The Colorado Department of Agriculture has released a number of biocontrol agents against diffuse and spotted knapweed including a two root boring beetles, two seed head feeding flies, seed head feeding weevils and seed head feeding moths. Several of these are well established in Colorado and are probably having an impact on the knapweeds.
The most successful agent for use against diffuse knapweed is the seed head feeding weevil, Larinus minutus. The beetle feeds on the seed heads but is also a foliage feeder as an adult. Large scale control against diffuse knapweed has been seen at a number of locations along the Front Range. The Colorado Department of Agriculture currently releases this agent as the primary biocontrol for diffuse knapweed.
The root boring weevil Cyphocleonus achates weakens plants by destroying the root system. This large weevil has been effective against both diffuse and spotted knapweed and collectable populations have recently been seen on the Front Range. The Department of Agriculture will distribute these beetles as number allow.