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 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 did 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 were 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 1, 2 and 3:
1) Compare the effects of biological control + sheep grazing in August to biological control alone (Control) on the number of adult biological control insects present, 2) on the occurrence of pupae and larvae in buds/flower heads and roots at spotted knapweed senescence, and 3) on the number of viable spotted knapweed seeds present per plant at spotted knapweed senescence.
Final insect surveys conducted in the summer of 2011 indicated that grazing in 2011 did not impact the 2012 insect populations. Viability assessments for seeds were completed in the lab and statistical analysis is in progress. Although final data for this project was collected in 2011, the additional years of funding leveraged by this project resulted in two additional years of treatment application that will be incorporated into a manuscript so it will reflect four years of grazing treatments. Final data will be collected in the summer of 2013, after which the final results will be incorporated into a journal article.
Objective 4: Present research results to interested groups and individuals through national, regional and local professional meetings; field days; and working group meetings.
A poster of the project was presented at the 2012 Annual Meeting of the Society for Range Management in Spokane, WA. Final project summaries were presented to the Montana Biological Control Working Group comprised of members from Montana State University, Bureau of Land Management, USDA Forest Service, USDA Agriculture Research Service, and many local weed and watershed organizations. Results were presented as part of a training protocol for the Bureau of Land Management Integrated Pest Management Training session held in Boise, ID. The research site was one of the locations visited during a 2012 Revegetation and Integrated Weed Management tour sponsored by Missoula County Extension. Approximately 30 landowners, Extension personnel, University researchers and tribal members visited the research site to view the effects of grazing on the plant and bug communities.
Funding from the Montana Department of Agriculture’s Noxious Weed Trust Fund was secured to apply two additional years of treatments to further assess the long-term impacts of combining sheep and biocontrol insects on populations of spotted knapweed. Consequently, the fourth year of grazing was completed in August 2012, and final vegetation data will be collected in summer 2013. The research site was one of the locations visited during a 2012 Revegetation and Integrated Weed Management tour sponsored by Missoula County Extension. Approximately 30 landowners, Extension personnel, University researchers and tribal members visited the research site to view the effects of grazing on the plant and bug communities.
Impacts and Contributions/Outcomes
This project is a cooperative project between Montana State University, the Confederated Salish and Kootenai Tribes, the Polson Cattlemen’s Grazing Association, and other producers and Extension professionals. The August 2012 tour sponsored by Missoula County Extension was a success and generated questions and conversations about next steps for restoration following knapweed reduction. Data derived from this grant helped secure additional funding through the Montana Noxious Weed Trust Fund to continue the project for two more years for a total of four years of grazing treatments, to track insect populations with repeated grazing. We have been invited numerous times to present this data to interested groups, and this project has prompted many discussions about the capabilities of integrating insect biological controls with targeted grazing.
Montana State University Extension
Livingston, MT 59047
Office Phone: 4062224517
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Office Phone: 4069944003