In the last two decades the weed species, diffuse knapweed (Centaurea diffusa) and spotted knapweed (C. maculosa) have gained prominence in the British Columbia interior. These aggressive species have invaded over 40,000 hectares (100,000 acres) of British Columbia range and forest land and are spreading at a rate of about 10% per year. Knapweed can reduce the native forage by as much as 80 to 90% and thus destroy the food supply for cattle and wild animals such as elk and deer. If the weeds spread to their ecological limit over 1 million hectares in British Columbia could be affected, causing an estimated agricultural loss of $13 million annually (Cranston, R. 1985, B.C. Min. of Agric. 85–1). The potential extinction of native plant and animal species is a cost too great to be measured.