Sustainable Agriculture Literature Review - Boulder County

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7.3 Pesticide Resistance
Due to the widespread use of pesticides, many target species including both insects and
plants, have evolved resistance to the pesticides designed to eradicate them. The number
of insect species known to display pesticide resistance has increased from less than 20 in
1950 to over 500 as of 1990. 595 The North American Herbicide Resistance Action
Committee and the Weed Science Society of America have recorded over 300 different
biotypes that have displayed resistance to one or more herbicides (Graph 21). Of these
300 resistant biotypes, the WSSA estimates that there are over 125 in the U.S. infesting
up to 18 million acres. 596
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Graph 21: Growth in Herbicide Resistant Biotypes 597
The WSSA states that there are currently four different types of herbicide resistant weeds
in Colorado. Local weed scientists estimate that there are 2,260 sites and more than
66,300 acres of barley, corn, and wheat fields infested with herbicide resistant weeds in
Colorado. The first herbicide resistant weed reported in 1982 was Redroot Pigweed
(Amaranthus retroflexus) with resistance to atrazine. The most widespread resistant weed
in Colorado is Kochia (Kochia scoparia), which evolved a resistance to atrazine (1982),
metsulfuron-methyl (1989), and triasulfuron (1989). Wild Oat (Avena fatua), with
resistance to diclofop-methyl is the most recently discovered (1997) new type of resistant
weed in Colorado. While only four resistant weeds have been found in Colorado, over 23
resistant biotypes have been found in Kansas, raising concern that other resistant varieties
may transfer to or develop in Colorado. 598 Weeds that are herbicide resistant have been
witnessed in Colorado since 1982, and are not solely associated with the adoption of GE
crops.
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