Why Reduced Seed Production is Not Necessarily Translated Into Successful Biological Weed Control

Insects introduced as biological control agents of diffuse knapweed, Centaurea diffusa, primarily reduce seed production. Knapweed densities appear to be resilient to greatly reduced seed production. We have used measurements of seedling density and survival, and production of seeds by flowering diffuse knapweed plants to develop functions for a model of knapweed populations. These simulations show that improved seedling survival can compensate for reduced seed production. The model indicates that the introduction of seed predators may even increase plant density under some relationships between seedling density and survival, and flowering plant density and seed production. We suggest that it is important to demonstrate that weeds are seed limited before introducing exotic herbivores whose impact is to reduce seed numbers. Successful biological control of knapweed may be achieved by insect herbivores that increase the mortality of later growth stages of plants such as the rosette stage for which survival is not as strongly related to density. Knapweed is likely to be typical of invasive weeds with high seed production and this study suggests that seed feeders are not likely to be successful control agents for most weeds.