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Re: [SANET-MG] Bt cotton does not imporve yield



to Dale, Wytze,  Steve and others,
One noteworthy feature from our discussion on Bt cotton and conventional cotton has been agreement that Bt cotton is not grown free of chemical pesticides but it is spayed with chemicals. The refuge strategy is used to delay appearance of boll worm resistance and that has been credited for the absence of established boll worm resistance in spite of the many years of Bt cotton production. The abstract below is typical of the discussion about the benefits of Bt cotton and absence of resistance without discussing the fact that Bt cotton is , in practice, sprayed with chemical insecticides providing a double "whammy" in which the chemicals eliminate Bt resistant insects. Even in scientific journal reports of studies of the commercial crops the combined use of chemical insecticides and Bt is seldom discussed. Full and truthful reporting should prevail and the combined use of Bt and chemicals should be fully reported and its implications for insect resistance, yield and cost of production brought to life.
sincerely,joe
Proc Natl Acad Sci U S A. 2005 Oct 25;102(43):15389-93. Epub 2005 Oct 14 Delayed resistance to transgenic cotton in pink bollworm.
Tabashnik BE, Dennehy TJ, Carriere Y.
Department of Entomology, University of Arizona, Tucson, AZ 85721, USA. brucet@ag.arizona.edu Transgenic crops producing Bacillus thuringiensis (Bt) toxins kill some key insect pests and thus can reduce reliance on insecticides. Widespread planting of such Bt crops increased concerns that their usefulness would be cut short by rapid evolution of resistance to Bt toxins by pests. Pink bollworm (Pectinophora gossypiella) is a major pest that has experienced intense selection for resistance to Bt cotton in Arizona since 1997. We monitored pink bollworm resistance to Bt toxin for 8 years with laboratory bioassays of strains derived annually from 10-17 cotton fields statewide. Bioassay results show no net increase from 1997 to 2004 in the mean frequency of pink bollworm resistance to Bt toxin. A synthesis of experimental and modeling results suggests that this delay in resistance can be explained by refuges of cotton without Bt toxin, recessive inheritance of resistance, incomplete resistance, and fitness costs associated with resistance.


Dale Wilson wrote:
Wytze,

I find this quote (and much of the whole story) quite
incomprehensible and it raises some questions (as does
much of the story) For example: Did Bt cotton farmers have
no problems with whiteflies and western tarnished plant bugs?
Why not, if Bt does not protect against these?

These other pests are not so devastating as the Bollworm.  If the
Bollworm is controlled by the narrow spectrum Bt, the need for broad
spectrum insecticide application is reduced.

Also: what insect diversity they looked at? the article only mentions
ants and beetles.

They were looking at very local effects along transects moving from the
cotton fields into the adjacent non-cropped areas.  It only makes sense
to sample groups of species with low mobility.

The paper itself is easier to understand than the abstract.

http://www.pnas.org/cgi/reprint/0508312103v1

Dale

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