The key to management is to prevent it from going to seed. Diffuse knapweed invades overgrazed pastures, forms dense stands and may be toxic to horses. A also readily control diffuse knapweed when applied at the rosette to early-bolting growth stage. Transline is a noncrop herbicide, while Curtail may be applied in pastures, rangeland and noncrop areas. The Colorado Department of Agriculture offers several biological control agents.
Females lay eggs in developing flowers.
As they feed on heads, the larvae incite gall formation, which reduces seed formation. The gall fly overwinters inside the seed head as larvae. The normal procedure for redistribution is to cut last year's dead stand that contain larvae and tie the bouquet to a fence post near knapweed stands. Adults will emerge in spring and repeat the life cycle. Using biocontrol that affects the vegetative growth stage may be most effective to decrease diffuse knapweed populations.