Biological control involves the introduction of organisms, usually natural competitors of the invasive species, into the invaded environment in order to control the invasive species. Since 1970, 12 insects have been released to control diffuse knapweed. Of these 12, 10 have become established, and 4 are widely established ("Urophora affinis" and "Urophora quadrifasciata", the root boring beetle, "Sphenoptera jugoslavica", and the weevil "Larinus minutus"). Research based on simulation models have shown that for biocontrol agents to be effective, they must kill their host, otherwise plants can compensate by having increased seedling survival.