On Fri, 8 Aug 2003 17:15:49 +0100, Oz
posted:

Torsten Brinch writes
On Fri, 08 Aug 2003 04:20:48 GMT, "Moosh:}"
wrote:

On Thu, 07 Aug 2003 07:20:22 -0700, Walter Epp
posted:
[Quoting Independent on Sunday (London) March 30, 2003:]
.. Environmentalists say that resistance develops all the faster
because the insects are constantly exposed to it in the plants, rather
than being subject to occasional spraying.

Occasional spraying will result in many occasions where dose is
sublethal. Ideal circumstances for resistance development.

However, reminding ourselves of the perils of assumption-based
reasoning, let us hear what the experienced farmers over at
sci.agriculture has to say about that.

There are two arguments:

1) Apply full dose and kill 99.999% except the 0.001% that have a
resistance gene and next season you will have a 100% resistant
population. If (as is common) you have a pest with a very high
reproductive rate then you are stuffed in a year or two.

This is what happened for dimfop resistant blackgrass.
This might be typical of single gene resistance (not tolerance).

This will happen whether or not the gene is less efficient than the
'natural' gene.

2) Apply a reduced rate, kill 99% of the pest, leave 1% of which 1:1000
have a resistance gene. Hope the resistance gives less efficient pest,
outbred by 'natural' genes, leaving a final pest population still with
about 0.001% resistance. So no change.

Most field weeds are more tolerant of pesticides than their wild
relatives, but often not by much.

Pesticides acting on single genes are MUCH more likely to become
completely useless due to single point mutation.

Pesticides with multiple-point action are pretty unlikely to develop
resistance.

Obviously simultaneously using several pesticides with different action
mimics multiple-point resistance.

If a pesticide targets a key site, that is hard for the pest to alter
because it is critical (perhaps used in many subsystems or is very
basic), then tolerance rather than resistance seems to be the normal
mode of action (eg hormone weedkillers, IPU). I haven't seen it stated,
but I suspect the progeny are less competitive.

Certainly resistant blackgrass seems to be highly susceptible to mildew,
for example.

Bottom line though is that BT expressed is no more likely fo cause
resistance development problems than intemittent application of BT.