Biological control is a form of weed control that uses a living agent to control weeds. A bio control agent can be insects, fungus, or grazing animals. A good bio control "bug" never completely destroys its plant host, but rather controls the population to a relatively lower population. Some bugs work better than others. Some years conditions are very hard on the bugs' ability to reproduce and thrive. Sometimes its weather, other times it may be due to lack of their specific weed host. The bugs population lags behind the host weed. When a lot of weeds build up, the next year there are a lot of bugs. Those bugs eat the weeds so next year there are fewer weeds to support bugs and their population crashes. It goes in cycles.
The Oregon Department of Agriculture Noxious Weed Control program administers the biological control program. There are many types of insects and fungus listed on their website for the various noxious weeds found in Oregon. The history of the weeds and release of biocontrol agents is included.
Most of the important biocontrol agents against weeds are already populating the noxious weeds in Oregon. If you inspect your weeds and do not find the bugs, you can visit other weed sites and collect the biocontrols and release them on your weeds. You can also purchase some of these biocontrols from a company in Montana. Integrated Weed Control (1- 888-319-1632 or firstname.lastname@example.org). Make sure you follow the ODA's recommendations for which biocontrol to use with each specific weed.
For information on using livestock as biological control agents against weeds see Livestock Grazing Guidelines for Control Noxious Weed in the Western United States