Sub-lethal doses of neonicotinoid insecticide impaired winterization of honey bees. Control and dosed bee colonies were similar until winter when dosed colonies collapsed.
We found honey bees in both control and neonicotinoid treated groups progressed almost identically through the summer and fall seasons and observed no acute morbidity or mortality in either group until the end of winter. Bees from six of the twelve neonicotinoid treated colonies had abandoned their hives, and were eventually dead with symptoms resembling CCD. However, we observed a complete opposite phenomenon in the control colonies in which instead of abandonment, they were repopulated quickly with new emerging bees. Only one of the six control colonies was lost due to Nosema like infection.The U.S. and United Kingdom which have not banned field application of neonicitinoid insecticides have been suffering from high rates of colony collapse disorder (CCD).European nations that have banned neonicotinoids have lower CCD rates than the U.K. Because honey bees are indispensable to pollination and crop production these insecticides need to be banned now, excepting uses that will not affect bees, in the U.S. and the U.K.
"We demonstrated that neonicotinoids are highly likely to be responsible for triggering 'colony collapse disorder' in honeybee hives that were healthy prior to the arrival of winter," said Chensheng Lu, an expert on environmental exposure biology at Harvard School of Public Health and who led the work.Update & Thanks for the excellent comments.
The discussion was very informative to me, especially the observations of a beekeeper who posts here as Araguato.
In general the winter-colony size threshold
for survival is the amount of bees needed to maintain the necessary temperature (90F +) at the core of the cluster, taking into account the local, temporal, heat losses which obviously vary between Atlanta and Alberta during January, as an example.
Bees need at least 45-50F to be able to move around. Below that they are in metabolic deficit and have only a short window of survival. Bees maintain the core temp of the cluster by having the outer bees "shiver" to generate heat in the outer layers as insulation for the inner layers. Bees circulate in complex ways among the layers as they become chilled. Bees don't heat their homes (hives), but they heat themselves and to some degree their close environs. My bees generated a remarkable amount of heat even during 20F temps. I could stick my kitchen instant read thermometer in the top ventilation hole and get readings as high as 85-90 F, a few inches from the cluster. (My hives are extraordinarily well insulated compared to most.)
(Some races of bees- Russians, for instance, which orginated in Siberia - are more effective at maintaining the core cluster temps in the winter with fewer bees. Italians are reputed to be less sturdy. I wouldn't know as my bees are mutts, of no certain pedigree.)
So challenges from mites, to lack of food, to poor long-term nutrition, or unsual temperature extremes or durations, or diseases, or subtle changes in comensal hive organisms might independently reduce the number of living bees capable of performing their winter hive job: shivering as needed. And that could lead to loss of a hive (colorfully termed "a deadout", in bee world) but it wouldn't necessarily lead to CCD, which isn't a hive with bees all present and dead. CCD is a hive with all, or nearly all, bees vanished and presumed dead.
The feral bees that used to be in my barn walls simply vanished, leaving hundreds of pounds of accessible, and apparently healthful and edible honey in their combs, the collective work-product of tens of billions of bees' lives. My new swarm-bees have been fed this honey, with no ill effects.