You're a shill - plain and simple.
The bee only dies immediately if exposed to a dose above a certain threshold. Frequently they'll be exposed to levels which will seriously damage them but allow them to survive for long enough to make it back to the hive .... if enough of them make it back, that can either directly kill off bees back at the hive or damage them enough to seriously impair the function of the hive - ultimately killing it / making it prone to infection with fungi / mites / attacks by predators. This scenario also applies when they carry back tainted pollen or are routinely exposed to low levels and enough workers become non-functional / diseased / damaged over time.
Toxicologists think that levels of neonicotinoids in the environment lower than we can reliably detect with our most accurate tests are enough to cause minor to moderate nervous system damage in bees and massively increase mortality.
The only safe amount of neonicotinoids (for bees or practically any other carbon based lifeform) is zero, as is sadly the case with so many pesticides.
Also, nice try re: it not seriously impairing immune function :-D
---- In future, before you and your colleagues burn them, you should read the studies which show people or other animals exposed to nerve agents exhibit major increases in disease and chronic ailments. Having said that, you probably know all of this and would much rather that people thought the nice poisons which you work with weren't to blame.