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We distributed a 16-question survey concerning noxious weed abundance, impacts, and management to livestock producers grazing on privately owned or leased grazing lands in Montana. The noxious weeds most commonly reported as being present on respondents’ grazing units were Canada thistle [Cirsium arvense (L.) Scop.] (64% of grazing units) and leafy spurge (Euphorbia esula L.) (45% of grazing units), and these species also reportedly caused the greatest reductions in livestock forage. Houndstongue (Cynoglossum officinale L.) was more prevalent than either spotted knapweed (Centaurea stoebe L.) or diffuse knapweed (Centaurea diffusa Lam.) (39% vs. 32% and 10%, respectively, of grazing units), but collectively C. stoebe and C. diffusa were reported to cause greater forage reductions than C. officinale. The top three strategies used to manage noxious weeds were chemical control, grazing, and biological control. Combining survey responses with forage-loss models derived from field data for C. stoebe and E. esula, we estimated the combined cost of noxious weed management and forage losses on privately owned rangeland to be $3.54 ha−1 yr−1, or $7,243 annually for an average size grazing unit (i.e., 2,046 ha [5,055 ac]). Our estimates of economic losses are lower than many estimates from previous studies, possibly because we focused only on direct costs related to private grazing land, while other studies often consider indirect impacts. Nonetheless, our estimates are substantial; for example, our estimated loss equates to 24% of the average per-hectare lease rate for Montana grazing land.

The use of biological control insects is a promising option for suppressing spotted knapweed, a nonindigenous perennial forb that infests more than 3 million hectares of North American rangeland. Efficacy increases when spotted knapweed is attacked by more than one phytophagous insect; however, combined herbivory by biological control insects has not achieved widespread suppression of spotted knapweed in North America. Here we expand the concept of combined herbivory beyond two or more species of biological control insects to include a vertebrate herbivore, specifically targeted grazing by domestic sheep. Our experiment on foothill rangeland in northwestern Montana evaluated spotted knapweed response to three treatments: (1) biological control insects only, (2) biological control insects + targeted sheep grazing applied in late July (spotted knapweed in late bud–early flower stage), and (3) biological control insects + targeted sheep grazing applied in mid-August (spotted knapweed in full-flower stage). We combined targeted sheep grazing with herbivory by three species of biological control insects: knapweed flower weevil, knapweed root weevil, and sulfur knapweed root moth. Treatments were applied during four consecutive years (2009 to 2012). Spotted knapweed fitness was suppressed more where targeted sheep grazing and biological control insects were combined vs. areas treated with biological control insects alone. Combined herbivory was effective when targeted sheep grazing was applied during either late July or mid-August, but July grazing was more effective. Spotted knapweed produced 96 to 99% fewer viable seeds in sheep-grazed areas. After 4 yr of treatment, total spotted knapweed plant density (seedlings, juvenile, and adult plants) was 86% less in July-grazed areas and 61% less in August-grazed areas than in areas treated with biological control insects alone. Combined herbivory by targeted sheep grazing and biological control insects reduced adult plant density and prevented compensatory recruitment of spotted knapweed, but treatment with biological control insects alone did not.

Extensive areas in the upper Midwest have been invaded by spotted knapweed, and effective management strategies are required to reestablish native plant communities. We examined effects of mowing, mowing plus clopyralid, or mowing plus glyphosate in factorial combination with hand pulling and burning on knapweed abundances on a knapweed-infested site in western Michigan. We applied mowing and herbicide treatments in summer 2008, and seeded all plots with native grasses and forbs in spring 2009. We conducted the knapweed pulling treatment from 2009 to 2012 in July. The prescribed burn was conducted in April 2012. By 2012, hand pulling reduced adult knapweed densities to 0.57 ± 0.12 m−2(0.053 ± 0.011 ft−2) (mean ± SE), which was 5.8% of nonpulled treatments, juvenile densities to 0.29 ± 0.07 m−2(2.1% of nonpulled treatments), and seedling densities to 0.07 ± 0.06 m−2(2.6% of nonpulled treatments). After 3 yr, hand pulling reduced seed bank densities to 68 ± 26 m−2as compared to 524 ± 254 m−2in nonpulled treatments and 369 ± 66 m−2in adjacent untreated areas of the study site. Without hand pulling, effects of mowing or mowing plus glyphosate were short-lived and allowed knapweed to rapidly resurge. In comparison, although a single mowing plus clopyralid treatment maintained significantly reduced densities of knapweed for 4 yr, by 2012 knapweed biomass in the nonpulled clopyralid treatment was approximately 60% of that in the other nonpulled treatments. Burning had minimal impacts on knapweed densities regardless of treatment combination, probably as a result of low fire intensity. Results demonstrated that persistent hand pulling used as a follow-up to single mowing or mowing plus herbicide treatments can be an effective practice for treating isolated spotted knapweed infestations or for removing small numbers of knapweed that survive herbicide applications.

Predicting site vulnerability to nonnative plant establishment remains a difficult goal. Seedling survival is an important component of population dynamics and can affect the success of control strategies. Field manipulations allow potential causal mechanisms of site vulnerability to be evaluated under realistic environmental conditions. We conducted field studies to determine the effects of plant competition and differing precipitation regimes on spotted knapweed seedling survival. We also examined the effect of herbivory on rosette survival and growth. Seeds were sown into plots with vegetation intact or removed at three sites. Seeds were also sown into plots where plant competition and precipitation were manipulated in a factorial design at a single site. Field studies demonstrated that site accounted for much of the variation in emergence rate, while herbivory and plant competition affected seedling survival rates. We observed a wide range in emergence rates, with site averages ranging from 13.1 to 42.5%. Survival the following year ranged from 0.5 to 9.4% of sown seeds. Rosette survival was significantly higher when herbivores were excluded from plots. Below average precipitation reduced seedling survival; however, even with supplemental water, dry-down of exposed sites resulted in low seedling survival. Of the 8,000 seeds added to plots in one study, by autumn, only eight plants resulted, seven of which survived in watered plots with intact vegetation. Collectively, these results show that seedling survival is a critical phase in spotted knapweed population dynamics and can vary among habitats on the basis of plant competition and precipitation. Furthermore, herbivory affects all stages of the lifecycle from the seedling onward. The observed differences help explain the reported variability in seedling survival in the literature and inform efforts to control spotted knapweed using plant competition and biological controls.

The objective of this study was to use fecal near-infrared reflectance spectroscopy (NIRS) to estimate the percent spotted knapweed in the diet of grazing ewes to fine tune grazing prescriptions. In trial 1, five ewes were randomly selected from a band of 900 ewes to estimate weekly variation in percent spotted knapweed in the diet. Fecal samples from these ewes were collected weekly from June 22 through August 17 (nine collections per ewe for a total of 45 observations). In trial 2, fecal samples were collected from 89 ewes in the band to estimate variability in percent spotted knapweed in the diet among sheep at two sampling dates corresponding to the bud (July 13) and postflowering (August 15) stages of spotted knapweed phenology. In trial 1, the percent spotted knapweed in the diet was similar (P > 0.05) on June 22, June 29, July 13, and July 20 but was greater (P < 0.05) on July 6 than June 22 and 29. Percent spotted knapweed in the diet was also greater (P < 0.05) from July 27 through August 17 than from June 22 through July 20. On July 13 in trial 2, 55% of ewes had 0 to 5% spotted knapweed in their diets, whereas 44% of ewes had 5 to 20% spotted knapweed in their diets. On August 15, only 1% of ewes had < 10% spotted knapweed in their diets, whereas 44% of ewes had 20 to 25% spotted knapweed in their diets. The estimated percent spotted knapweed in the diet of the 89 ewes in trial 2 was similar to that of the five ewes during the same time period in sampling trial 1 (5.3% compared with 5.0% for sampling trials 1 and 2, respectively, on July 13 and 22.0% compared with 20.7% on August 15 and 17 for trials 1 and 2, respectively). This is the first study to estimate percent spotted knapweed in the diet of individual grazing ewes. These data suggest that the best time to graze spotted knapweed–infested pastures would be in late July or early August, when spotted knapweed is flowering, but before viable seed production.

Invasive plants have devastating effects on ecosystems and biodiversity that early intervention can prevent. Eradication or containment of new invasions is difficult to achieve because of constraints posed by the low density and detectability of individuals. Domestic dogs trained to cue on distinctive scents might provide an effective method to detect spotted knapweed. The objective of this study was to compare the accuracy and detection distances of dogs to humans in locating new spotted knapweed (Centaurea stoebe) invasions. Three dogs, trained to detect knapweed using scent discrimination and tracking techniques, were compared with human surveyors. Seven sampling units (0.5 ha [1.2 ac]) were delineated in a grazed dryland pasture. Dogs, with their handlers, and human surveyors performed line-transect surveys in fall 2005 and spring, summer, and fall 2006. Dog accuracy for large-size knapweed targets (infestations 0.52 m3[18.4 ft3]) was similar to human accuracy and better than humans (94 vs. 78%) for medium-size targets (infestations 0.13m3). Dog accuracy (67%) was greater (> 81% probability) than humans (34%) for small targets (plants; 0.02 m3). Overall dog accuracy (81%) andF-measure scores (86%) were better than human scores, 59% and 74%, respectively. Human precision was greater (100%) than dogs at 94%. Dogs detected a larger percentage of small targets (80%) at distances greater than 7.9 m (26 ft) compared with humans at only 20%. Our results indicate dogs are more accurate than humans are, especially at critical detection of small spotted knapweed plants, and from greater distances. Invasive plant monitoring using detection dogs can provide greater overall accuracy of plant detection.

Invasive plants need to be managed after wildfire to suppress the invasive plant and to maintain or restore a desired plant community. Our study tested treatments that influence species availability and performance following a disturbance (wildfire). The overall objective was to determine the ability of herbicide and revegetation treatments to restore spotted knapweed–infested areas to desired plant communities after wildfire. The study consisted of a factorial combination of three herbicide application treatments (broadcast application, spot application, and no herbicide) and three seed mixture treatments (grass-only seed mix, a grass and forb seed mix, no seeding). Picloram was used for the herbicide. Both the broadcast and spot picloram application methods decreased spotted knapweed cover and density up to 80% while increasing desired grass cover and density up to 20% compared with the control. However, broadcast spraying picloram decreased species richness from 5.7 to 3.6 species 0.1 m−2 and decreased desired forb density and cover compared with spot-applied picloram treatment. Spot spraying resulted in an increase in other undesired forbs compared with broadcast spraying. Seeding with desired species had no effect on spotted knapweed cover or density. Spot spraying may help maintain desired species richness while managing spotted knapweed.

Periodic sweep-net sampling and capitula sampling were conducted between April and October 2007 to determine the seasonal phenology of Urophora quadrifasciata (Meigen) (Diptera: Tephritidae) on spotted knapweed, Centaurea stoebe subsp. micranthos (Gugler) Hayek (Asteraceae), in the Arkansas Ozarks of the United States of America. Urophora quadrifasciata produces up to three generations in Arkansas, rather than being bivoltine as in the northern United States of America. The first, second, and third generations of U. quadrifasciata reached peak adult numbers around 26 May, 29 June, and 27 July, respectively. Males tended to emerge earlier than females but the female:male ratio was approximately 1:1 for most of the season. The majority of the offspring of the three generations emerged between June and October, while approximately 38% entered diapause and emerged as adults in April-June of the following year; 3.4 ± 0.1 (mean ± SE) (range 1–12) flies emerged from each infested capitulum. The absence of other, competing knapweed biological control agents as well as very low rates of parasitism, mild weather conditions, and a longer knapweed growing season likely contributed to the adaptation and establishment of U. quadrifasciata on spotted knapweed in Arkansas.

Hybridization between species has the potential to change invasion dynamics. Field observations suggest that spotted knapweed and diffuse knapweed, two ecologically and economically destructive invasive plants, hybridize in their introduced range. As a first step towards understanding whether hybridization has affected the dynamics of the invasion of these species, we conducted field surveys in the introduced (North American) and native (European) ranges to discern patterns of hybridization and measured fitness-related traits among field hybrids and parental species. In North America we detected plants with hybrid morphology in 97% of the diffuse knapweed sites (n= 40); such hybrid plants were taller and more often exhibited polycarpy than plants with typical diffuse knapweed morphology. Hybrids were not detected in North American spotted knapweed sites (n= 22). In most regions surveyed in Europe, diffuse knapweed and spotted knapweed were isolated from each other and existed as distinct, nonhybridizing species. However, in Ukraine, the two species frequently coexisted within a site, resulting in hybrid swarms. On average, the plants from the North American diffuse knapweed sites (including plants with both diffuse and hybrid morphology), were larger than the apparently pure diffuse knapweed in the native range. The cross-continental patterns of hybridization likely are explained by differences in cytology. It recently has been confirmed that the spotted knapweed in North America is tetraploid whereas the diffuse knapweed is diploid. Genetic incompatibilities associated with these two cytotypes likely prevent ongoing hybridization. We hypothesize that hybrid individuals were introduced to North America along with diffuse knapweed. Because plants with hybrid morphology are found in nearly all North American diffuse knapweed sites, the introduction of hybrids likely occurred early in the invasion of diffuse knapweed. Thus, although the presence of hybrids might facilitate the ongoing invasion of diffuse knapweed into North America, elevated concern regarding their presence might not be warranted. Because such individuals are not likely to represent a new hybridization event, currently effective management strategies used in diffuse knapweed sites should not need alteration.

The influence of spotted knapweed on surface runoff and sediment yield was determined under simulated rainfall conditions near Garrison, MT. Comparisons were made in 12 paired plots: one of each pair dominated by bunchgrass and the other dominated by spoted knapweed. Runoff and sediment yield were 56% and 192% higher, respectively, for the spotted knapweed, rather than the bunchgrass vegetation trpes. Spotted knapweed invasion onto bunchgrass range of western Montana is thus detrimental to the protection of soil and water resources. Nomenclature: Spotted knapweed, Centaurea maculosa Lam. #3 CENMA. Additional index words: Erosion, CENMA.