News - March 27, 2014

Imidacloprid is not cause of bee deaths in winter

Roelof Kleis

The insecticide imidacloprid is not the cause of bee deaths in winter, it appears from a multi-year experiment carried out by the Wageningen bee researcher Sjef van der Steen. In field trials, there was little effect on healthy colonies of chronic exposure to high doses of imidacloprid. Van der Steen’s experiment was commissioned by the Ministry of Economic Affairs.

Neonicotinoids such as imidacloprid have come under huge attack as many people blame the disappearance of bees on the excessive use of these insecticides. In the Netherlands, they are used as a coating on some varieties of maize seed, for instance. Various lab experiments have shown the neonicotinoids to have a harmful effect but less is known about how bee colonies respond in the wild. Van der Steen has now put it to the test.  


The Wageningen bee expert gave 60 relative small free-flying colonies (averaging 5500 bees) sugared water contaminated with a dose of imidacloprid that was twice as high as in the nectar in fields. The exposure lasted twelve weeks, from June until early September. ‘A worst-case scenario,’ says Van der Steen. ‘Oilseed rape, for instance, only flowers for three weeks.’ An equally large control group of bee colonies was given the sugared water without imidacloprid. The development in the colonies was then monitored closely until after the winter.

Bee colonies seem to be sufficiently robust to cope with the effects
Sjef van der Steen

The trial was a follow-up to a similar experiment carried out the year before. The results of the two studies are striking. Van der Steen: ‘Though imidacloprid does have some effect on the number of bees, the quantity of bee bread (stocks of pollen) and the brood, there was no impact on their ability to survive the winter. Bee colonies seem to be sufficiently robust to cope with the effects. The winter mortality rate among the imidacloprid colonies was 12 percent, in line with the norm in Europe. Imidacloprid also has no impact on the colonies’ vitality and the transition from the summer colony to the winter colony.’

Less swarming

But that does not mean imidacloprid has no impact at all on bees. The 2012 study showed that the bees exposed to the insecticide were significantly less likely to swarm. That was evident from the formation of swarm cells. In these cells, a new queen is created so that the old queen and her followers can swarm off. Van der Steen does not know why they are less interested in swarming. ‘Further research is needed to determine whether imidacloprid is the direct cause or whether it’s a secondary effect of the exposure.’  

There is not always a simple explanation. For instance, in the summer more bees died in the control group than in the imidacloprid group. Van der Steen thinks that may also be linked to the greater urge to swarm. But one thing he is sure of is that imidacloprid is not the main culprit when it comes to winter deaths among bees in this setup. ‘But that doesn’t mean imidacloprid has no effect. If a bee colony is weak, it may be the final straw that the bees are unable to deal with.’