Integrating Biological Control with Targeted Sheep Grazing to Suppress Spotted Knapweed

2011 Annual Report for SW09-068

Project Type: Research and Education
Funds awarded in 2009: $49,865.00
Projected End Date: 12/31/2012
Region: Western
State: Montana
Principal Investigator:
Rachel Frost
Montana State University
Dr. Jeff Mosley
Montana State University

Integrating Biological Control with Targeted Sheep Grazing to Suppress Spotted Knapweed


Spotted knapweed is an extremely aggressive competitor that threatens the ecological sustainability of western rangelands and the economic sustainability of western ranches, animal production systems and rural communities. In response to the high costs, environmental concerns and health risks associated with herbicides, landowners and land managers have explored the integration of biological control and targeted (or prescribed) sheep grazing on spotted knapweed invasions. It is unknown if sheep grazing decreases the insects’ efficacy or if the two control methods may be synergistic. This multi-disciplinary project, which is widely supported by stakeholder groups in the West, proposes non-pesticidal, environmentally sound tactics. The goals of this three-year field project are to evaluate the potentially synergistic effects of combining targeted sheep grazing and biological control to suppress spotted knapweed and to determine the potential effects of targeted sheep grazing on biological control insect presence and activity.

Plots were treated with biological control and biological control + sheep grazing in August. Density of biological control agent populations were monitored pre- and post-grazing and at senescence. Targeted sheep grazing does not reduce Larinus or Cyphocleonus abundance the following year. Sheep removed 92% of spotted knapweed buds and flowers in August. Sheep grazing and biological controls together reduced viable seed production of spotted knapweed 99% more than insect biological controls alone. Results will be disseminated to ranchers, livestock producers and land managers through professional meetings, field tours, short courses and Extension programs to facilitate the control and prevention of spotted knapweed on rangelands.

Objectives/Performance Targets

1) Compare the effects of biological control + sheep grazing in August to biological control alone (Control) on the

a) number of adult biological control insects present,

b) on the occurrence of pupae and larvae in buds/flower heads and roots at spotted knapweed senescence,

c) on the number of viable spotted knapweed seeds present per plant at spotted knapweed senescence.

Final insect surveys conducted in summer 2011 indicated that grazing in 2010 did not impact the 2011 insect populations. Viability assessments for the 2010 seeds were completed in the lab and statistical analysis was completed in 2011.

2) Present research results to interested groups and individuals through national, regional and local professional meetings, field days and working group meetings.

Frost was invited to present final results at the Montana Weed Control Association, and an abstract has been accepted to present a poster at the 2012 Annual Meeting of the Society for Range Management in Spokane, WA. Final project summaries were presented to the Missoula County Weed Board and members of the Salish Kootenai Tribal Council.

A tour with Grazing Association Members, the Tribal Council, Extension personnel, weed board members and other producers was conducted at the research site in August 2011. Mosley, Frost and Roeder highlighted the methods and findings of the project. Jim Story, retired MSU Research Scientist, rounded out the day with a crash-course in insect identification and a history of the spotted knapweed biological control program in Montana. Despite the high temperatures, people stayed for well over four hours discussing the potential application of the research results and exchanging ideas and contact information.

After acquiring funding to graze for an additional year, it was determined to delay the manuscript to allow for inclusion of the supplementary data. While this delays publication approximately one year, it will produce a stronger manuscript that generates solid conclusions from the data and a better understanding of the true potential to integrate these control methods.


Final data collection was completed for this project and laboratory analysis of seeds and statistical analysis of data is complete. Final data was presented at several weed education workshops and to project cooperators. A field day was held in August 2011 to allow landowners and managers to tour the site and see the results of the project. Funding from the Montana Department of Agriculture’s Noxious Weed Trust Fund was secured to apply an additional year of treatments to further assess the long-term impacts of combining sheep and biocontrol insects on populations of spotted knapweed.

Impacts and Contributions/Outcomes

This project is a cooperative project between Montana State University, the Polson Cattlemen’s Grazing Association and other producers and Extension professionals. The August 2011 tour with Grazing Association Members, Tribal Council and other producers was a great success. All of the attendees were encouraged to hear that spotted knapweed research was being continued and that alternative control methods were being explored. Data derived from this grant helped secure additional funding through the Montana Noxious Weed Trust Fund to continue the project for one more year to track insect populations with repeated grazing.


Tracy Brewer Mosley

Park County Ag Extension Agent
Montana State University Extension
Livingston, MT 59047
Office Phone: 4062224517
Rachel Frost

[email protected]
Research Scientist
Montana State University
101 Linfield Hall
P.O. Box 172900
Bozeman, MT 59047
Office Phone: 4069944003