Agriculture Reference
In-Depth Information
pests become predominant in the pest population (see box, “Economics of
Resistance Management”). Developers of Bt crops and other researchers
recognized early on that insect resistance to Bt toxins could develop. Measures
to delay the onset of such resistance (such as refuges) were taken and, so far,
the emergence of insect resistance to Bt crops has been low and of “little
economic and agronomic significance” (NRC, 2010), but there are some
indications that insect resistance is developing to some Bt traits in some
areas.
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Also, since many users of HT crops rely solely on glyphosate to
control weeds, resistance to this herbicide was anticipated by some
researchers. Thus far, overreliance on glyphosate and a reduction in the
diversity of weed management practices adopted by crop producers have
contributed to the evolution of glyphosate resistance in 14 weed species and
biotypes in the United States.
Insect Resistance to Bt Crops
Prior to the availability of Bt crops, entomologists and other scientists
successfully argued that mandatory refuge requirements—planting sufficient
acres of the non-Bt crop near the Bt crop—were needed to reduce the rate at
which targeted insect pests evolved resistance. Such refuges slow the rate at
which Bt resistance evolves by allowing target insects that are susceptible to
the Bt toxin to survive and reproduce. To be effective, the refuge must be
positioned appropriately and be large enough to ensure that insects that survive
on the Bt acres mate with insects that survive on the non-Bt acres. Such
interbreeding increases the chances that their progeny are susceptible, having
inherited Bt resistance as a recessive trait.
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The U.S. Environmental
Protection Agency (EPA) instituted mandatory refuge requirements as a
condition of the registration of Bt corn and Bt cotton varieties for commercial
use in the United States. This was the first time regulations were used to
manage resistance to a pest control technology. Bt crop growers were required
to sign a contract with their technology provider to comply with minimum
refuge requirements, and technology providers were required to monitor and
enforce grower compliance. An analysis of more than a decade of monitoring
data suggests that the minimum refuge requirement, as well as natural refuges
that also serve as hosts for target insect pests, has helped delay the evolution of
Bt resistance (Tabashnik et al., 2008).
