Biological Control of Weeds in the United States and Canada

The worldwide problem of nonindigenous (NI) pest invasions is as old as human migrations and trade between ecologically isolated areas. This problem began with the discovery and settlement of, and trade among, the Americas, Australia, southern Africa, the Pacific islands, and other areas by Europeans. These settlers brought with them alfalfa, cabbage, wheat, cattle, and chickens as well as seeds that escaped, grew, and later became weeds that devastated indigenous (IN) plants; rats that ate stored food; cats that destroyed birds; cabbage worms and Hessian flies; and smallpox and measles. Some pests traveled back to Europe, for example, ragweeds, potato blight, and syphilis. The former British colonies can be identified by plagues of prickly-pear cacti brought from the Americas to grow cochineal insects used to dye the British Army’s red coats. Homesick settlers brought many garden ornamentals; several of these became serious weeds. We cannot blame settlers for bringing these plants, which had never been weeds in their IN range; no one could imagine that these plants would become problems elsewhere.