According to the professor at U Wis extension course several weeks ago there is no such data by any pesticide harming bees when used properly or in combination. His point was: THERE IS NO DATA, just anecdotal stories, beliefs and such.
However, the whole subject is highly controversial with claims made all over the place but there is no data to back up any claim - it just has not been studied in any depth and most products have never been looked at in connection with bees.
The whole herbicide/pesticide thing is just what I said but in different words.
I’m afraid our Extension friend is mistaken. There is a whole body of peer reviewed research that discusses synergism in pesticide use.
Note well that I do not conclude (as is the case with most reputable researchers) that CCD is caused by the neonicotinoids.
But to suggest that insecticides that are clearly labeled as anywhere from moderately to highly toxic to bees aren’t toxic to bees is simply wrong. But that’s not the point, the point is to clearly understand that if a pesticide with mode of action “X” is applied to a crop, and another pesticide with mode of action “Y” is applied to hives for control of varroa, then the result of exposure to the combination of the two would be X+Y. It clearly is not. Here is an abstract that discusses such a scenario:
“Recently, the widespread distribution of pesticides detected in the hive has raised serious concerns about pesticide exposure on honey bee (Apis mellifera L.) health. A larval rearing method was adapted to assess the chronic oral toxicity to honey bee larvae of the four most common pesticides detected in pollen and wax - fluvalinate, coumaphos, chlorothalonil, and chloropyrifos - tested alone and in all combinations. All pesticides at hive-residue levels triggered a significant increase in larval mortality compared to untreated larvae by over two fold, with a strong increase after 3 days of exposure. Among these four pesticides, honey bee larvae were most sensitive to chlorothalonil compared to adults. Synergistic toxicity was observed in the binary mixture of chlorothalonil with fluvalinate at the concentrations of 34 mg/L and 3 mg/L, respectively; whereas, when diluted by 10 fold, the interaction switched to antagonism. Chlorothalonil at 34 mg/L was also found to synergize the miticide coumaphos at 8 mg/L. The addition of coumaphos significantly reduced the toxicity of the fluvalinate and chlorothalonil mixture, the only significant non-additive effect in all tested ternary mixtures. We also tested the common ‘inert’ ingredient N-methyl-2-pyrrolidone at seven concentrations, and documented its high toxicity to larval bees. We have shown that chronic dietary exposure to a fungicide, pesticide mixtures, and a formulation solvent have the potential to impact honey bee populations, and warrants further investigation. We suggest that pesticide mixtures in pollen be evaluated by adding their toxicities together, until complete data on interactions can be accumulated.”
http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0077547
This is a case of the left hand (Beekeepers) not knowing what the right hand (Farmers) is doing. The important takeaway is for both parties to be fully aware of what each other is using and when.