First, if anyone wants a pdf of papers we are discussing, just email me off-list and tell me which ones you'd like. If you are trying to follow this discussion it would be a lot better if you had your own copy to refer to. It would not be appropriate to send the pdfs to the whole list (plus I don't think BEE-L lets us do that) but individual requests can be honored without copyright infringement.
About imidacloprid metabolites studied in the Suchail paper "Discrepancy between acute and chronic toxicity induced by imidacloprid and its metabolites in Apis mellifera":
Here, mortality was measured over a total of 250 hours in the chronic trial where bees got one of the metabolites in their 1:1 ad libitum sugar supply. The metabolites are 5-hydroxyimidacloprid, olefin, 4,5 dihydroxyimidacloprid, desnitroimidacloprid, 6-chloronicotinic acid, and a derivative that they call a "urea derivative". (Except that it's not urea...it's still a two-ring hydrocarbon like imidacloprid, and it still has the benzyl position carbon linking the two rings. One of the rings has a urea group incorporated into it.) All of the trials showed about the same mortality...a surprise. Mortality began after 72 hours in each trial and maximum mortality in all trials at 250 hours was about 60%. The controls did not have mortality above 15% over that time (stated but not shown).
About imidacloprid metabolites studied in the Suchail "In vivo distribution and metabolisation of 14C-imidacloprid in different compartments of Apis mellifera L" paper:
(Note. The LD50 should be considered an acute test of toxicity, not a chronic one. Mortality is usually measured after no more than 48 hours when doing LD50 trials. That's what these authors confirmed, as well.)
This study used radioactive carbon on the benzyl position of the imidacloprid to track the IMI metabolites in the bee after ingesting imidacloprid in the "acute dose" manner discussed for the previous paper (i.e. they got one dose and that was all, they were then switched back to 1:1 sucrose). I checked with a chemist friend, and she said this C14 method of tracking the various metabolites using the benzyl position carbon would be reliable, and that the radioactive carbon in that benzyl position is quite stable and unlikely to "flip" with another carbon to give misleading results. Of these metabolites, in Table 1 of the paper it can be seen that the urea derivative is toxic at the level of >1,000 ng, based on the LD50, and so are the 6-chloronicotinic acid and the 4,5 dihydroxyimidacloprid. The olefin is highly toxic in acute doses as is the 5-hydroxyimidacloprid, and of course the imidacloprid itself. Table 1 shows that the toxicity is olefin > imidacloprid > 5-hydroxyimidacloprid.
Randy, you are right in that only two of the metabolites remain, according to the graphs in Plate 2, after 72 hours. These are 5 hydroxyimidacloprid and 4, 5 dihydroxyimidacloprid and they are in the thorax. (They call 5 hydroxyimidacloprid "4/5 hydroxyimidacloprid" in some places but they refer to it elsewhere as "monohydroxyimidacloprid"...I guess the H moves around?)
Thus, the second most toxic metabolite of IMI remains in the thorax for more than 72 hours, along with a second, also toxic metabolite, both at more than 2% of the total ingested dose.
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So is 2% retention of a couple of metabolites for several days (4% total AChR binding potential) enough to explain delayed mortality?
And how does this compare to chronic (continuous) exposure to each individual metabolite, which causes high mortality starting around 72 hours?
We have to make sure we don't get confused by "chronic" vs "acute" experiments.
I think that a) all the metabolites are toxic enough (continuous binding to AChR receptors) when administered chronically at low doses to kill the neurons and then the bees by 72 hours ("Discrepancy" paper) and b) at least two of the metabolites are persistent enough in the bee's systems after an ACUTE dose to be measurable after 72 hours in the thorax (C14 paper).
Christina
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