The city of Helena has used insects to combat the spread of weeds for more than a decade, and for the first time Helena recently offered up some of its bugs to take the battle across the state.
Montana has two varieties of invasive toadflax, both growing distinctive yellow “snapdragon” flowers. Yellow toadflax is distinguished by more spear-like leaves while Dalmatian toadflax grows taller with broader leaves.
As invasive weeds without natural controls, both species often spread quickly, outcompeting native plants for their spot on the hillside. In the early 2000s Helena began a “biocontrol” program, releasing approved insects captured from weeds’ native range in Eurasia. Since then the city has made about 180 releases of six different insects to combat multiple weeds, including toadflax, spotted knapweed and leafy spurge.
“This has been a long time coming – insects are just not a quick form of management,” said Greta Dige with Helena Parks and Recreation. “It’s exciting to have this kickoff for our first collection, to give back what’s been given to us as a city so that others can use them on their property.”
The city owns roughly 3,200 acres in Lewis and Clark and Jefferson counties. Controlling weeds on the land is a constant and potentially costly job, and biocontrols have become a crucial tool that has helped keep several weeds from spreading into a massive infestation.
“In the early days we purchased insects for release but the more we got into the program, we went and participated in collections,” Dige said.
Biocontrols are not always the answer. For example, the city still struggles with invasive thistles but is reluctant to use nonnative insects because they also feed on native thistles, Dige said.
With established populations of weevils helping keep the spread of Dalmatian toadflax at bay, land managers and extension agents from across Montana hiked up Mount Ascension this week. They toted plastic tubs and a cooler with ice to collect and store the insects for transport to other areas of the state where the weevils will be released.
Collecting weevils starts with hiking the hillside in search of toadflax. Once discovered, collectors hold the tub underneath and give the plant a tap just hard enough to knock the bugs down but not so hard as to hurt the plant or knock other debris into the tub.
While biocontrol seems simple – acquire the weeds’ natural enemies and release them – the approval process for individual insects takes about five years, said Sharlene Sing, research entomologist with the Forest Service’s Rocky Mountain Research Station who specializes in biocontrols. That is to ensure the bugs will not have any unintended consequences once out in the wild.
Because insects evolve with the plants they feed on, approved bugs like weevils for toadflax are “host specific,” meaning they only feed on one species. That means that two different weevils have been imported to suppress each species of toadflax.
Using insects to control invasive weeds is not necessarily a replacement for herbicides but has proven advantageous in many circumstances, Sing said. Insects are more cost effective, particularly for larger infestations, and are safer to use near water. Once released, the bugs can reproduce and naturally spread across varying terrain.
Sing emphasized that the effectiveness of biocontrols has been highly site-specific with varying degrees of success.
“Monitoring and evaluation is so crucial,” she said, to gauge the effectiveness and potential need to release more insects.
Melissa Maggio, coordinator of the Montana Biocontrol Project, echoed the need for patience and a long-term vision when releasing insects to suppress weeds.
“Biocontrol is not instant gratification, you’re looking at a minimum of three to five years,” she said.
While attempting to control invasive weeds is difficult enough, the two species of toadflax in North America have hybridized in many places, said Sarah Ward, professor of genetics at Colorado State University. As researchers learn more about the hybrids, the news is not positive.
Hybridization is not particularly novel in nature, but when it comes to toadflax, the hybrids occurring in North America are not happening in the plants’ native ranges.
Ward and graduate students have researched the DNA structures of hybrid toadflax. On the ground, a first generation – half yellow and half Dalmatian – is relatively easy to distinguish based on its structure. But as the genetics shift to more or less of one species it begins to take on the characteristics of the dominant plant, but still carrying DNA of both. That raises significant questions about whether the plants will evolve faster than their natural enemies.
“Seeing a (hybrid) population look like one or the other has implications for biocontrol, and that may be one reason biocontrol fails, because there’s enough DNA to throw off the biocontrol agents,” Ward said.
The hybrid plants themselves have also shown the alarming characteristics outcompeting pure strains of toadflax. By every metric the hybrids growing bigger and faster and with more seed pods and flowers, Ward said.
“It didn’t matter where we grew them but hybrids outperformed their parents,” she said.
With hybridization very likely to occur wherever yellow and Dalmatian toadflax intersperse, it makes it all the more critical to hit those populations fast and hard before they have a chance to spread.