GENTECH archive


Reply to Herve on diversity+Bt

Herve:  Sorry, I didn't understand all of your message, but here are a few

>Rick Roush wrote :
>> For example, resistance to kurstaki in
>> diamondback moth has been shown to be due to a single gene conferring
>> resistance to Cry 1A and relatives.

Herve replied>
>OK for a given resistance. But other resistances to the same toxin may
>be due to  an other genes.

True, but my point was only to illustrate that Bt sprays typically rely on
just one Cry protein for toxicity

>Moreover, at the time when Bt toxins were developed, they did not know
>all those 140 toxins (and surely much more right now). That must be the
>reason why they did not use this whole range of Bt that is large and
>so diminishes the probability of appearance (and of propagation) of
>a resistance.

Dozens of toxins have been known since the early 1980's (see the Hofte and
Whitely citation sent earlier) but are still not developed in sprays.

>Moreover, production of Bt wil b e all year long. It's not the very
>same as 2 to 4 aspersions (i've been confirmed that in France it's the
>order of magnitude).

Sprays are both a blessing and curse when it comes to resistance
management. They have less persistence, but also give poorer control of
dose. We have already found in experiments that resistance can occur faster
to sprays than transgenic plants.

>> Far more relevant as a cause of lack of resistance is that Bt is used so
>> little, the best proof of which is widespread resistance in the one case in
>> which Bt is used much at all (for diamondback moth).
>False : on may only compare the frequancies of appearance of resistance for a
>very same toxin, a very same insect, under the very same  conditions.....

Herve, not false; I can give you precisely that experiment. The experiments
we did were with diamondback moth, comparing both plants and sprays with
the Cry1Ac protein, under exactly the same conditions (all in one
greenhouse, from the same starting colony of insects, including that 20% of
the plants were neither sprayed nor transgenic).  Resistance, as measured
by control failures, occurred first for the sprays.

If anyone is really interested, I can send you the data as an attachment.