Chronic and indirect exposure to a common pesticide coupled with stress from parasitic infection has lethal effects on a majority of queen bees, a French study has found.

The French National Institute for Research and Analysis (INRA) presented Wednesday its findings, which reflected a 90-100 percent mortality rate for queen bees subjected to the interaction between both factors.

"In light of the rise in mortality rates for worker bees, the fertility of the queen is essential for the renewal of colonies' populations and, ultimately, for their survival," the INRA said.

The study concluded that the combination of neonicotinoid insecticide imidacloprid with the widespread parasite 'Nosema ceranae' altered honeybee queens' physiology and negatively affected the chances of survival of queens and, by extension, their colonies.

The French team of researchers published its paper in 'Nature' magazine following a two-year study that involved the breeding of four groups _ each made up of 10 queen bees _ under varying conditions.

Neonicotinoids are a class of insecticide chemically similar to nicotine that attack insects' nervous systems and were not fully tested for long-term chronic effects when approved for use, environmental group Friends of the Earth have said.

The first group included queens fed by worker bees exposed to the insecticide; the second one was exposed to the parasite; the third group was exposed to both; and the fourth one, as a control group, had no exposure to either.

The queens were then placed on small honeycombs in fields to allow them to mate and lay eggs.

Results showed a change in the queens' physiology when exposed to 0.7 micrograms per liter of imidacloprid _ a typical dose a honeybee could be exposed to in the wild _ combined with a 'Nosema ceranae' infection, as well as a death rate of over 90 percent in a period lasting between 45-90 days.

Honeybee ('Apis mellifera') colonies are constantly exposed to multiple inimical factors, such as pesticides and pathogens, whose combined effect is believed to be partly responsible for the sharp global decline suffered by bee colonies in recent years.

The study's results could explain the "loss in colonies' ability to recover, since the queen's disappearance implies a halt to egg production and therefore a lack of new workers," the institute added.

It is estimated that 300,000 bee colonies die each year in France, according to the French National Apiculture Union (UNAF).

UNAF claims the average honeybee mortality in the country stands at 30 percent, although it has been known to reach 50 or even 80 percent in some regions.