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Depending on the weed, you might use targeted grazing or beneficial insects, scientists advise.

Heather Smith Thomas

April 15, 2021

6 Min Read
The root weevil -- Cyphocleonus achates -- is native to southern Europe and is used as an agent of biological control against noxious knapweeds.Paul Brusven

Many ranchers and rangeland managers try to find ways to halt prolific spread of invasive weeds. The first step in any control program is knowing which weed is taking over your pastures, fields or rangelands. Depending on the weed, you might use targeted grazing, and/or insects that feed on those particular weeds.

Shelby Filley, Professor, PhD, PAS (Oregon State University Extension, Douglas County Regional Livestock & Forages Specialist) says some of the “bugs” that feed on a certain weed may already be there but we don’t notice them. “People often call us, asking where they can buy the bugs, and we help educate them about what these biological control agents look like,” she says.

The Oregon Department of Agriculture is responsible for the biological control program in Oregon. “They tell us these bugs are everywhere and that we can collect them if we think we don’t have enough. You can collect bugs from a patch that has them, and take them to your place where you might need more of them,” she explains. 

“Oregon State University has a new integrated pest management program called Oregon IPM Center. I’m not sure if all Departments of Agriculture in the different states have a similar program, but use of biocontrol agents is now quite popular. There’s at least one commercial business that sells them,” says Filley.

She thinks counties could have field days to catch some of these bugs and show people what they look like, the plants they feed on, and which bugs might be useful to reduce viability of certain invasive plants.  “People could learn what time of year they can find them, etc. because they are all different,” she says. Proper timing is also important for placing them in weed patches.

“There are many types of larvae that feed on certain plants. I am not sure whether you’d be catching and transferring the larvae or the adults, but the Oregon Department of Agriculture has an entomologist who could work with you on this.”

Most people don’t understand the lag time between plant population fluctuations and the insect populations. “These cycles are in continual flux. When there’s a lot of weeds, the insect population increases; there are many bugs eating them.  Then the weed population drops, and the bug population drops because the weeds are their food source. They don’t kill all the plants because they depend on them for food. They control these weeds but don’t eliminate them,” she explains.

Thus there is a period of time when the bugs are in low numbers and the weeds start coming back. “Then we get phone calls from people saying the bugs aren’t working. We have to tell them this is the lag phase—a natural part of the cycle—and they just need to be patient.”

Some environmental conditions such as hot, dry weather or extremely cold weather can be hard on these insects and their population may drop even more.

Weeds have diminished

People need to realize that since introduction of some of these biological control agents, weed populations in many regions have diminished. “For instance, in years past, tansy ragwort was responsible for many cases of poisoning in livestock, and millions of dollars in death losses," Filley says. "Then the ODA and OSU released the cinnabar moth and the population of tansy ragwort dropped, and so did the death losses.” 

There are two other biocontrol agents that are now even more important for tansy ragwort control than the beautiful moth. “Since these releases years ago, we haven’t seen major livestock losses,” she says.

"Many of the invasive weeds in North America originated in Europe and Asia in a similar climate, where they are part of the natural landscape. They evolved through time along with their natural checks and balances. Some of their natural enemies do not exist on our continent, so we’ve brought some of them here to combat their host weed," she explains.

"Invasive weeds came as seeds in grain or immigrants’ belongings or clinging to livestock hair--imported from other continents," she says. "We have to go to the home country to find the biocontrol agents that are host specific for the target weed. These agents (mainly insects) are rigorously tested to ensure safety to native plants, agricultural crops or endangered species before we release them here.

"Over the years there have been many biological agents brought to the U.S. after researchers found them in their native countries and made sure they were safe to bring to this continent.  “Before a certain biological control agent is accepted for release, the testing is very intense,” says Filley.

“A few years ago a fungus was found that was good at controlling non-native blackberries. These are hugely invasive here on the coast. The fungus is more viable on the coast where there’s more moisture, but wasn’t able to survive very far inland in drier weather. Then the researchers found that the fungus was specific for certain types of blackberries and didn’t work on various subspecies. \ Luckily it was not a problem for the commercial blackberry industry. \ But this illustrates the difficulty in matching a biological control agent with the weeds you want to target,” she says.

Set-aside areas needed

People also need some set-aside areas to preserve biological controls, just as we now have some set-aside fields for pollinator insects (where no pesticides are used). The biological control insects need some areas where they have their desired plants to feed on. “If there’s a tansy ragwort plant in your pasture and plenty of forage for livestock, just leave that plant alone and don’t remove it.  It may help keep the bug population alive to help control any new plants coming in,” says Filley.

“One experiment on controlling tansy ragwort with herbicides versus biological control showed the biocontrol was still helping years later. The spraying only lasted one season and the tansy ragwort came back and was even worse.  So we need to be careful to leave some of those plants alone. This includes some of the native plants like milkweed, that the monarch butterfly depends on,” she says.

“We need to think about the insects we are dealing with and how to keep them for when we need them.”

Along with biological control agents there is also need for pasture management as part of the strategy to reduce unwanted weeds.  “A healthy pasture provides more competition for weeds and keeps them at lower levels.  It really is an integrated pest management strategy,” she explains.

Targeted grazing of weed patches with livestock can also help.  Sheep or goats can eat some weeds that are toxic to cattle.  In some situations cattle can be grouped (using temporary electric fencing, herding or virtual fences) and concentrated in small areas to graze an area and knock down/eat the targeted weed.

[Heather Smith Thomas writes from Salmon, Idaho]

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