Biotic suppression of invasive weeds in Washington state: a half-century of progress

Washington state has long employed natural enemies for the management of invasive aquatic and terrestrial plant species infesting state, federal, tribal, and privately owned water bodies and lands. During the last 55 years, a total of 72 natural enemies have been used for the biological control of 30 nonindigenous weeds. Over 85% of the organisms deployed have been insects, the remaining bioagents consisting of fungi, mites, and nematodes. Of these accidental and intentional introductions, 81% have readily established and yielded various levels of suppression against 27 annual, biennial, and perennial weeds. In recent years, populations of Dalmatian toadflax, Linaria genistifolia ssp. dalmatica, diffuse knapweed, Centaurea diffusa, and other undesired plant species have been reduced by a diverse assemblage of biotic agents.