The volume of the debate surrounding the death of up to a third of all honey bees in the United States during the winter, in a phenomena known as colony collapse disorder (CCD), was increased a notch this year as the results of a new Harvard study were released in May.

A press release from the study said, “Two widely used neonicotinoids—a class of insecticide—appear to significantly harm honey bee colonies over the winter, particularly during colder winters…. The study replicated a 2012 finding from the same research group that found a link between low doses of imidacloprid and CCD, in which bees abandon their hives over the winter and eventually die. The new study also found that low doses of a second neonicotinoid, clothianidin, had the same negative effect.”

The companies that make the two pesticides were quick to say the Harvard study is flawed. Dr. Julian Little, spokesman for Bayer CropScience UK, told the Farmer’s Weekly website that the study exposed test bees to amounts of the two pesticides that were 10 to 100 times higher than the bees would ever experience in a real-world situation and that there was no chance of a bee colony suffering that much exposure.

The Harvard researchers studied 18 hives divided into three groups. One group was treated with clothianidin, another was treated with imidacloprid and the third was not treated.

From the press release: “There was a steady decline in the size of all the bee colonies through the beginning of winter—typical among hives during the colder months in New England. Beginning in January 2013, bee populations in the control colonies began to increase as expected, but populations in the neonicotinoid-treated hives continued to decline. By April 2013, six out of 12 of the neonicotinoid-treated colonies were lost, with abandoned hives that are typical of CCD. Only one of the control colonies was lost—thousands of dead bees were found inside the hive—with what appeared to be symptoms of a common intestinal parasite called Nosema ceranae.

“While the 12 pesticide-treated hives in the current study experienced a 50% CCD mortality rate, the authors noted that, in their 2012 study, bees in pesticide-treated hives had a much higher CCD mortality rate—94%. That earlier bee die-off occurred during the particularly cold and prolonged winter of 2010-2011 in central Massachusetts, leading the authors to speculate that colder temperatures, in combination with neonicotinoids, may play a role in the severity of CCD.

Because of uncertainties about the effects of neonicotinoids on bees, the European Union has banned the use of the two chemicals, as well as a third one, thiametoxam, for two years beginning in December 2013. The ban was based on research of the European Food Safety Authority, which said there were gaps in data about the effects of the chemicals. The use of the chemicals will be considered again after the two-year ban.

The Harvard study adds more fuel to a group of beekeepers and environmental groups in the United States, which last year petitioned the Environmental Protection Agency (EPA) to immediately ban clothianidin because it presents an “imminent hazard” to bees and the bee-keeping industry that could mean billions of dollars in lost bees and honey.

The EPA did not act on the petition, but evidence continues to mount that pesticides are at least part of the problem with the crash of honeybee populations in the United States and many other countries and other studies back the Harvard one.

A group called the Task Force on Systemic Pesticides in June concluded a four-year analysis of more than 800 peer-reviewed studies regarding neonicotinoids, and found they are causing “significant damage to a wide range of invertebrate species and are a key factor in the decline of bees.”

One of the lead authors of the work, Dr Jean-Marc Bonmatin of The National Centre for Scientific Research in France said, “The evidence is very clear. We are witnessing a threat to the productivity of our natural and farmed environment equivalent to that posed by organophosphates or DDT. Far from protecting food production the use of neonics is threatening the very infrastructure which enables it, imperiling the pollinators, habitat engineers and natural pest controllers at the heart of a functioning exposure to pesticides widely used in farming.”

Further, a paper published in the journal Nature on July 9, argues that the pesticides are not only harming insects but also birds. Scientists from Radboud University and the Dutch Centre for Field Ornithology and Birdlife in the Netherlands found that areas with high amounts of imidacloprid experienced yearly declines in bird populations.

One of the authors said there was clearly a parallel between the current neonicotinoid issue and the DDT issue brought to light by Rachel Carson in the 1960s.

The companies that manufacture the pesticides dispute such findings and insist they are safe, but the matter has the attention of President Barack Obama. In June he ordered the Environmental Protection Agency to “assess the effect of pesticides, including neonicotinoids, on bee and other pollinator health and take action, as appropriate,” within 180 days.