Letter: Isolating GM crops

Sir: Dr Sean Munro (letter, 27 May) probably knows that there are actually several twists in the Bt toxin tale. Natural and organic it certainly is, having been used by organic farmers for 40 years as a biopesticide. But the version inserted into transgenic crops is anything but natural, incorporating as it does aggressive promoters and enhancers not found in the organic version.

Hence, possibly, the major upsurge in Bt resistance among insect pests in those parts of the US where transgenic crops have been released - and the major Bt cotton crop failures experienced there. It will not be enough to proceed on a case-by-case basis if the basic approach is flawed from the outset.

JEREMY BEVAN

Coventry