A team of Harvard biologists has come closer to cracking the mystery of honeybee Colony Collapse Disorder (CCD), eight years after its appearance.

CCD persists in transforming bee colonies around the world into ghost towns: by the end of each winter, some colonies wind up littered with dead bees and emptied of many more, with no signs of renewal.

"One of the defining symptomatic observations of CCD colonies is the emptiness of hives in which the amount of dead bees found inside the hives do not account for the total numbers of bees present prior to winter when they were alive," states the report, published May 9 in the Bulletin of Insectology.

The exact mechanism behind these collapses remains dauntingly unclear, but they have been linked with pathogen infestation, malnutrition, and pesticide exposure. This week's report strongly indicates that two neonicotinoid insecticides that are widely used on crops can decimate honeybee colonies' winter survival rates, whether or not mites or parasites are present.

The two chemicals, imidacloprid and clothianidin, both block insects' central nervous systems, killing them by paralysis. Imidacloprid is the world's most widely-used insecticide, and has been registered for use in the US since the 1994; clothianidin was registered in 2003 by the US Environmental Protection Agency, which found that it had passed honeybee-specific toxicity tests.

These scientists studied the health of 18 bee colonies in central Massachusetts over a six-month period spanning the winter of 2012-2013. Six of the colonies were fed sugar spiked with sub-lethal doses of imidacloprid, six had theirs laced with clothianidin, and six less-unfortunate control colonies ate clean sugar, starting in October.

All of the colonies went about their apian routines in good form throughout the fall. But by late January, six of the 12 poisoned colonies experienced collapses with CCD-like symptoms, like en-masse disappearance and the presence of un-hatched young. Of the six control hives, only one failed to survive the winter, seemingly due to an infestation by Nosema Ceranae parasites.

"The honey bee clusters in the six surviving neonicotinoid treated colonies were very small, and were either without queen bees or had no brood," reports the study, suggesting the poisons harm the animals' abilities to raise and train new young. In contrast, the five surviving control hives replenished their populations quickly, as the winter gave way to spring.

According to the report, these results "reinforce the conclusion that sublethal exposure to neonicotinoids is likely the main culprit for the occurrence of CCD."

The finding raises a large question for further study: Why do honeybees, who don't normally abandon their hives during winter, do so when poisoned by neonicotinoids?

The finding may point to "the impairment of honey bee neurological functions, specifically memory, cognition, or behavior, as the results from the chronic sublethal neonicotinoid exposure," suggest the authors. "Although the failure to initiate brood rearing and the vanishing of the worker caste in the neonicotinoid-treated colonies might be governed by completely different mechanisms, they suggest the possible involvement of cascading events prior to the occurrence of CCD."

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The study comes amid a busy spring season for honey bee research. Last month a team of Kenyan biologists found that African honey bees seem be impervious to the pests, Varroa and Nosema, which plague honey bee populations in Europe, Asia, and the United States.

And earlier this week Brazilian scientists identified two substances in honeybees' brains, that appear to vary as the insects move through space and time, guiding their age-related division of labor.