Integrating Biological Control with Targeted Sheep Grazing to Suppress Spotted Knapweed

2010 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
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. Objectives include comparing the effects of biological control vs. biological control + sheep grazing on: 1) the number of biological control insects present; 2) the occurrence of insect activity on buds and roots; and 3) the number of viable spotted knapweed seeds present per plant. The final objective is 4) extending results to appropriate groups and individuals. Plots will be 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. Results of this study will demonstrate the potential benefits or pitfalls of participating in integrated weed management techniques to land managers. 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

Objectives 1, 2 and 3 were completed in summer of 2010. Forty sheep grazed the four August treatment plots for approximately one week. Sheep removed 95% of the buds and flowers from knapweed plants in August. Viable seed produced in grazed plots was 96% lower than the amount of viable seed produced in plots with only biocontrol agents present. Abundance of bio-control agents, with the exception of C. achates, decreased immediately following sheep grazing, even in ungrazed plots. C. achates was equally prevalent in grazed and ungrazed paddocks, both before and immediately following sheep grazing. Pre-grazing insect surveys conducted in summer 2010 indicated that grazing in 2009 did not impact the 2010 insect populations.

Objective 4 is the dissemination of results. Preliminary data was presented to the Montana State University Sheep Advisory Committee. Data analysis is ongoing with manuscript preparation to immediately follow.


The second year of field work was successfully completed for this project and laboratory analysis of seeds and statistical analysis of data is ongoing. Preliminary data was presented to the Montana State University Sheep Advisory Committee and at several weed education workshops.

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 preliminary results have increased the interest in grazing sheep on Tribal-owned lands and their potential to be another tool in the battle against noxious weeds. A tour with Grazing Association Members, Tribal Council and other producers is being planned for summer of 2011. Preliminary data was presented to the Montana State University Sheep Advisory Committee. Additional funding has been sought through the Montana Noxious Weed Trust Fund to continue collecting data for additional years 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
Research Scientist
Montana State University
101 Linfield Hall
P.O. Box 172900
Bozeman, MT 59047
Office Phone: 4069944003