Centaurea stoebe (Asteraceae), a forb native to Eastern Europe, now occupies millions of ha in rangelands of North America. Land managers spend millions of dollars annually in control costs and recovery of lost forage, and meanwhile spotted knapweed perseveres as a detriment to the native biodiversity and soil stability of infested ecosystems. Despite intense scientific inquiry aimed at understanding and explaining its unusual dominance, substantial uncertainty remains in our understanding of control mechanisms, and sustainable methods to control the plant at a regional scale remain controversial. In greenhouse, enclosure, and field studies, I developed a top-down (herbivory) and bottom-up (soil resources/plant competition) approach to test hypotheses on how the intensity and interaction of these biotic and abiotic factors influence spotted knapweed survivorship, growth, and reproductive output, at different ontogenetic stages.

In a greenhouse experiment, competition with a native grass reduced spotted knapweed seedling germination and survivorship. While high soil nitrogen (N) improved survivorship and growth for seedlings in monoculture, unexpectedly, low soil N conditions improved survivorship in the presence of grasses possibly through a relaxation of grass growth and competition for water. Seedling recruitment in an intact plant community was extremely low, as was the viability of seed from the existing soil seed bank. Thus, while low soil N may facilitate seedling survival, the overall ability of competition to greatly suppress growth and survivorship outweighs this benefit under most conditions.

In enclosure and field experiments I found that herbivory by the root feeding weevil (Cyphocleonus achates) and flower head weevil (Larinus minutus) reduced reproductive output, growth, and survivorship of juvenile and adult plants. Spotted knapweed growth was reduced to the greatest extent under conditions of low N, higher neighboring plant cover, and more intense herbivory. Plants undergoing herbivory by both insects produced a less than 50% of the biomass of those with only one or neither species. In support of the cumulative stress hypothesis, these results indicate that the use of multiple specialist biological control herbivores in combination with cultural management techniques that increase plant competition, should lead to decreases in spotted knapweed abundance to levels of less ecological and economic concern in many rangelands across the west.