeders and the rest are seed-feeding weevils, flies and moths,” says Brusven.

Spotted knapweed is a tough perennial, which makes it harder to eliminate. “With a biocontrol agent continually hammering at the root system, and another agent hammering the seed, we slow the spread and interfere with its ability to pull up nutrients and moisture due to root damage,” he explains.

Leafy spurge can often be controlled with grazing. Sheep and goats can make a big difference, especially in large areas that would be difficult to spray.

“We are also having good luck with biocontrol using the aphthona beetle, a type of flea beetle that defoliates the plant. The beetle larvae develop in the root system of leafy spurge, and when adults emerge, they start eating the leaves. They damage the roots and then defoliate the plant,” says Brusven. There are six different species of aphthona beetles that can work for this.

“We’re also having good luck with a redheaded leafy spurge stem-boring weevil. For range and pasture management, those two agents are often used for leafy spurge control,” he says. 

Another big problem is Russian knapweed. “We are now rearing and providing some of the newer biocontrol agents for this weed. One is Aulacidea acroptilonica, a gall wasp that creates galls on the plant — in the junction where the branches come off the main stem. This reduces the biomass of the plant. The other is a gall midge, Jaapiella ivannikovi, that creates galls on the ends of the main and side stems, reducing the seed load,” he says.

“We also have a biocontrol agent for Dalmatian toadflax. From 2007 to 2014, we saw more than 70% reduction across Idaho, which is a huge success,” says Brusven. 

Weeds in sensitive areas, such as near streams, wetlands, lakes and other areas where it’s not a good idea to use chemicals, must be controlled with methods other than spray. “A weed called purple loosestrife grows in wet areas, but we have a very effective biocontrol agent for that one — little flea beetles. We are also growing a root-feeding weevil. We are rearing them on an artificial diet and distributing them in states throughout the U.S.,” he says.

Using biocontrol agents

To find out what you might be able to do on your ranch or range, you must identify the invasive plant and see if there is a biocontrol agent. You can get help from your county weed superintendent or Extension office. 

There is also a USDA APHIS (Animal and Plant Health Inspection Service) office in every state. “We get some of our base funding through USDA, and coordinate things across state boundaries,” says Brusven. Interstate permits are managed and regulated through USDA APHIS. Local counties also work with state departments of agriculture.

Ranchers can obtain biocontrol agents through insectaries, and sometimes from county control programs. Costs for biocontrol agents may range from $35 to $200 per release, depending on the insect species and availability. “Releases may be free through scheduled educational workshops,” Brusven says.

For a successful release, dump the insects (50 to 1,000 of them, depending on the type of insect) at one spot on the edge of the weed infestation rather than sprinkling them through that area. “Keep initial releases together and concentrated as long as possible to promote mating and egg laying. Early in the morning or in the evening is best, when air temperatures are cooler, to increase chances of successful establishment,” he says.

Once released, the insects establish on the host weed and continue to multiply over time. “After the biological control agents are released, do not spray directly over this area for at least three years. The agent needs adequate food and proper habitat to sustain and build its population. These insects will die without their host weed. Leave at least 1 acre (ideally 5 acres) for your new agent to increase in population. Perimeter of the release area can be sprayed to keep the weed from spreading while the agent population builds in numbers,” Brusven says.

Finding successful control agents

“The cost to study and test a new biological control agent [just one insect species] and get it approved is about $1.5 million, due to the regulatory process to make sure we are not importing another invasive species,” says Brusven. “These programs have an outstanding safety record for over 100 years because we’ve been making sure the agents we are using are host-specific to just the target weeds,” he says.

Biocontrol research is ongoing to find the best way to control certain invasive weeds, says Jeremey Varley, Agriculture Section manager, Noxious Weeds, Idaho State Department of Agriculture. “An organization called CABI [Center for Agriculture and Bioscience International] from Switzerland is doing a lot of research and looking for species that will work on certain weeds. These researchers go to the native land of a plant we consider a noxious weed here, and find its native predators. They do tests to determine the life cycle of the predator agent, and make sure it only feeds on that plant. We want to make sure that agent can’t complete its life cycle on any other plant.”

Various groups find funding for this research. “One group is concerned with rush skeleton weed, looking for a new biocontrol. After they find one, it will be brought here for more testing. Biologic control agents have to pass through USDA APHIS and the Technical Advisory Group [TAG]. After all the studies are done, we can be sure that a certain biocontrol agent can be approved for release,” he says. 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. In some cases, they work well, but in other situations are not as effective as hoped. “We keep studying them, to make sure they are working how we want them to. In some cases they do, by reducing seed count or plant vigor in the target weed,” says Varley.

Worldwide, 133 weed species have been targeted, and more than 350 biological control agents introduced into 70 countries. “We are seeing increased support of this control method. Land managers have realized there are very few effective and sustainable tools other than biological control to manage invasive weeds on large landscapes,” Brusven says.

Smith Thomas is a rancher and freelance writer based in Salmon, Idaho.

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