According to this Canadian Government document:
http://www.cdpr.ca.gov/docs/emon/pubs/fatememo/indoxacarb.pdf
Indotoxacarb is a potent bee killer on contact. But, once the spray is dry three hours after application it was bee safe on alfalfa. I have no idea if a fresh dew or light rain would make it bee active again after it has once dried.
It is also possible that the time between the bees dying and the time it took to get samples to the lab gave enough time for any residue to degrade. Samples often need to be collected really fast and samples kept frozen to prevent degradation. The contact LD50 is 0.18 microgram per bee. It takes about 6000 bees to make a kilogram. If I did the math right that means the lethal dose in bees would be about 100 ppb which is well above the detection limit the lab lists. It seems unlikely that it could have degraded to that extent in the samples even if not frozen.
It sounds like a long time between the spray and first notice that bees were dying. Were the bees confined by weather or low temperatures for that week?
Things like this are really frustrating. Something killed your bees other than old age.
Dick
" Any discovery made by the human mind can be explained in its essentials to the curious learner." Professor Benjamin Schumacher talking about teaching quantum mechanics to non scientists. "For every complex problem there is a solution which is simple, neat and wrong." H. L. Mencken
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