Some of the world's most commonly used pesticides are killing bees by damaging their ability to navigate and reducing numbers of queens, research suggests.
Scientific groups in the UK and France studied the effects of neonicotinoids, which are used in more than 100 nations on farm crops and in gardens.
The UK team found the pesticides caused an 85% drop in queen production.
Writing in the journal Science, the groups note that bee declines in many countries are reducing crop yields.
In the UK alone, pollination is calculated to be worth about £430m to the national economy.
And the US is among countries where a succession of local populations has crashed, a syndrome known as Colony Collapse Disorder.
Many causes have been suggested, including diseases, parasites, reduction in the range of flowers growing wild in the countryside, pesticides, or a combination of them all.
The neonicotinoids investigated in the two Science papers are used on crops such as cereals, oilseed rape and sunflowers.
Often the chemical is applied to seeds before planting. As the plant grows, the pesticide is contained in every part of it, deterring insect pests such as aphids.
But it also enters the pollen and nectar, which is how it can affect bees.
Dave Goulson from the UK's University of Stirling and colleagues studied the impact of the neonicotinoid imidacloprid on bumblebees.
They let bees from some colonies feed on pollen and sugar water containing levels of imidacloprid typically found in the wild, while others received a natural diet.
Then they placed the colonies out in the field.
After six weeks, colonies exposed to the pesticide were lighter than the others, suggesting that workers had brought back less food to the hive.
But the most dramatic effect was on queen production. The naturally-fed hives produced around 14 queens each - those exposed to the pesticide, just two.
"I wouldn't say this proves neonicotinoids are the sole cause of the problems bees face," said Dr Goulson, "but it does suggest they're likely to be one of the causes, and possibly a significant one.
"The use of these pesticides is so widespread that most bee colonies in areas of arable farming are likely to be exposed to them, so there is potential for them to be playing a significant role in suppression of bee populations on a pretty staggering scale."
The French research group investigated the impact of a different neonicotinoid, thiamethoxam, on the number of bees able to make it back to the colony after release.
Using tiny tags attached to the bees' backs, they showed that significantly fewer insects came back if they had previously been exposed to levels of thiamethoxam that they might encounter on farms.
Calculations showed the impairment was bad enough that the capacity of colonies to survive could be severely compromised.
"What we found is that actually if colonies are exposed to pesticides, the population might decline to a point that would put them at risk of collapse due to other stressors," said lead scientist Mickael Henry from the French National Insitute for Agricultural Research (Inra) in Avignon.
Dr Henry told BBC News that it was time for authorities to re-design the safety tests that pesticides have to pass.
"To date, the tests mostly require that the doses found in nature do not kill bees," he said.
"But those authorisation processes ignore possible consequences for the behaviour of bees, and we hope the people in charge will be more careful."
Neonicotinoids are a multi-billion dollar business worldwide. Even though some countries have banned them partially, a complete global prohibition, as some environmental groups advocate, might be impossible.
May Berenbaum, head of entomology at the University of Illinois and one of the leading US experts on CCD, said the chemicals should be used more carefully.
"There is no question that neonicotinoids are being used recklessly, for want of a better word," she said.
"Fifty years of experience should have taught us that overuse of a single class of compounds is an inherently unsustainable practice, and that pre-treating seeds when pest problems might not even be present is collossally unwise.
"But neonicotinoids could be banned everywhere in the world, and honeybees would still have problems with pathogens, parasites, habitat degradation and overuse of just about every other class of chemical pesticide."
At EU level, the Agriculture and Rural Development Committee has asked the European Commission to increase research and produce an action plan to conserve bees.
"When the action plan is produced, we are ready to give member states a deadline to use or not use a specific pesticide - until then it is up to individual states," said Paolo de Castro MEP, the committee's chairman.
In the UK context, Dr Goulson added, it would certainly be worth re-considering neonicotinoid use in gardens.
"Personally I would ban insecticides completely in gardens," he said.
"There are very few serious insect pests in Britain as far as gardening's concerned, it's too cold; and if roses have a few aphids on, then tough, it's not a big deal."
His research team now plans to expand their study to other bee species, while Dr Henry's group will try to discover exactly how thiamethoxam does its damage.
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