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Re: [SANET-MG] insect resistance management in gm crops



Hi Dale,
In reply to your comments. First, The main Bt corn includes borer and
root worm protection (yielgard plus) while earlier yieldgard was borer
or root worm specific. My understanding is that root worm protection
generally involves both Bt and soil insecticide treatment.
http://www.uky.edu/Agriculture/Entomology/entfacts/fldcrops/ef147.htm
"The grower may apply insecticide treatments for control of corn root
worm larvae with the refuge corn "
http://apply-mag.com/mag/farming_corn_rootworm_technology/
"The corn refuge can be treated with a non-Bt insecticide to control
late-season pests such as corn borer; however, the YieldGard Rootworm
corn also must be treated".
My comment that Canada does not allow chemical insecticide treatment of
the refuge was this year changed in Ontario to allow insecticide.
"http://www.gov.on.ca/OMAFRA/english/crops/field/news/croptalk/2004/ct_0104a6.htm
Soil-applied insecticide treatments for control of corn rootworm larvae
may be applied to the refuge area. Insecticide use for adult control is
not permitted."
My understanding is that chemical insecticide use is costly but used a
great deal to market corn from the refuge.
Nest,turning to your comment about registering Bt corn with levels
sufficient to eliminate heterozygous  resistant insects.In genetic
terminology, the corn insects are described as being recessive meaning
that the resistant allele was quite (not expressed) in the heterozygote.
If the allele was partially expressed the heterozygote would be called
partially dominant.Corn resistance to Bt has been described as recessive
while cotton resistance was observed to be recessive at high toxin
levels but to be partially dominant in strains expressing low toxin levels.
I disagree with your view that the study I mentioned could not detect
resistance in a short time span. The study was capable of detecting
resistance but failed to mention that refuges were sprayed with
chemicals that likely killed the resistant individuals.
Finally, both borer and root worm corn have used chemical insecticides
that certainly render the "refuge" more public relations than resistance
control. The promoters of gm corn for used in developing countries seem
to ignore the need for chemical insecticides and costly licenses for
using gm corn.



Dale Wilson wrote:
Joe,


The paper below proclaims that GM crops bearing insecticidal
proteins have been used for a number of years without giving
rise to resistant insects. The report deals with the refuge
strategy and the use of high toxin dosage along with pyramiding
toxin genes as factors preventing insect resistance from
arising.


I take this paper with a grain of salt simply because 8 years is too
short a time to draw any conclusions about the success or failure of
resistance managment.


However, the main reason that insect resistance has not
been detected was not mentioned in the article. The
main reason that insect resistance has not appeared is that the
US EPA allows the gm crop and refuge to be sprayed with chemical
insecticides. The refuge strategy of insect resistance provides
crop areas in which the target insect pest may thrive and breed
with resistant insects from the gm crop (insect genes for
resistance are usually recessive). Spraying chemical insecticides
allows taking damage free crops from the refuge but it also rubs
out any resistant insects.


That might be true in theory, but in the case of European corn borer,
the insectidal remedies are not very effective.  Plus, most growers use
standard economic thresholds to decide whether to spray. Since corn
tolerates a fair amount of ECB, many refuge fields will not be sprayed.
 Plenty of moths will be produced in areas under conventional
insecticidal control, especially first generation.


Another unmentioned factor that effects resistance is the use of
synthetic toxin genes in the various commercial gm crops that vary in
both toxin production and the actual sequence of the toxins.


The strategy has been to register only Bt corn with sufficiently high
toxin production to kill ECB heterozygous for the resistance gene.  I
think this is a sound strategy.


However, that factor is far less imposing than the combination of
GM crop toxin and chemical insecticides.


Bt corn is far more effective than insecticide for control of ECB.
Spraying insecticide onto Bt corn is a waste of money.

Dale



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