Using insects to control invasive weeds on pasture

Houndstongue biocontrol project in B.C. has been highly successful

Once invasive plants reach a point of domination, elimination is generally not feasible. Land managers sometimes turn to biologic control, a last resort for reducing a widespread weed’s populations to below an economic or environmental damage threshold. Eradicating the weed may be impossible, but biocontrol agents may reduce it to manageable levels.

Since 1951, 82 arthropod species (mainly insects) have been released against 30 target species in Canada. Of these released species, 58 have become established, with 18 of them effectively reducing target weed populations. Biocontrol programs for leafy spurge in the Prairies, and for knapweed in British Columbia, have been the largest projects. Each is responsible for the establishment of 10 new arthropod species on rangelands.

Dr. Rosemarie De Clerck-Floate, research scientists with Agriculture and Agri-Food Canada (AAFC) in Lethbridge, works on weed control using insects that target a specific host plant. The Canadian research program for weed biological control is based at AAFC’s Lethbridge Research and Development Centre, where De Clerck-Floate and Dr. Rob Bourchier have been providing Canada with new biocontrol agents.

Many producers are aware of this government-supported strategy for weed control.

“British Columbia cattlemen are big supporters of biological control. The hounds-tongue biocontrol project is an example. The B.C. cattlemen came forward 30 years ago, asking the provincial and federal government to support this project,” she says.

The houndstongue biocontrol project has been highly successful in British Columbia. This weed has been greatly reduced in many areas, particularly the southern interior, where it was a problem. Federal and provincial governments kicked in most of the funding to find and test potential biocontrol agents for houndstongue.

Hawkweed is a more recent problem in British Columbia.

“We are finally getting biocontrol agents approved for release to help control this group of weeds that are all closely related, but we still have a lot to work out before we can get effective control of all of them,” she says.

It’s not always easy to find an insect that will target a specific weed. It depends on what’s available in the native land of the weed. The insects must also be specific to the host plant.

“We do a lot of testing in the native country after we find a possible control agent, to determine what plants they will feed on and develop on.”

There can also be jurisdictional considerations. For example, the houndstongue control project has caused border tensions. The U.S. has a different set of laws and is concerned about threatened and endangered native plant species. Some of those plants are closely related to houndstongue and there’s concern they may be affected by the root weevil biocontrol agent, which has moved across the border on its own.

Yellow toadflax with a stem gall caused by a rhinusa pilosa weevil. The weevil has been released in seven provinces since 2014. photo: Supplied by Dr. Rosemarie De Clerck-Floate

De Clerck-Floate is working with the Americans on this issue. She says they’ve studied the weevil’s effect on the hounds-tongue’s close relatives. Although the European root weevils can develop on these native plants, they don’t prefer them, she says.

“They don’t appear to affect native plant populations adversely. Based on our monitoring, years after weevil releases were made, they have not been found feeding on the native plants at all,” she says.

That’s typical of most of these host-specific insects, she says. De Clerck-Floate and Bourchier test the insects’ behaviour in the lab by placing them on different plant species, including native plants that are of concern in North America. They want to see whether the insects can feed and lay eggs on these plants, when given no alternative, as they don’t want them to damage native plants in the field.

“Sometimes in order to survive they will develop on these other plants that are closely related to the host weed. But out in the field where they have many options, they find and use only their host plant,” she says.

The insects have biological filters that guide them to the target plant, she says. That means the host range in the field is usually much narrower than in the lab. In the field, they often don’t even recognize the substitute plant, dying before they feed on it.

“They are like little robots. If they can’t see or detect their host plant in the environment, they won’t even go to it,” says De Clerck-Floate.

Researchers test insects on many plants. It takes years of testing and can be expensive to find the right agent. Groups interested in using biocontrol to manage specific invasive weeds have come together to fund the overseas exploration and testing of new agents, says De Clerck-Floate. For example, the houndstongue project was funded by groups in Canada and from Idaho, Montana and Wyoming, where houndstongue is a problem, she says.

After release onto their host weed, it takes a long time for some agents to build an adequate population to be effective. Releasing them into an annual crop doesn’t work because the plants and insects are disturbed every year.

It also makes more sense economically to use biocontrol on extensive, topographically varied areas such as native rangelands where it is difficult to physically apply herbicides, she says. Insects can naturally propagate and expand in weed-infested areas, hindering weeds.

“It takes patience, however. You won’t see results immediately. And sometimes a program works for a while and after a few years doesn’t work as effectively or as quickly.”

One of the weeds cattlemen are currently worried about is spotted knapweed, which is a resurging problem. Although first targeted for biocontrol in the 1960s, it now appears to be increasing and expanding in regions such as the Okanagan and Nicola Valleys of British Columbia, says De Clerck-Floate. Researchers want to find out if anything has changed in the environment or management methods to cause those biocontrols to no longer work as well.

“We may be searching for new biocontrol agents but we need more information before we can make new decisions on management of those weeds. Sometimes we need to include the use of chemical as well,” she says.

Researchers are also looking for new agents for some emerging weed problems, such as ox-eye daisy and common tansy.

“We are close to being able to petition for release of agents for these two weeds,” says De Clerck-Floate. “One that looks promising for common tansy is a weevil found in Russia that feeds in the stem. For ox-eye daisy, an agent in Europe is a moth whose larvae feed on the roots.”

If something hasn’t been approved yet by government regulators for release, researchers can get a permit to bring it in for testing in a secure quarantine facility that has been certified. Researchers try to pick insects that will work, but lab tests are not ideal for being able to predict what may happen under natural field conditions.

“We don’t want to waste money and time releasing something that isn’t going to work, and also have to consider the safety for other plants that might be affected. Regulators who look at our petitions want some evidence of efficacy as well as safety,” she says.

Sometimes they do private land releases, if it fits in with a field experiment, but there are no guarantees that the insect agent will be effective at that stage.

“The province has a database for every individual release we make, as well. We provide our release information to them, and vice versa, especially for early stages of a release project,” she says.

De Clerck-Floate and her colleagues test biocontrol agents, but they don’t administer the biocontrol programs. In British Columbia, for example, the provincial government runs the program and works closely with municipal governments and local weed management councils.

Recently De Clerck-Floate has been working with the British Columbia government on a number of releases of a new agent for yellow toadflax.

“This is a stem-feeding weevil that’s very specific to yellow toadflax. It forms a big gall on the stems, and sucks a lot of nutrients, energy and water from the plant for the developing larvae inside the gall,” says De Clerck-Floate. If the plant develops enough galls, it dies, she adds.

Since receiving approval for the weevil in 2014, it’s been released in seven provinces.

“In Nova Scotia it was released in pastures and grazing land, and we also did trials in strawberry crops. Province by province, we’ve tried them on various grasslands, and a lot of releases on ranches in the foothills in southern Alberta,” she says.

The insects initially did well at some sites, but then their numbers dropped.

“It is too early to tell, but the population drop may be related to hot, dry conditions, which we hope to investigate.” De Clerck-Floate says they hope to develop recommendations on where to use them over time, so they can establish populations that expand.

A suitable habitat for insect agents includes plenty of host weeds, without predators such as ants to prey on them. The insects should be kept at a proper temperature while transported — not in a closed plastic container on the vehicle’s dashboard, in the sun.

“As one of the B.C. people told me, these insects are like little livestock — you need to take care of them,” she says.

In some ways, the weevils are very hardy. “You can suck them up with a shop vacuum cleaner and bounce them around and they survive just fine,” she says. “That’s how we collected houndstongue weevils in the spring, to move to new sites.”

It’s important to know as much as possible about the insect you are working with. It also helps to release them in fairly large groups in the weed patch, she says, as most insect species need to be able to find each other easily to mate and lay eggs.

De Clerck-Floate says they’ve also had success releasing a group of closely related root beetle species on leafy spurge. Some of those beetles prefer hotter areas and south-facing slopes, while others prefer cooler, moister areas.

“It takes them a few years to get ramped up, but they’ve been quite reliable,” she says, adding people must choose the beetle types best suited for their own situations.

In Alberta, they had spurge beetle days, De Clerck-Floate says. Producers filled small cloth bags with beetles to release on leafy spurge patches on their own ranches. By releasing them in the middle of the patch and marking the release point, they can monitor how fast the beetles disperse.

“We’ve figured these things out over the years, with producers’ help.”

Testing an agent for oxeye daisy

Researchers may have found a European moth to control ox-eye daisy. The process for evaluating the moth’s suitability is one example of how complicated these trials can be. Researchers decided to test the moth on a little daisy that grows on Yukon mountaintops, to make sure the insect doesn’t harm similar native species. Collecting the daisy last summer was “quite an ordeal,” says De Clerck-Floate.

“The province of B.C. hired a retired botanist who hiked up the mountains looking for these little daisies.” The botanist had to go up several times to gather plants at the right stage of development.

Co-ordinating the moth shipment was tricky, too. The moths emerge earlier in Europe than Canadian daisies, so they had to stall the insect’s development until the daisies were at the right stage. They did this by keeping the pupae in a root cellar in Europe. The moths emerged, mated and produced eggs for the Canadian researchers in time for shipment. By the time De Clerck-Floate received them, they were on the verge of emerging as caterpillars, just in time to set up the test.

The daisies were also a challenge to grow because they were collected at the peak of summer in the northern Yukon, where day length was 20 hours of sun. Researchers had to replicate those conditions in the lab, but the insects had never encountered that type of environment.

“It was a shot in the dark, but we managed to get it all together to complete the test,” says De Clerck- Floate.

De Clerck-Floate finds these challenging tests interesting, as she’s both an entomologist and botanist.

“You have to be able to understand both the plants and the insects and how they interact to test them. We also propagate them in our lab so we have enough numbers to send out when we get the green light to release them into the environment,” she says.

Researchers are now looking at whether the moths will survive Canadian winters, and what environments will be most conducive to establishing colonies and surviving in the field. The goal is to match them to environments that are similar enough to their European homeland for them to survive.

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