> I suspect that the problem is related to the amount of seed dressing

Except that the amount of neonics in nectar is practically undetectable. 

> Overall, field studies show that neonicotinoid residues are extremely variable. Many found no detectable residues, and others found levels lower than 10 ppb. The rates used in recent laboratory based studies, therefore, appear not to be typical of concentrations in nectar and pollen. Rather than “field realistic”, they seem more representative of a “worst case” scenario. 

> Laboratory based studies show that neonicotinoid insecticides can have adverse sub-lethal effects on bees, but so far these effects have not been observed in the field.

Norman L Carreck & Francis L W Ratnieks (2014) The dose makes the poison: have "field realistic" rates of exposure of bees to neonicotinoid insecticides been overestimated in laboratory studies?, Journal of Apicultural Research, 53:5, 607-614, DOI: 10.3896/IBRA.1.53.5.08

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