its awfully hard to prove a negative.
for example, Coffee has never been proven to be carcinogenic, but its never been proven to not be carcinogenic and therefore its listed by the United Nations (we know how scientific they are) as a possible carcinogen. If we follow your advise, we should ban the use of coffee until we can prove beyond a shadow of a doubt that it or any of its components (pre and post roasting, water used in making it, soil nutrients of the soil it's grown in, etc) do in fact not cause cancer. Only after exploring and exhausting every hypothetical possibility can the coffee be allowed to be grown and consumed.
If you went with that approach everything would be banned.
That being said, the leading (and obvious) indicator that Neonic pesticides do not contribute to CCD (also observed by Richard above) is that the footprint for neonic pesticide use does not correspond to CCD impacted areas. On top of that some of the heaviest use areas of neonic applications (such as miles and miles of Canola fields in North Dakota) have thriving bees.