>  Bayer claim that 185 ppb is the LD50 level, according to the article 
> Brian Ames posted earlier.

Actually, Bayer claim that is the acute toxicity level (the lethal dose for 
50% of the bees
in 24 hours.  There is also the LD50 for 48 hours, which is lower, and the 
LD50 for ten
days (a more chronic exposure) which is lower still.

Perhaps a better measure is the NOLEC (no observed lethal effects 
concentration).  This
is the highest level at which chronic lethal effects are not observed.  From 
a discussion
on this last year here is what I posted (taken from a paper by Richard 
Schmuck, Bayer's
main "imidacloprid and bees" researcher:

> http://bee-quick.com/reprints/imd/Schmuck_2004.pdf

>It certainly seems from this paper that Suchail et al's
figures are out of line with other results.

However, to summarize the survey of investigators
that Schmuck lists, the NOLEC (no observed lethal
effects concentrations) from different investigations
using lab feeding are (each number=one paper):

4 ppb, 24 ppb, 48 ppb, 10 ppb, 10 ppb, 0.1 ppb
from chronic dietary toxicity studies.

From tunnel and field feeding studies (chronic):

100 ppb, 20 ppb, and >20 ppb.

So, while 20 ppb (seems roughly a middle figure)
is well below the 140 ppb for acute LD (or
192 if we take your average) it shows that the
chronic toxicity is much lower (one order of
magnitude) than the acute.

end of quote

So, if the citrus plantations are having high residual levels
of imidacloprid in the soil for years, then all the things
growing in that soil are potentially toxic, and the exposure
of bees is certainly chronic, beyond even just the flowering
of the oranges.

Then of course, there are the sub lethal effects.....

Stan

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