Bt is self-limiting

August 2016
Warnings about the potential for 'Bt' insecticidal GM crops to trigger rapid pest resistance have been voiced since these novel plants first arrived on the market.

Unlike a chemical spray designed to wipe out all the pests in the treated area, but short-lived, Bt-toxin generated inside a GM plant is there all the time but not necessarily in sufficient quantities to kill: these are perfect conditions for the evolution of resistance.

Moreover, assumptions that Bt-resistance would come with a self-limiting fitness cost to the pest, or that Bt resistance would require a pair of genetic mutations which happened to come together rather than one single dominant mutation, or that all farmers would go to the trouble of planting a non-Bt plot to harbour a supply of normal, susceptible pests, have all proved limited in practice.

The biotech industry answer to pest resistance is, of course, more Bt.
GM maize strains producing four different Bt toxins targeting western corn rootworm, a major pest problem, have been marketed. Latterly, these same genes have been 'pyramided' into a single crop all the better to deal with resistant pests.

However, the new multi-Bt crops are growing in a background of pests already exposed to the single-Bt versions and already evolving resistance.

The authors of a new study on emerging western corn rootworm resistance have pointed out another resistance-promoting factor. Bt toxins come in three sections each contributing a vital action: one is involved in forming pores in the pest gut, one is involved in binding to specific receptor sites in the pest gut, and the third is involved in stabilising the Bt toxin and in its binding ability. The problem is that the four commercialised Bt toxins are, to some extent, a mix-and-match of the same three sections. This reduced individuality makes for cross-resistance between different Bt-'protected' crops, including the pyramided varieties.

OUR COMMENT

Interesting. Despite the size and complexity of a Bt protein, only four marketable versions have been created against a major pest of a major crop, and these four aren't in fact so very different from one another.

Conclusion?

GM is a lot more difficult and limited than the biotech industry would have you believe. 

Time to suggest to your representative in government that your taxes should be diverted away from the very expensive GM road to nowhere, and towards a sustainable diversity of chemical-free, GM-free crops produced using sustainable agro-ecological principles.

SOURCE:

• Sive R. K. Jakka, et al., 2016, Broad-spectrum resistance to Bacillus thruingiensis toxins by western corn rootworm (Diabrotica virigfera virgifera), Nature Scientific Reports, 14.06.16