| Biological control insects can be a sustainable management strategy to control negative effects of invasive plants. However, population-level effects of spotted knapweed biological control insects are debated. I used a combination of field manipulations and modeling to examine how herbivory and precipitation effect population dynamics. Field studies demonstrated that recruitment rates were reduced by herbivory and plant competition, but varied by site. The best model to explain observed population declines included the effects of biological control agents. The results also show that the ability to achieve control of knapweed (i.e., conditions where lambda ≤ 1) depend on high densities or large per capita impact of biological controls, both of which are known to vary at this field site and elsewhere. In certain habitats (e.g., disturbed riparian areas) insect herbivory alone is unlikely to control densities, but knapweed densities in most other habitats can be reduced by biological control agents. |
