Neonicotinoids are pesticides, typically used to treat seeds before planting, has been lined to the mass die-offs of bee colonies. Previous studies have made the same correlation between the common pesticide and bee deaths and the European Union has banned their use. Now additional evidence from researchers at Harvard, published in the Bulletin of Insectology, may cause that ban to spread.

The team from Harvard’s School of Public Health (HSPH) lead by Chensheng (Alex) Lu tested two common neonicotinoids over the winter. Out of 18 bee hives they exposed 6 hives to each of the neonicotinoids imidacloprid and clothianidin and left 6 bee hives unexposed as a control group.

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. In the beginning of winter all of the colonies began to decline in population. This is typical of all bee colonies in winter.

In January of 2013 the colonies in the control group began to bounce back as expected but the neonicotinoid-treated colonies continued to decline. By spring half of the neonicotinoid colonies were lost, with abandoned hives that are typical of Colony Collapse Disorder (CCD). Only one of the control colonies was lost, showing signs of the common parasite Nosema ceranae.

“We demonstrated again in this study that neonicotinoids are highly likely to be responsible for triggering CCD in honey bee hives that were healthy prior to the arrival of winter,” said Lu, associate professor of environmental exposure biology at HSPH in a statement.

Bees are responsible for the pollination of one-third of all crops globally and the economic value of their impact on food production would be hard to underestimate. There have been calls for the US to emulate the European ban. However, despite the recent Harvard study, the evidence is not cut and dry.

According to Forbes, the number of bee colonies in the US and globally has begun to bounce back. Meanwhile, in Europe where neonicotinoids have been banned since early 2013 bee colony decline has continued in many places. Although no explanation for geographical differences has been presented to date, colony deaths in northern countries such as the UK, Sweden, Finland and Norway were much higher than in southern countries according to a recent report.