Some studies have reported that summer bees rather than winter bees were found in colonies going into the winter. The difference is in the smaller fat reserves, leaving summer bees with much shorter lifespans.
When the fat reserve is used up, a bee leaves the colony to die away from the hive.
Lots of bees leaving at the same time leave behind an empty hive, the beekeeper calls it CCD or Marie Celeste or dwindling disease.

The trigger for winter bee production in a healthy colony is the shortening of daylength after the summer solstice, June 21st.
Depending on your type of bee, winter bee production should commence soon or slightly later after that.

If the colony is under the influence of neurotoxins, like neonicotinoids, the bees might not be able to perceive the change of daylight and continue production of summer bees.

It has already been proven that neonicotinoids reduce the memory of bees in field studies (reutrn rate of foragers) and in the lab (training experiments).

A compromised memory would make perception of daylight changes impossible, and as many colonies are exposed to neonics at the critical time of year for winter bee procuction, we should look into this possibility more closely.

sources of neonics that could therefore be responsible for CCD:

@ fields of corn/maize
@ flowering trees like lime trees on golf courses and in parks, where the lawn/turf has been treated with the pesticides
@ other treated crops flowering around midsummer and after
@ contaminated water (runoff) that bees collect for brood rearing