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[SANET-MG] Resistance to Bt Cotton Remains Rare After A Decade of Exposure
The paper below seeks to locate resistant allele to Bt cotton in boll
worm. Such alleles were absent. Does that mean that the "refuge"
strategy works? Actually the refuge works by mating between wild type
insect and recessive resistant insects yielding heterozygotes. Such
heterozygotes should not be eliminated but they should achieve a stable
Hardy-Weinberg equilibrium value provided that there is not selection
against the heterozygotes which must be defective in that case. There
should have been heterozygotes in the insect pool if "refuge' had worked
as it should.
However, the absence of resistance heterozyotes for the recessive
resistance genes is more likely related to the use of powerful
insecticides in the refuge, which is approved and generally done in
order to harvest the refuge. The practice also tends to keep down
secondary pests that have damaged Bt cotton in other countries. Of
course, "refuges" are not really refuges but they are more like
killing grounds or death camps. As they used to say "Arbeit Macht Frei".
DNA Screening Reveals Pink Bollworm Resistance to Bt Cotton Remains Rare
After A Decade of Exposure
- Tabashnik, Bruce E. et al. 2006. Journal of Economic Entomology. 99(5)
1525 - 1530.
Transgenic crops producing toxins from the bacterium Bacillus
thuringiensis (Bt) kill insect pests and can reduce reliance on
insecticide sprays. Although Bt cotton (Gossypium hirsutum L.) and Bt
corn (Zea mays L.) covered 26 million ha worldwide in 2005, their
success could be cut short by evolution of pest resistance. Monitoring
the early phases of pest resistance to Bt crops is crucial, but it has
been extremely difficult because bioassays usually cannot detect
heterozygotes harboring one allele for resistance.
We report here monitoring of resistance to Bt cotton with DNA-based
screening, which detects single resistance alleles in heterozygotes. We
used polymerase chain reaction primers that specifically amplify three
mutant alleles of a cadherin gene linked with resistance to Bt cotton in
pink bollworm, Pectinophora gossypiella (Saunders), a major pest. We
screened DNA of 5,571 insects derived from 59 cotton fields in Arizona,
California, and Texas during 2001-2005.
No resistance alleles were detected despite a decade of exposure to Bt
cotton. In conjunction with data from bioassays and field efficacy
tests, the results reported here contradict predictions of rapid pest
resistance to Bt crops.
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