June 30 (UPI) -- With the addition of a pair of new studies, the evidence is now overwhelming: neonicotinoid pesticides kill both honeybees and wild bees.
One of the research efforts involved a series of field studies in England, Germany and Hungary. Scientists exposed three bee species to oilseed rape crops grown from seeds coated with clothianidin, a common neonicotinoid pesticide.
Exposure reduced the overwinter success -- the survival rate from one growing season to the next -- of bee colonies in both England and Hungary. The experiments took place in a series of large fields, and scientists accounted for mitigating factors such as disease and habitat quality.
"Neonicotinoid seed dressings do have positive attributes: they target insects that damage the plant, can be applied to the seed at low dosage rates but protect the whole plant and reduce the need for broad spectrum insecticide sprays," lead researcher Ben Woodcock, scientist at the Centre for Ecology and Hydrology, said in a news release. "Their use as an alternative chemical control option is also useful in controlling pests where insecticide resistance to other pesticides is already found, so play an important role to play in food production."
But the findings -- detailed in the journal Science -- show the seed coating can have a negative effect on bees, not only increasing mortality rates of workers, but depressing the egg laying and reproductive abilities of queen bees.
Researchers believe the German colonies proved more resilient because disease was less prevalent and their habitat boasted a greater diversity of flowers on which to feed.
Previous research has highlighted the compounding effects toxic insecticides and disease, like parasite infections, can have on bee health.
"There may be opportunities to mitigate negative impacts of neonicotinoid exposure on bees through improved honeybee husbandry or availability of flowering plants for bees to feed on across non-cropped areas of the farmed landscape," Woodcock said. "Both these issues require further research."
A second study, also published in the journal Science, focused on exposing bees to "realistic" levels of neonicotinoids. Some have claimed bees are exposed to unrealistic levels of neonicotinoids in studies exploring the risks of pesticides.
To ensure realistic field conditions, researchers at York University in Canada studied the health of bees colonies living adjacent to to corn grown from neonicotinoid-coated seeds for just four months. A control group of bee colonies living far from agricultural fields served as a control.
The field tests showed neonicotinoids had a negative effect on the mortality rates of both queens and workers. Surprisingly, most the bees' neonicotinoid exposures was traced to flowers growing adjacent to the corn fields.
"This indicates that neonicotinoids, which are water soluble, spill over from agricultural fields into the surrounding environment, where they are taken up by other plants that are very attractive to bees," said Nadia Tsvetkov, a PhD student at York.
Researchers also found the addition of a common fungicide, called boscalid, increased the negative impacts of neonicotinoid exposure.