This replicated field experiment was conducted on foothill rangeland in northwestern Montana. We evaluated the effects of combining targeted sheep grazing and bio-control insects to suppress an invasive plant, spotted knapweed (Centaurea stoebe). Populations of the three bio-control insects we studied (Larinus spp., Cyphocleonus achates and Agapeta zoegana) were not harmed by targeted sheep grazing applied in either July or August. When integrated together, targeted sheep grazing and bio-control insects provided greater control of spotted knapweed than bio-control insects alone. Targeted sheep grazing was effective when applied in either July or August, but spotted knapweed reproduction was suppressed best when targeted sheep grazing was applied in mid- to late-July, when spotted knapweed was in the late bud–early flower stage. Project results were disseminated to diverse audiences via seminars, field days and scientific conferences.
Objective 1: Compare the effects of bio-control insects + targeted sheep grazing in July (Treatment 1) vs. bio-control insects + targeted sheep grazing in August (Treatment 2) vs. bio-control insects alone (Control) on the abundance of bio-control insects.
Objective 2: Compare the effects of bio-control insects + sheep grazing in July (Treatment 1) vs. bio-control insects + targeted sheep grazing in August (Treatment 2) vs. bio-control insects alone (Control) on plant community fitness.
Objective 3: Compare the effects of bio-control insects + targeted sheep grazing in July (Treatment 1) vs. bio-control insects + targeted sheep grazing in August (Treatment 2) vs. bio-control insects alone (Control) on spotted knapweed reproduction.
Objective 4: Present research results to interested groups and individuals through national, regional and local professional meetings; field days; seminars; the worldwide web; and a research journal article.
Spotted knapweed is a perennial, invasive forb that reduces livestock and wildlife forage, reduces biodiversity, increases surface-water runoff and soil erosion, and inflicts dramatic economic damage (Sheley et al. 1999). Spotted knapweed was introduced from Eurasia to British Columbia circa 1883, and first collected in the U.S. in western Montana in 1935 (Müller-Schärer and Schroeder 1993). Today, spotted knapweed infests more than seven million acres of grazing land in 46 states and seven Canadian provinces. It continues to spread exponentially at 10-20% per year, despite more than 40 years of widespread control efforts (Duncan 2005).
Herbicides have been widely used to suppress spotted knapweed, but herbicidal control is expensive and herbicides must often be reapplied every three to five years (Griffith and Lacey 1991). In response to the high costs, environmental concerns and health risks surrounding herbicides, increasing numbers of landowners and weed managers have explored alternatives, including bio-control insects and targeted sheep grazing.
Previous research has documented that bio-control insects can harm spotted knapweed, especially when spotted knapweed is attacked by more than one bio-control agent. Combined herbivory by Cyphocleonus achates, a root-boring weevil, and Larinus spp., a flower weevil, has been the most effective (Story et al. 2006; Seastedt et al. 2007; Knochel et al. 2010). However, North American spotted knapweed plants are much more resistant to bio-control insects than spotted knapweed in its native habitat (Ridenour et al. 2008), suggesting that bio-control insects alone may never be able to achieve significant control of spotted knapweed in North America. Previous research by us and others also has proven that targeted grazing by sheep or goats can suppress spotted knapweed, and that it is best applied during the late bud-early flowering stage or full-flower stage (Williams and Prather 2006; Thrift et al. 2008; Benzel et al. 2009; Surber et al. 2011; Henderson et al. 2012).
Bio-control insects and targeted sheep grazing could potentially be integrated to achieve greater weed control (DiTomaso 2000). It is unknown, however, whether sheep grazing decreases or increases the efficacy of the insects. In some instances, landowners and weed managers have resisted using targeted grazing where they have already established bio-control insects for fear that the sheep may harm the insects. Research is needed to examine the potential benefits or pitfalls of integrating targeted sheep grazing and bio-control insects to suppress spotted knapweed. Responding to this need, our project examined whether targeted sheep grazing and bio-control insects were compatible and whether, when integrated together, targeted sheep grazing and bio-control insects would provide greater control of spotted knapweed than bio-control insects alone.
This four-year field study was located on spotted knapweed-infested foothill rangeland on tribal lands of the Confederated Salish and Kootenai Tribes in northwestern Montana. Larinus spp., Cyphocleonus achates and Agapeta zoegana were prevalent within the heavy spotted knapweed infestation (spotted knapweed = 44% of vegetative composition). Field densities of the bio-control insects were not manipulated, and insects were allowed access to the entire study site following the procedures of Jacobs et al. (2006) and Knochel and Seastedt (2010).
Twelve, 0.26-ha paddocks were constructed, plus one 2.2-ha paddock where sheep grazed each month and year for five days to acclimate to the forage and study site before the grazing treatments were applied. Four paddocks (Treatment 1) were grazed by sheep when spotted knapweed was in the late bud–early flower stage (mid- to late-July) each year (2009, 2010, 2011 and 2012), and four paddocks (Treatment 2) were grazed by sheep each year when spotted knapweed was in the full-flower stage (mid-August). The four remaining paddocks were not grazed by sheep, representing the effects of bio-control insects alone (Treatment 3). Ten yearling Rambouillet wethers grazed each of the four paddocks in Treatments 1 and 2 for about seven days per year (total = 40 wethers/month/year). Sheep remained grazing in the paddocks until desirable grasses reached a 3- to 4-inch residual stubble height or when 90-100% of spotted knapweed buds/flowers/seed-heads were removed, whichever occurred first. Yearling wethers averaged 68 kg/animal, and stocking rate was 1.3 AUM/ha. Each month and year, all wethers were randomly assigned to the treatment paddocks.
Bio-control insect abundance and plant community fitness were sampled before sheep grazing in July 2009, 2010, 2011 and 2012, and again in July 2013. All spotted knapweed plants were counted and categorized by age class (seedling, adult). Spotted knapweed seedlings were counted within ten 50 × 50-cm quadrats per paddock, spaced at 3-m intervals along a 30-m transect near the center of each paddock. Adult plants were counted within three 1.0 × 1.0-m quadrats per paddock, spaced at 10-m intervals along each transect. All adult spotted knapweed plants rooted in each quadrat were excavated, taproots were dissected, and the absence or combined presence of Cyphocleonus and Agapeta larvae or feeding tunnels was recorded (Lejeune et al. 2005; Knochel and Seastedt 2010). Larinus was sampled with sweep nets within three 1-m × 3-m quadrats per paddock. All insects were counted in the field and then promptly returned to their respective quadrats.
Plant community composition was sampled within ten 50 × 50-cm quadrats per paddock. Quadrats were spaced at 3-m intervals along a 30-m transect near the center of each paddock, and plant canopy cover percentage was estimated by lifeform (perennial graminoids, annual grasses, spotted knapweed, other forbs). At plant senescence, but before seeds (achenes) dehisced in August 2009, 2010, 2011 and 2012, all buds and seed-heads (capitula) on spotted knapweed plants were counted and collected within ten 50 × 50-cm quadrats per paddock. These quadrats also were spaced at 3-m intervals along a 30-m transect located near the center of each paddock. In the laboratory, seeds were extracted from buds and seedheads using a rub board, counted, and tested for viability using the tetrazolium test.
Experimental design was a split-plot in time. The whole plot factor was defoliation treatment (bio-control insects alone, July grazing + bio-control insects, and August grazing + bio-control insects) and the subplot factor was year (2009, 2010, 2011 and 2012). The cumulative effects of four consecutive years of treatment were analyzed using analysis of variance and covariance in the Generalized Linear Model of SAS software (Version 9.3, SAS Institute, Cary, NC). Analyses of covariance used a variable’s July 2009 pre-grazing abundance as its covariate. Production of spotted knapweed seeds, production of viable spotted knapweed seeds and spotted knapweed seedling density were analyzed with analysis of variance because no covariate measure was made in 2009. Treatment means were compared using Fisher’s Protected LSD Test, and all differences were considered significant at P ≤ 0.05. Plant community comparisons between 2009 and 2013 were made with a t-test. Percentage data were arcsine square root transformed before statistical analysis. Density data were evaluated for deviations from normality using the Shapiro-Wilkes test (P ≤ 0.05). Densities of bio-control insects and densities of viable spotted knapweed seeds were transformed with the square root of Y + ½, whereas densities of all spotted knapweed seeds and densities of spotted knapweed seedlings were transformed with log10(Y + 1) (Steel and Torrie (1980). Means and standard errors presented in the text and tables are from untransformed data.
Our results demonstrate that four consecutive years of targeted sheep grazing did not harm the bio-control insects that we evaluated. Densities of Larinus spp. (P = 0.11), Cyphocleonus achates (P = 0.83) and Agapeta zoegana (P = 0.86) did not differ among treatments. Mean densities were 3.5 Larinus/m2, 0.4 Cyphocleonus/m2 and 1.5 Agapeta/m2.
In addition to worries about the effects of targeted sheep grazing on bio-control insect populations, landowners sometimes worry that targeted sheep grazing may decrease the abundance of desirable grasses and forbs while causing annual grasses to increase. None of these concerns materialized in our study. Plant community composition did not differ among treatments after four consecutive years of targeted sheep grazing), with perennial graminoids averaging 18% composition (P = 0.15), annual grasses 25% (P = 0.33), spotted knapweed 44% (P = 0.48) and other forbs 13% (P = 0.87). All three treatments, however, reduced the density of adult spotted knapweed plants 47%, from 16.3 plants/m2 in 2009 to 8.6 plants/m2 in 2013 (P < 0.01).
Targeted sheep grazing removed 96% of spotted knapweed buds/flowers/seed-heads in both July and August and dramatically impacted spotted knapweed reproduction. Compared with bio-control insects alone, 74% fewer spotted knapweed seeds were produced in July-grazed paddocks and 97% fewer in August-grazed paddocks (P < 0.01). Similarly, production of viable spotted knapweed seeds averaged 86% less in July- and August-grazed paddocks (P = 0.03) than in paddocks treated with bio-control insects alone. Spotted knapweed seedling density also was dramatically reduced by targeted sheep grazing compared with bio-control insects alone, with spotted knapweed seedling density 93% less in July-grazed paddocks and 68% less in August-grazed paddocks (P < 0.01). Finally, the densities of spotted knapweed seeds produced in the sheep-grazed paddocks (i.e., 45 seeds/m2 in July-grazed paddocks and five seeds/m2 in August-grazed paddocks) were far less than the 160 seeds/m2 needed to sustain spotted knapweed populations (Story et al. 2008), whereas the quantity of seeds produced with bio-control insects alone (175 seeds/m2) exceeded the minimum amount required.
In summary, targeted sheep grazing and the bio-control insects we studied were compatible and, when integrated together, targeted sheep grazing and bio-control insects provided greater control of spotted knapweed than bio-control insects alone. Targeted sheep grazing was effective when applied in either July or August, but spotted knapweed reproduction was suppressed best when targeted sheep grazing was applied in mid- to late-July when spotted knapweed was in the late bud–early flower stage.
Benzel, K.T., T.K. Brewer, and J.C. Mosley. 2009. Defoliation timing effects on spotted knapweed seed production and viability. Rangeland Ecology and Management 62:550-556.
DiTomaso, J.M. 2000. Invasive weeds in rangelands: species, impacts, and management. Weed Science 48:255-265.
Duncan, C.A. 2005. Spotted knapweed, Centaurea stoebe L. ssp. Micranthos (Gugler) Hayek. In: C.A. Duncan and J.K. Clark [EDS.], Invasive Plants of Range and Wildlands and their Environmental, Economic, and Societal Impacts. Lawrence, KS, USA: Weed Science Society of America. p. 51-68.
Jacobs, J., S.E. Sing, and J.M. Martin. 2006. Influence of herbivory and competition on invasive weed fitness: observed effects of Cyphocleonus achates (Coleoptera:Curculionidae) and grass-seeding treatments on spotted knapweed performance. Environmental Entomology 35:1590-1596.
Griffith, D., and J.R. Lacey. 1991. Economic evaluation of spotted knapweed [Centaurea maculosa] control using picloram. Journal of Range Management 44:43-47.
Henderson, S.L., T.K. Mosley, J.C. Mosley, and R.W. Kott. 2012. Spotted knapweed utilization by sequential cattle and sheep grazing. Rangeland Ecology and Management 65:286-291.
Knochel, D.G., N.D. Monson, and T.R. Seastedt. 2010. Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe). Oecologia 164:701-712.
Knochel, D.G., and T.R. Seastedt. 2010. Reconciling contradictory findings of herbivore impacts on the growth and reproduction of spotted knapweed (Centaurea stoebe). Ecological Applications 20:1903-1927.
Lejeune, K.D., K.N. Suding, S. Sturgis, A. Scott, and T.R. Seastedt. 2005. Biological control insect use of fertilized and unfertilized diffuse knapweed in a Colorado grassland. Environmental Entomology 34:225-234.
Müller-Schärer, H., and D. Schroeder. 1993. The biological control of Centaurea spp. in North America: Do insects solve the problem? Pesticide Science 37:343-353.
Ridenour, W.M., J.M. Vivanco, Y. Feng, J. Horiuchi, and R.M. Callaway. 2008. No evidence for trade-offs: Centaurea plants from America are better competitors and defenders. Ecological Monographs 78:369-386.
Seastedt, T.R., D.G. Knochel, M. Garmoe, and S.A. Shosky. 2007. Interactions and effects of multiple biological control insects on diffuse and spotted knapweed in the Front Range of Colorado. Biological Control 42:345-354.
Sheley, R.L., J.S. Jacobs, and M.L. Carpinelli. 1999. Spotted knapweed. In: R.K. Sheley and J.K. Petroff [EDS.], Biology and Management of Rangeland Weeds. Corvallis, OR, USA: Oregon State University Press. p. 350-361.
Steel, R.G.D., and J.H. Torrie. 1980. Principles and procedures of statistics: A biometrical approach. 2nd Ed. New York: McHraw-Hill. 633 p.
Story, J.M., N.W. Callan, J.G. Corn, and L.J. White. 2006. Decline of spotted knapweed density at two sites in western Montana with large populations of the introduced root weevil, Cyphocleonus achates (Fahraeus). Biological Control 38:227-232.
Story, J.M., L. Smith, J.G. Corn, and L.J. White. 2008. Influence of seed-head attacking biological control agents on spotted knapweed reproductive potential in western Montana over a 30-year period. Environmental Entomology 37:510-519
Surber, L.M.M., M.E. Rude, B.L. Roeder, T.K. Mosley, A.V. Grove, J.W. Walker, and R.W. Kott. 2011. Percent spotted knapweed (Centaurea stoebe) in the diets of grazing sheep. Invasive Plant Science and Management 4:95-101.
Thrift, B.D., J.C. Mosley, T.K. Brewer, B.L. Roeder, B.E. Olson, and R.W. Kott. 2008. Prescribed sheep grazing to suppress spotted knapweed on foothill rangeland. Rangeland Ecology and Management 61:18-25.
Williams, S., and T. Prather. 2006. Goats: a tool for controlling spotted knapweed. Journal of Extension 44(5):Research in Brief Number 6. 5 p.
Our project created the new knowledge needed to enable landowners and weed managers to confidently integrate targeted sheep grazing and bio-control insects to suppress spotted knapweed. On-campus university teachers, Extension educators, government agency personnel and others can use our results to educate students, landowners and weed managers who may resist applying targeted sheep grazing where bio-control insects are established for fear that bio-control insects may be harmed. Because of our project, more acres of spotted knapweed infestations will be suppressed; suppression will be more effective than when using bio-control insects alone; herbicide use and associated environmental risks will decrease on some of the 7+ million acres of spotted knapweed infestations in North America; and landowners and weed managers will spend less money controlling spotted knapweed.
One direct outcome of our project is that the Confederated Salish and Kootenai Tribe has reduced its herbicide use and expanded its integrated use of targeted sheep grazing and bio-control insects to suppress spotted knapweed on tribal rangelands. In addition, our project fostered cooperative working relationships among diverse groups, including the Tribal Council, tribal members and employees of the Confederated Salish and Kootenai Tribes; Montana State University Extension; the Montana Agricultural Experiment Station; U.S. Fish and Wildlife Service; Bureau of Land Management; Montana State University Seed Laboratory; Polson Stockmen’s Association; and the Montana Woolgrowers Association.
Educational & Outreach Activities
Project results were presented to 35 different audiences. Presentations included two field days, 29 seminars/workshops and four scientific conferences. Attendees included livestock producers, other landowners, county weed board members, the Tribal Council of the Confederated Salish and Kootenai Tribe, government agency personnel, university Extension educators, university students, researchers, consultants, county weed supervisors and commercial herbicide applicators. We estimate that at least 1,385 people were reached by these presentations. Information gleaned from this project will continue to be disseminated for many years via Extension teaching presentations. For example, another seminar will be presented to the Montana Wool Growers Association in December 2013. In addition, one more scientific conference presentation has been accepted and will be made in February 2014, and one refereed journal manuscript is currently in preparation. Listed below are the publications from the project:
Frost, R., J. Mosley, B. Roeder, T. Mosley, and J. Marks. 2011. Can biological control and targeted sheep grazing be integrated to suppress spotted knapweed? Abstract. Montana Weed Control Association Annual Meeting, Great Falls, Montana.
Frost, R. 2012. Grazing management—a tool for invasive species management. Abstract. Society for Range Management Annual Meeting. Spokane, Washington.
Frost, R.A., J.C. Mosley, B.L. Roeder, T.K. Mosley, and G. Marks. 2012. Can biological control and targeted sheep grazing be integrated to suppress spotted knapweed? Abstract. Society for Range Management Annual Meeting, Spokane, Washington.
Frost, R.A., J.C. Mosley, T.K. Mosley, and B.L. Roeder. 2013. Alternative grazing strategies for industry diversification and rangeland improvement. Proceedings of the Western Section American Society of Animal Science 67:31-35.
Frost, R.A., J.C. Mosley, B.L. Roeder, T.K. Mosley, and G. Marks. 2013. Can targeted sheep grazing be integrated with bio-control insects to suppress spotted knapweed? Refereed journal manuscript in progress.
Mosley, J.C., R.A. Frost, B.L. Roeder, T.K. Mosley, and G. Marks. 2014. Is targeted sheep grazing compatible with biological control of spotted knapweed? Abstract accepted. Society for Range Management Annual Meeting, Orlando, Florida.
Targeted sheep grazing to suppress spotted knapweed on Montana rangeland is estimated to cost $4-8/acre. This amount is considerably less than costs for mowing spotted knapweed ($50/acre) or applying herbicide once every three to five years at an annualized cost of $13-18/acre. Consequently, landowners and weed managers will spend $5-14/acre less for suppressing spotted knapweed when targeted sheep grazing is used rather than herbicides, and $36-45/acre less when targeted sheep grazing is used rather than mowing.
About 590 farmers and ranchers attended our outreach teaching seminars, workshops and field days. These landowners own or manage approximately 4.5 million acres of grazing land. We recommend that farmers and ranchers consider adding targeted sheep grazing to areas where bio-control insects have been established to suppress spotted knapweed, and we recommend that targeted sheep grazing be applied when spotted knapweed is in the late bud-early flower stage (mid-July) or full flower stage (mid-August), before spotted knapweed seeds dehisce. We recommend that sheep be moved to a new area when desirable grasses reach a three- to four-inch residual stubble height or when 90-100% of spotted knapweed buds/flowers/seed-heads are removed, whichever occurs first.
Areas needing additional study
The next opportunity for integrated management of spotted knapweed is to explore combining targeted cattle grazing with bio-control insects. Integrating bio-control insects with cattle, versus sheep or goats, is preferable for several reasons. First, targeted sheep or goat grazing is not suitable for many areas due to predation risks and potential disease transfer to wildlife. Second, many cattle producers do not have the expertise, desire or financial means to add the infrastructure and labor needed to adapt their cattle operation to include sheep or goats. The most compelling reason, however, is because cattle are far more plentiful than sheep or goats in the U.S.; therefore, many more acres of spotted knapweed infestations potentially can be treated with cattle.