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Issue : Development of Pest Resistance
Widespread and intensive use of GM crops
that are resistant to pests have the potential of selecting for pests that
are resistant to the GM crops. While the possibility of pest developing
resistance to GM crops like Bt corn is only a theory, pests have a long
history of developing resistance to any pest management tactic that is
used for a long period of time over a wide area. The example pests being
able to overcome pest management strategies are too numerous to list!
Consider the western corn rootworm beetle in Illinois and Indiana. For
more than 20 years it was effectively controlled through the use of a
corn-soybean rotation. The eggs that were laid one summer in a corn field
would hatch the following year in what has become a soybean field. This
pest had been a problem only with continuous corn. But it adapted. Now
only a portion of the female beetles that lay their eggs in soybean fields and rootworms
are a serious problem in first-year corn in this area. Don't underestimate
the ability of insect pests to adapt!
Cornborer
To counter the ability of pests to develop
resistance, farmers are required to use resistant management strategies.
For example, the required resistance management plan with Bt corn is to
plant some acreage with non-Bt hybrids. This is called the refuge strategy.
In it, non-Bt acreage on each farm serves as a refuge, allowing some Bt-susceptible
corn borers to survive. Use
of resistance management strategies are required and will delay or prevent
the development of pest resistance.
Earworm
Changes in resistance of target pests
Tabashnik et al., (2005)
report on changes if any in resistance of the pink bollworm after eight years
of widespread cultivation of bollworm resistant Bt cotton in
Arizona
. No increase in resistance was
observed from the 1997 to 2004 growing seasons.
Part of this lack of an increase in observed resistance to Bt toxin in
pink bollworm over the eight year period is attributed to the proper usage of
refuges.
Tabashnik, B.E., T.J. Dennehy, and Y. Carriere. 2005. Delayed resistance to
transgenic cotton in pink bollworm. PNAS 102:15389-15393.
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