Bt corn flap

Rob and Pam robnpam at acieed.fsnet.co.uk
Wed Mar 29 14:52:40 EST 2000


On Wed, 29 Mar 00 14:14:10 GMT, Neil at nwjones.demon.co.uk (Neil Jones)
wrote:

>
>The so called Toxin expressed in the BT corn is NOT identical to that
>produced in organic control agents. 

Well, it's the delta-endotoxin gene from Bt var kurstaki, which is the
strain used for lepidoptera control. How about this : The toxin (and
it is a toxin, see later) expressed in Bt corn is identical to the
major component of the toxin produced in one of the commonest organic
control agents.

>Bacillus thurigiensis is a specialised
>bacerial predator of invertebrates.

We don't know too much about the 'normal' ecological niche for Bt but
it seems to be a bacterium that lives as a conventional saprophyte a
lot of the time and only occasionally kills insects. It's certainly
not an obligate specialised predator.

 It exists is a variety of strains which
>differ from each other. These natural predators are one of the factors which
>influence the population dynamics of many species. For example Bacillus
>species are a major natural cause of mortality in the endangered Jamaican
>Swallowtail Papilio homerus.
>

Intersting. Do you have a reference for the P. homerus stuff? I'd like
to know more. Most of the reports I'm aware of of Bt causing much
mortality in wild populations comes from stored product pests, which
live in environments that are particularly good for Bt spores and
toxin to survive in (dry, no UV), and it doesn't seem to be important
in the population dynamics of many species in natural environments.

>Engineered corn contains PART of a gene from ONE strain of Bacillus
>thurigiensis. This is an important factor to take into consideration.
>
>The variability of wild Bacillus thurigiensis is an important tool in its
>armoury. The "toxin" (actually a tool for enabling it to gain access to
>the insect's body) exists in nature in such a range of forms that it is
>difficult for insects to develop immunity.

You seem to have some resistance with calling it a 'toxin'. Consider
the following

1) When an insect ingests Bt toxin, the toxin binds to the gut wall if
the insect is of a species susceptible to that strain of Bt. It
creates large pores in the cell membranes, disrupting their water
balance and causing them to rupture. The gut lining of an insect
that's ingested Bt toxin looks like someone's fired a shotgun at it if
you look at it with an SEM.

2) Some species of insect are killed just by the action of the toxin
(i.e. Bombyx mori, Plodia interpunctella). Many others are killed by a
combination of the toxin action and septicaemia from the bacterium.

3) If it isn't a toxin, then why are plants expressing the gene
protected from insect pests?

Sounds like a toxin to me...

>However the ecological consequences of flooding an ecosystem with high
>levels of one specific form of the toxin are difficult to predict.

Well not really. What will happen is:

>...  resistance will rapidly spread in the
>populations of many species. It is also likely that this will facilitate
>the developement of resistance to other wild strains. This could easily
>occur once the majority of the population of a species carries a gene
>that that works against one strain. Natural selection works just
>like this.
>The consequences are that we actually decrease the ability of Natural
>predators to control pests. That is not a smart thing to do.

Quite right. Unfortunately as far as I'm aware the organic industry
doesn't seem to be doing a great deal to manage the resistance that
may develop from their use of 'high levels of one specific form of the
toxin', which many of them are doing, I'm afraid.

>
>What also worries me is that the industry has chosen repeatedly to
>describe Bacillus thurigiensis as a "soil bacterium" this is
>not truthful at all. I tend to distrust people who are not truthful.
>

Well Neil, the thing is that Bt IS a soil bacterium. If you want to
find Bt, look in the soil. Most isolates of Bt come from the soil.
It's a damn sight easier to find it in the soil than it is in diseased
insects, for example. It is also found elsewhere, I'll grant you that,
but if you want to describe Bt and you only have two words, 'soil
bacterium' is pretty good. 

I would be interested to know your source for this information: maybe
someone in the organic food industry is not being truthful. Remember,
it is an industry, and whereas some organic growers are dedicated
selfless protectors of the environment there are others who are in it
for the money, and they stand to lose big wads of cash if the public
decides that one of their most effective control agents is no longer
acceptable.

Best wishes

Rob Knell


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