Even low doses of popular pesticides can reduce bees' survival and reproduction, two new studies show.

The findings bolster evidence that such chemicals may be partly responsible for recent declines in populations of honeybees and bumblebees in North America and Europe, which have caused alarm due to the insects' importance as crop pollinators.

The researchers suggest the widespread use of the pesticides needs to be re-evaluated in light of the findings.

The two studies, published Wednesday in the journal Science, looked at the effects of pesticides called neonicotinoids on bumblebees and honeybees, respectively.

Neonicotinoids, first introduced in the 1990s, are used to kill aphids and other sap-sucking insects. According to a news release from Science, they are now some of the most widely used crop pesticides in the world.

Bayer Crop Sciences, which is the leading producer of this type of pesticide, told The Associated Press that it is used on 90 per cent of the corn grown in the U.S. and is safe.

In the first study, led by Penelope Whitehorn at the University of Stirling in Britain, colonies of buff-tailed bumblebees were fed doses of a neonicotinoid pesticide called imidacloprid in doses similar to those that they would be exposed to in the wild when foraging among crops sprayed with the pesticides. The bees were then allowed to forage for six weeks. The researchers found that bumblebees exposed to the pesticide had nests that were an average of eight to 12 per cent smaller than colonies that weren't exposed. They also produced 85 per cent fewer queen bees.

That could have a huge effect on bumblebee populations, because all bumblebees except the queens die when winter sets in. Bumblebee populations rely on the queens to survive the winter and found new colonies in the spring.

In the second study, led by Mickaël Henry of the French National Institute for Agricultural Research in Avignon, France, honeybees were fed small doses of a different neonicotinoid pesticide called thiamethoxam. They were then tracked with small microchips called RFID tags.

The researchers found that 10 to 31 per cent of bees exposed to the pesticide did not return to their colony after being released to forage for the day. That was up to double the estimated normal mortality rate for a honeybee on a given day, about 15 per cent.

The pesticide appears to interfere with the bees' ability to navigate and find their way back to the colony, an effect that has been shown in previous studies.

Henry noted that currently, in order to get a pesticide approved, the manufacturer must show that the product does not directly kill bees when applied to a field.

"But they basically ignore the consequences of doses that do not kill them but may cause behavioral difficulties," he said in a statement.

David Goulson, who co-authored the British study with Whitehorn, said the use of neonicotinoid pesticides "clearly poses a threat" to the health of bees and "urgently needs to be re-evaluated."

However, Bayer eco-toxicologist David Fischer said the honeybee study used doses of pesticides far higher than those used on crops bees normally pollinate. The study had described the dose as "field-realistic."

Many bee species have been declining in North America and Europe, and some have even gone extinct or are believed to be close to extinction. Meanwhile, honeybees, which are used to pollinate important crops such as raspberries, cherries, and almonds have been suffering since around 2006 from a phenomenon called Colony Collapse Disorder, in which worker bees disappear suddenly.

Pesticides are only one of the possible causes. Recent studies have suggested that fungi and viruses or parasites may be to blame.