Jim wrote:

 So, to summarize, my understanding of the current scientific consensus on
> imidacloprid is as follows:
>
> The imidacloprid acute oral LD50 for bees is 192 ppb.

This is way high.  Bayer's own researchers have placed it as low as 20 ppb
at one time (Kirschner), but now place it somewhat higher, but not that
high.  I just posted a reference to a paper on chronic toxicity by the 
author
which you referred to (Suchail) which gives specific LD50's for certain
lengths of time. I am sure you have a reference for 192, but "concensus"?

> At very high levels like 500 ppb, one can start to see "memory and 
> learning"
> effects, but such high doses are also fatal.

Bayer's own book on imidacloprid research for 1999 shows a picture of
Pham-Delegue's proboscis extension reflex test and a graph showing that
"memory and learning" effects start at 4 ppb (or less, that was the lowest
concentration tested after control).  Pflanzenschutz Nachrichten Bayer 99
She is an expert on "memory and learning" effects.  She has a book on
the subject, and you can read some of her papers in full on the apidologie
web site.

> At 50 to 100 ppb, multiple studies have shown various negative impacts on
> honey bees.

Actually, at that concentration bees are doing what Bayer calls "trembling
dances", and foragers will often avoid contaminated forage or stop foraging.

> No one has claimed that bees would be exposed to levels anywhere near 20 
> ppb
> in the field.  Most people toss around "typical" numbers like
> "5 ppb" as a worst-case number.

The above statement is first of all logically non sequitor.  ("No one....
most people..") But more to the point,  the same authors you just quoted
above, (Cynthia Scott Dupree....) found greater than 5 ppb in their testing
of nectar and pollen of canola in Ontario.  The authors of paper we were
recently discussing on risk assessment of systemics chose imidacloprid
because it is so well researched and it is so well known that the pollen and
nectar of seed treated plants like sunflower and canola and corn (pollen)
WILL contain more than 5 ppb.

> Long-term exposure does not imply a significantly greater risk than
> short-term exposure as neonicotinoids are rapidly metabolized by bees
> and do not bio-accumulate.

I posted an abstract of a paper by the author who you quoted to make
this statement about risk.  That is a paper by that author (Suchail) on
chronic toxicity.  It is obvious that the metabolization of imidacloprid
does not change the fact that it is chronicly toxic as demonstrated in the
other paper by the same author.  The abstract of the paper you referred to
does not lead to any conclusion like the one you are claiming about
long term exposure.  Could you please post quotes from the summary
or conclusions of the paper that would back up this claim.

Stan

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