Suchail et al (2000) also noted that unlike many other pesticides, neonicotinoids, in this case Imidacloprid,  are more toxic to bees by oral consumption than by contact.

Characteristics of imidacloprid toxicity in two Apis mellifera subspecies,
    Séverine Suchail,
    David Guez,
    Luc P. Belzunces

http://onlinelibrary.wiley.com/doi/10.1002/etc.5620190726/abstract

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