From the paper Medhat mentioned:

In summary, we have detected low levels of pesticide
residues in stored pollen, mainly acaricides used
in homemade formulae to control varroosis [varroa].

Imidacloprid (or metabolites) was not detected, suggesting
that those products or other agrotoxics commonly
used in Spain are not directly related with the
generalized reemergent problem of honey bee colony
loss detected since autumn 2004.

Source: Journal of Economic Entomology, 103(6):1964-1971. 2010.

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