As they buzz from flower to flower, bees, moths and hoverflies carry out a vital job. Around one third of agricultural crops are pollinated by these busy insects, a service that is worth £440 m a year to the UK economy alone.

But in recent years these valuable pollinators have been struggling, with populations plummeting worldwide. Honeybees in particular have been suffering, with colony collapse disorder (CCD) – a phenomenon where the bees desert the hive – becoming more common in Europe and North America.

Controversy has swirled around the issue, and everything from mobile phones to GM crops have been blamed. Now new studies indicate that insecticides are playing a significant role.

The most recent studies have exposed a variety of insects to varying doses of neonicotinoid insecticides over long time periods – 12 months or more. Neonicotinoid insecticides are widely used worldwide; they work by acting on the central nervous system of the insect. The chemicals have little affinity for vertebrate nervous systems, so they are much less toxic to mammals and birds.

The researchers found that the total dose of insecticide required to kill the insect was smaller if administered over a longer time period (Ecotoxicology (2009) 18:343–354). In the case of honeybees, up to 6000 times less insecticide was required to kill them if it was administered in multiple tiny doses over a long time period.

According to Henk Tennekes, a researcher at Experimental Toxicology Services (ETS) in the Netherlands, these findings make perfect sense. “Start by considering a high exposure level,” he said. “It may cause an early effect, such as cancer or mortality. At a much lower exposure level you may get a late effect. However, as it turns out, in the latter case you need much less of the stuff (in total) to produce the effect.” Tennekes describes the findings in a forthcoming paper in Toxicology.

So how do these insecticides achieve such a powerful long-term effect? The answer lies in the way that they work. Neonicotinoids bind irreversibly to receptors in the central nervous system of insects. “An insect has a limited amount of such receptors,” explained Jeroen van der Sluijs, a scientist at Utrecht University in the Netherlands, who has also worked on the problem. “The damage is cumulative: with every exposure more receptors are blocked until the damage is so big that the insect cannot function anymore and dies.”

Even small doses over a short time period can cause serious problems. At low doses insects have been observed to become disorientated and less co-ordinated in their movements, making them easier prey for predators. Sub-lethal effects such as this weaken the insect; they particularly jeopardize social insects, which depend on the entire colony being healthy for survival.

Right now it still isn’t possible to say if neonicotinoids are the sole cause of CCD in honeybees, but it seems likely that they play a significant role. “It explains the rapid increase in CCD since 2004, which coincides with the rapid growth in worldwide use of neonicotinoids – the most widely used class of insecticides,” said van der Sluijs.

Currently the insecticides are commonly used to coat seeds, regardless of whether there are many insect pests or not. They leach easily into soil and water and are taken up readily by plants, making the entire plant toxic to insects. And as the new research shows, even at very low levels they have the potential to cause huge damage to insect populations. “I think these insecticides need to be replaced by less long-lived alternatives that are less toxic to honeybees and less prone to leaching,” said Tennekes.