Joe,
But using insecticidal controls in the refuge of non-Bt corn
would be more likely to kill insects *susceptible* to Bt corn,
right? The resistant ones (if any are present) are in the Bt
corn! This is a numbers game. As long as there are way
more non-resistant individuals flying around than heterozygotes
(that is heterozygous for the resistance gene), the chances of
heterozygotes mating is very small."
The heterozygous resistant insects are equally susceptible to Bt
corn as are the wild type insects. Since resistance is recessive only
the homozygous recessive insects are resistance to Bt toxin. However,
all are equally sensitive to chemical insecticides. The chemical
insecticides will efficiently eliminate homozygous resistant,
heterozygous and wild type insects meaning that use of chemical
pesticides certainly acts to prevent Bt resistance from taking hold.
Since chemical pesticides are mainly used on the refugia, they
potentially make the refugia less effective, and could conceivably
promote the evolution of resistance. For various reasons I posted, I
don't think this is a problem in the case of ECB and rootworm in corn.
The main point I have been making is that the scientific papers
should be reporting that chemical pesticides are allowed and used
on Bt corn whether yieldgard borer, yieldgard root worm or
yieldgard plus that combines the two. The same story is true
for the other Bt corn varieties.
But Joe, the pesticides applied on the Bt corn itself are not used to
control the same pests that are controlled by the Bt corn. Low rate
Poncho is used on rootworm-resistant corn to control *different pests*.
Such use cannot affect evolution of resistance in the ECB or rootworm.
There is no doubt that the chemical insecticides will act
to prevent Bt tolerant insects from being established in
the field.
Not if they have no effect on the pests in question. Rootworm requires
heavy-handed chemical application to provide mediocre control, and
chemical control of ECB requires carefully timed application to provide
mediocre control. Putting on low dose Poncho or Cruiser, using
planterbox permethrin, or spraying for cutworm will have NO effect on
rootworm or ECB.
Certainly both the scientists who report and the journal reviewers
seem to agree to ignore the impact of the chemical use.
They ignore this chemical use because it is not relevant to the
resistance discussion. Granted, some farmers, ignorant or oversold by
chemical company hype, may apply insecticidal controls that duplicate
the control provided by Bt corn. But that cannot be widespread or
important (farmers are not that stupid). And anyway, it is unclear
what such duplication would do to the evolution of resistance.
Farmers from other countries should be informed about the use of
chemical insecticides in Bt corn and other Bt crops.
That's right, they should be informed that they do not need to use
chemical insecticides on Bt corn, especially if they use varieties with
the Herculex 1 trait(1), and they need to know that Bt cotton can
reduce insecticide application by about 70% (2)
Dale
1. Lowland tropical and subtropical corn experiences severe pest
pressure from Fall Armyworm (Spodoptera). Standard Bt corn (Mon 810)
provides some control.
http://www.fcla.edu/FlaEnt/fe84p37.pdf#search='Bt%20corn%20and%20armyworm'
Another Bt trait Cry1F (Herculex 1) provides better Spodoptera control:
http://www.pioneer.com/usa/agronomy/insects/1214.htmvv
2. Bt cotton does not control all the pests, but it helps a lot
http://www.isaaa.org/kc/Global%20Status/crop/gmcotton/experience.htmv
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