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.

Nomenclature: Knapweed flower weevil, Larinus spp.; knapweed root weevil, Cyphocleonus achates; sulfur knapweed root moth, Agopeta zoegana; spotted knapweed, Centaurea stoebe L.

Management Implications: Combined herbivory by targeted sheep grazing and biological control insects suppressed spotted knapweed more than biological control insects alone. Drought was not a contributing factor during our study. Sites inhabited by Cyphocleonus, Larinus, or Agapeta can be treated effectively with targeted sheep grazing during either late July (spotted knapweed in the late bud–early flowering stage) or mid-August (spotted knapweed in the full-flowering stage), but July grazing was more effective. After 4 yr of treatment, biological control insects combined with targeted sheep grazing in late July reduced spotted knapweed plant density 86% compared with areas treated with biological control insects alone. Spotted knapweed populations should decrease exponentially wherever weed management can (1) significantly reduce adult plant density and (2) keep spotted knapweed recruitment below the threshold needed to replace adult plants that die. Sheep-grazed areas in our study met both of these criteria, but areas treated with biological control insects alone did not. Combined herbivory by more than one species of biological control insect may be sufficient to suppress spotted knapweed populations in places where few spotted knapweed seedlings establish and transition to adults. However, mammalian herbivory may need to be combined with biological control insects to suppress spotted knapweed in those North American habitats where spotted knapweed seedlings and juvenile plants thrive.