Bees may be getting hooked on nectar laced with widely used nicotine-related chemicals in pesticides they cannot even taste, in the same way humans are addicted to cigarettes, new research has found.
Adding to evidence of potential harm from the chemicals, another field-scale study published on Wednesday also found that exposure to so-called neonicotinoids affects reproduction and colony growth in some bee species.
Europe has placed restrictions on three such pesticides, citing concerns for bees, but debate continues about the impact of low doses on these and other non-target insects.
Supporters of neonicotinoids - made by companies including Bayer and Syngenta - say they have a major benefit because they destroy pests and boost crop yields.
Critics, however, fear they contribute to a decline in bees, which are crucial for crop pollination.
To find out more, Geraldine Wright of Newcastle University and colleagues offered bees a choice of sipping on pure sugar water or a sugar solution containing very low doses of neonicotinoids.
Wright said she was shocked to find that honeybees and bumblebees drank more from pesticide-containing solutions, implying that naturally foraging bees would do likewise.
"There's a conundrum that they are attracted to the stuff that actually is having a negative impact on their motor function and their ability to collect food and forage," she told reporters.
The most likely reason lies in the similarity of the chemicals to nicotine, which itself is produced by tobacco plants to prevent against attacks by insects. In large amounts it is toxic, but a little bit acts as a drug.
"As soon as it gets into their blood they are getting a little buzz, as it were, and they are responding to that," Wright said.
Separately, a team of Swedish researchers found that oilseed rape sown from seeds coated in neonicotinoids reduced wild bee density, solitary bee nesting and bumblebee colony growth.
Neonicotinoid exposure did not significantly impact honeybee colonies, possibly because they have larger colonies with more workers and are better equipped to deal with damage, but scientists said the evidence against the chemicals was growing.
"At this point in time it is no longer credible to argue that agricultural use of neonicotinoids does not harm wild bees," said David Goulson, a biologist at the University of Sussex, who was not involved in either of the research efforts.
Both bee studies were published in the journal Nature.
(Editing by Louise Ireland)