We do not know with certainty if neonics killed the bumble bees – and assuming neonics are responsible for their death, we do not know how or why it killed the bees.
It's also a great chance to draw attention to the facts surrounding neonic use and its effect on insects:
1) Neonic usage above a particular concentration is lethal to insects.
2) It has been shown that certain neonics have harsher consequences than others. Some of the 'milder' neonics break down into the 'harsher' neonics over time within the insect's body.
3) There is a statistical linkage, but no conclusive evidence or scientific consensus on whether chronic sublethal neonic concentrations have a lethal effect on insect populations.
4) To make the claim that neonics kill bumblebees, we need to first see repeatable research demonstrating what the medical reason behind a lethal effect of sublethal neonic concentration penetration is, as well as how much the required concentration to induce the lethal effect is.
Disclaimer: I am no expert on insectology or on toxicology. In my opinion, it is irresponsible to use a chemical that has a potentially disasterous effect on the ecological system – in particular pollinators – as long as the safety of that chemical is not scientifically proven.
For the scientifically inclined, this report presents what, in my opinion, is a rather unbiased overview of the current opinions and studies available:
Reshared post from +Pamela Zuppo
The Case of the Vanishing Bees
That's what happened one June morning at a Target parking lot in Portland Oregon in 2013. A pest-control company (unbeknownst to Target) had sprayed neonictinoids on surrounding Linden trees to protect them from aphids; but nobody warned the bees to stay away. In the end, an estimated 50,000 bumblebees perished. http://ow.ly/x4N6V
The tragedy wiped out as many as 300 bumblebee colonies, robbing the area's trees and flowers of a key pollinator species. And it shows why stomping out the use of neonics is paramount to the survival of bees. You can read more about this case here >> http://ow.ly/x4N6V