Twenty Years of Success: Managing Tansy Ragwort in Northwestern Montana

High temperatures, windy conditions and lack of precipitation created extreme wildfire conditions in northwest Montana during the summer of 1994. The Little Wolf Fire began in August of that year, burning over 15,000 acres of national forest and private timber lands.  

“Tansy ragwort was probably already present as scattered plants when the wildfire burned,” explains Dan Williams, weed coordinator in Lincoln County. “But, two years after the fire, the ragwort infestation exploded within the burned area.”

Open sites created by the burn and disturbance from fire-fighting activities provided ideal habitat for tansy ragwort. Initial estimates suggested that about 1,000 acres were infested by the weed; but subsequent surveys recorded 15,000 acres of tansy ragwort scattered within a 500,000-acre perimeter. 

The County Weed District and other partners organized a cooperative weed management area (CWMA) and developed management plans to contain tansy ragwort. The goal of the Tansy Ragwort CWMA is to restrict populations of tansy ragwort to existing areas and prevent further expansion through an integrated approach. 

Management components include: 

1)    inventory/mapping and monitoring

2)    control and containment

3)    use of biological control agents and related research

4)    road closures and grazing restrictions

5)    public education

6)    a plan for cross jurisdictional cooperation

“The project has always been a cooperative effort between the Forest Service, county weed districts, state Departments of Agriculture and Natural Resources, a large private timber company, local landowners and other partners,” Williams explains. “We were fortunate that several biological control specialists with the Forest Service and Montana State University were able to secure insects from Oregon and establish release sites in the area early in the project.” 

Three insects including the cinnabar moth, tansy ragwort seed fly and tansy ragwort flea beetle were collected and tested for survival and suitability for tansy ragwort control in northwestern Montana (See on this page, Controlling Tansy Ragwort). The cinnabar moth was established on the site by 1999 followed by the other two insects. The cinnabar moth has proved to be the most successful of the three insects in reducing tansy ragwort density in Montana. 

Biological control agents were integrated with the herbicide treatment program to reduce larger core populations of tansy ragwort. Milestone® or Transline® herbicides are applied to tansy ragwort along roadsides, small newly established infestations, and on the perimeter of some of the larger tansy ragwort infestations where insects were released. 

“We apply Milestone at 6 fluid ounces per acre (fl oz/A) on the majority of infestations and have had good results,” says Williams. “Transline at 1 pint per acre (pt/A) is used under sensitive trees and shrubs.” Both herbicides are applied to tansy ragwort from rosette through bloom stage, and in the fall. 

Williams explains they are getting complete control of tansy ragwort with Milestone regardless of plant growth stage. “Plants in the mid to late bloom stage will likely produce viable seed if flower heads aren’t clipped. But, we do an excellent job of controlling rosettes and seedling germination with late summer and fall applications of Milestone,” Williams explains. 

The CWMA achieved dramatic reductions in tansy ragwort populations by integrating effective biological control agents with herbicides treatments. Their success has led to creation of other cooperative weed management areas in northwestern Montana. 

“Consistent surveys, monitoring, and public education are key components for meeting our goal,” says Williams. “By mapping the infestation we can track the spread or decline of the infestation and adjust management methods to maximize control. Monitoring previously treated areas and surveying the outer edge of the infestation for new plants also let us know if our containment efforts are working. Establishing biological control agents in core ragwort populations, and using herbicides on the perimeters of infestations, along roadsides and on newly invading plants reduces the occurrence of new infestations outside of the infestation perimeter.” 

Counties in northwestern Montana rely heavily on proceeds from timber production, livestock and wildlife forage production, recreational access, and agriculture—all of which are at risk from tansy ragwort invasion. The success of this project is critical to safeguard the social and economic base of communities in this region. 

Published September 2016; reviewed and updated June 2019.

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Active ingredients for herbicide products mentioned in this article: Milestone (aminopyralid), Transline (clopyralid).