Controlling crop weeds with beneficial insects

Flea beetles that eat leafy spurge are only the first wave of biological weed control

What is it about weeds? No matter how hard you try to eradicate them, they always come back. Worse still, each method of weed control has its drawbacks. Spraying with herbicides can lead to weed resistance if the same product is used continuously. Tillage can result in soil erosion (remember the Dirty ’30s?), and although cursing makes you feel a bit better, it has absolutely no effect at all on the weeds themselves.

But there is another form of weed management that can be cheap and effective if you’re willing to wait a long time. It’s called biological control and it involves using specific insects to eat certain weeds, if not into oblivion, then close to it.

“It’s definitely one of the tools in the tool box,” says Beverly Dunlop, a range and forage biologist with Agriculture and Agri-Food Canada in Brandon, Man.

Dunlop would know. She works with the Leafy Spurge Stakeholders Group, a coalition of government, agricultural and conservation groups examining ways to prevent and control leafy spurge, a noxious weed, in Manitoba.

There are many ways to control leafy spurge, with varying degrees of success. Herbicides will contain but not eradicate it. Cultivation is practical only on cropland, not rangeland where most of the weed is found. (Besides, fragments of cultivated roots will quickly form new plants.) Digging or hand pulling is impractical except in small patches. Mowing has to be done repeatedly to keep leafy spurge plants from going to seed. Sheep and goats will eat leafy spurge but having enough of those animals to provide widespread control is unrealistic.

Enter the lowly flea beetle.

Since the 1990s, the Leafy Spurge Stakeholders Group (LSSG) has been releasing batches of insects which feed solely on leafy spurge in selected locations as a means of weed control. Although 14 different foreign insects have been released against leafy spurge over the years, the most successful have been five flea beetle species in the genus Apthona. The larvae feed on the roots of leafy spurge and the adults defoliate the weed before it can set seed. In the early 2000s, entire hillsides in western Manitoba once covered with leafy spurge had become bare of the weed, thanks to those insects.

Since that time, research projects have abated and leafy spurge has crept back. Dunlop is trying to revive projects and revisit former sites to see if beetle populations can be re-established.

“What we need to do is to get more interest in biological control, show some good case studies and get interest built up again,” Dunlop says. “The results are there.”

Employing flea beetles to control leafy spurge, dubbed by the LSSG as Manitoba’s most unwanted weed, is one example of using an organism to regulate the population of another pest organism. It’s what scientists call classical biological control — controlling pests introduced from another region by importing specialized natural enemies of those pests from their native home.

“These are insects that have co-evolved with the particular target weed in their country of origin and they’re screened to ensure their specificity in order to bring them over here,” says Robert Bourchier, an entomologist and biocontrol researcher with Agriculture and Agri-Food Canada in Lethbridge, Alta.

group of agricultural researchers in a field

AAFC’s Robert Bourchier, seen here with his research team, says finding natural insect predators for weeds is the first step in biocontrol efforts.
photo: Supplied

There are several advantages to biological control. It’s very specific because the predator insect will control only the pest it’s meant to target, unlike chemicals which can destroy good plants as well as bad ones. It takes little effort to keep a biological control system running, because the beetles can sustain themselves. It’s self-perpetuating and the insects need to be applied only once, unlike chemicals which must be applied continuously. It can also be very effective. Dunlop says she has seen 20-acre sites in Saskatchewan once covered in leafy spurge which the insects have since chewed down to nearly nothing.

Even so, biological control has one great disadvantage. It’s a very slow process which takes time to become effective.

The first challenge is to find the proper insects for the task. Bourchier says it can take 10 years of testing to make sure the insects being released are the right ones. It then requires several years to establish populations which can survive our winters. Finally, populations must be large enough when released to have a noticeable impact. It takes time to multiply these populations.

But when the insects are finally turned loose, the effect can be noticeable in a fairly short time. Bourchier says one long-running project in southern Alberta, involving different insects on hound’s tongue (a noxious biennial taproot weed), managed to eliminate the weed at certain locations within two to three years.

It can be expensive to set up a biological control system. Bourchier says the cost of getting a biocontrol agent on the ground is somewhere between one million and two million dollars over 10 years. But the system is cheap in the long run, especially compared to developing a new herbicide, which costs tens of millions of dollars or more.

Bourchier says biological control isn’t the best method to use on a new weed because quick action is critical to keep it from becoming established. But in the case of leafy spurge, which arrived in North America in the 19th century and subsequently became established in Western Canada, biological control to slow its spread can be useful.

Bourchier says biological control is not commonly used in commercial crops because the weeds are not suitable targets. But it can be effective in environmentally sensitive areas, such as riparian zones, wetlands or parks, where you don’t want to apply herbicides too close to water or people.

Although only a small player in an integrated pest management program, biological control could play a more important role at a time when pesticide costs are rising and herbicide-resistant weeds are becoming more prevalent, Bourchier suggests.

“By nature it’s specific and targeted toward a single species, whereas herbicides tend to be more broad spectrum, which is a benefit but also a cost,” Bourchier says.

Controlling weeds with insects made its debut in Canada in the early 1950s with the release of an agent to control St. John’s wort in Ontario and British Columbia. Since then, about 20 weeds have been targeted with 75 different insects throughout the country.

Supporters of biological pest control call it an environmentally safe way to control pests without using toxic chemicals. But there are risks. A classic example is the cane toad, introduced into Australia in 1935 to control beetles in sugar cane crops. Without natural enemies, this poisonous creature has since become an invasive species spreading throughout northern and western Australia, forcing the Australian government to develop a national cane toad control plan.

Such examples have some environmental groups warning against “frankenbugs” and raising fears that biological control may be a cure worse than the disease.

But Bourchier rejects those concerns. He points out the type of screening done for biocontrol agents these days is very different. Regulations and oversight have changed dramatically since the 1930s and programs select for “specificity” — agents that work only on a single pest and cannot survive without it.

Says Bourchier: “You would never get approval to release cane toads in Australia today.”

This article was originally published as “You’ll like these insects” in the April 2015 issue of Country Guide

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