Plummeting bee populations across the world have mystified and worried scientists for decades, but now they believe they can be sure of at least one of the answers. Common crop pesticides have been shown for the first time to seriously harm bees by damaging their renowned ability to navigate home.
The new research strongly links the pesticides to the serious decline in honeybee numbers in the US and UK — a drop of about 50 percent in the last 25 years. The losses pose a threat to food supplies, as bees pollinate a third of the food we eat, such as tomatoes, beans, apples and strawberries.
Scientists found that bees consuming one pesticide suffered an 85 percent loss in the number of queens their nests produced, while another study showed a doubling in “disappeared” bees — those that failed to return from food foraging trips. The significance of the new work, published in the journal Science, is that it is the first carried out in realistic, open-air conditions.
“People had found pretty trivial effects in lab and greenhouse experiments, but we have shown they can translate into really big effects in the field. This has transformed our understanding,” said David Goulson, a professor at the University of Stirling, Scotland, and a leader of one of the research teams. “If it’s only 1m from where they forage in a lab to their nest, even an unwell bee can manage that.”
Mickael Henry, a professor at the National Institute for Agricultural Research in Avignon, France, who led a separate research team, said: “Under the effects we saw from the pesticides, the population size would decline disastrously, and make them even more sensitive to parasites or a lack of food.”
The reason for the huge decline in bee numbers has for many years remained uncertain, but pesticides, the varroa mite and other parasites, and destruction of the flower-rich habitats are believed to be the key reasons.
Pesticide manufacturers and the British government deny that a class of the chemicals called neonicotinoids cause significant problems for bees, but Germany, Italy and France have suspended key insecticides over such fears.
A spokesperson from Britain’s Department for Environment, Food and Rural Affairs said the research did not change the government’s position.
“The UK has a robust system for assessing risks from pesticides and all the evidence shows neonicotinoids do not pose an unacceptable risk to honeybees when products are used correctly. However, we will not hesitate to act if presented with any new evidence,” the spokesperson said.
Henry said the research showed that approval processes for the pesticides were inadequate.
“We now have enough data to say authorization processes must take into account not only the lethal effects, but also the effects of non-lethal doses,” he said.
The pesticides investigated in the studies are applied to seeds and flow through the plants’ whole system. The environmental advantage of this is it reduces pesticide spraying, but chemicals end up in the nectar and pollen on which bees feed. Goulson’s group studied a type called imidacloprid, primarily manufactured by Bayer Cropscience and registered for use on more than 140 crops in 120 countries.
Bumblebees were fed the toxin at the same level found in treated rape plants and it was found that these colonies were about 10 percent smaller than those not exposed to the insecticide. Most strikingly, the exposed colonies lost almost all of their ability to produce queens, which are the only bee to survive the winter and establish new colonies.