Researchers use insects to fight invasive plants
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Minnesota has some new research efforts to find natural methods that will control invasive plants and insects.
Slowing the spread of exotic plants and insects with chemicals is expensive and often has negative environmental side effects. Finding a natural enemy to kill a plant or insect is often more efficient and more sustainable.
Minnesota Department of Agriculture Research Scientist Monika Chandler swept a net through a patch of purple flowered spotted knapweed in rural Becker County looking for a small insect called a root-boring weevil.
Finally catching one, she said they hold on to the flowers and look just like the seed head.
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"They're really masters of disguise so they're very difficult to see," she said.
The root weevil is a natural enemy of the invasive spotted knapweed. Along with a seed head weevil, it's helping control knapweed on this 40-acre prairie at the Tamarac National Wildlife Refuge.
"It was just solid knapweed everywhere," Chandler said. "And it was much taller and thicker. These are scrawny knapweed plants by contrast."
Knapweed spread unchecked in Minnesota for decades because it had no natural enemies. The plant came here by accident from Eurasia according to Chandler, and that's where researchers found the weevils that are its natural enemy.
Tamarac National Wildlife Refuge biologist Lowell Deede wanted to create good habitat here for nesting upland birds on this former farm field. But spotted knapweed had crowded out all of the native plants.
"We wanted to try and control that and the bio-agents that were released have done a very good job with that," Deede said. "That's probably been, oh gosh, within four, five, six years you can tell that it is being controlled."
Now, most of the field is tall prairie grasses with scattered native wild flowers.
There are only a few patches of spotted knapweed left. Monika Chandler collects weevils here to send to other areas in Minnesota where spotted knapweed is spreading rapidly.
"So, for example in the southeast part of the state we're trying to do a lot of releases along Highway 61 where there's knapweed," Chandler said. "Because we know the knapweed will spread and what we're trying to do in those regions is have the bio-agent spread with the knapweed."
The goal of biological control is not to eradicate an invasive plant or insect, but to control and contain it so expensive annual chemical control isn't necessary.
Minnesota is expanding research on biological control thanks to some new infrastructure. The state has the only biological containment or quarantine research facilities in the Midwest, where exotic insects and plant pathogens can be safely studied.
David Ragsdale heads the entomology department at the University of Minnesota and sees growing interest in biological control of invasive species.
One insect high on the list for researchers is the soybean aphid. In the past decade it's become a significant problem in Minnesota, infesting millions of acres of soybeans and the only control option now is repeated insecticide applications.
"So we're adding a lot of broad spectrum insecticides to the landscape," Ragsdale said. "There are detrimental side effects of using chemical management year in and year out. We're trying to replace that with a more permanent and sustainable method of control."
But finding that biological control can be a long, tedious process and it's not always successful. A tiny wasp released a couple of years ago to kill the soybean aphid hasn't been very successful according to Ragsdale.
Another aphid predator will likely be ready for release next year.
And the spotted knapweed control effort shows how difficult it can be to predict what will happen in nature. The insects used to control the knapweed are very effective, but according to Ragsdale, the weevil maggots wintered in seed heads.
Mice found those maggots a good food source and ate them along with the seeds. Owls then ate the mice and where the owl droppings fell, new spotted knapweed plants often sprouted. Still, biological control is effectively slowing the spread of spotted knapweed in Minnesota.
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Biological control works one-third to one-half of the time, said Ragsdale. But when it works it really works. A classic example is purple loosestrife, an invasive plant that choked Minnesota wetlands. Insects released by researchers now effectively control the plant.
"If we had done nothing with purple loosestrife, the department of agriculture and the DNR would be spending about a million dollars a year to control this weed," he said. "They currently spend nothing. If you look at return on investment, every dollar invested in biological control returns $36 in benefits."
The savings on purple loosestrife control alone paid for the new containment facility to allow expanded bio-control research according to Ragsdale.
And researchers have plenty of subjects. Eurasian Water Milfoil, Common Buckthorn, Gypsy Moth, Emerald Ash Borer. As the list of invasive species grows, so does the urgency to find effective ways to control the populations.
