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Neonicotinoids: Are they killing our bees?

In April, the EU banned the use of neonicotinoid pesticides for two years starting in December because of concerns over their effect on bees.  The use of these pesticides will not be allowed on flowering crops that attract bees or by the general public, however winter crops may still be treated. Fifteen countries voted for this ban, with eight voting against it (including the UK and Germany) and four countries abstaining.

Neonicotinoids were originally thought to have less of an impact on the environment and human health than other leading pesticides. They are systemic insecticides, which means they are transported throughout the plant in the vascular system making all tissues toxic to herbivorous insects looking for an easy meal. The most common application in the UK is to treat seeds before they are sown to ensure that even tiny seedlings are protected against pests.

Image by Kath Baldock
The major concern over neonicotinoids is whether nectar and pollen contains levels of pesticide is high enough to cause problems for bees. It has already been shown that they do not contain a lethal dose, however this is not the full story. Bees live in complex social colonies and work together to ensure that there is enough food for developing larvae and the queen. Since neonicotinoids were introduced in the early 1990s bee populations have been in decline and there is a growing feeling of unease that the two may be connected. Scientific research has provided evidence both for and against a possible link leaving governments, farmers, chemical companies environmentalists and beekeepers in an endless debate about whether or not a ban would save our bees.

Several studies on bees have shown that sublethal levels of neonicotinoids disrupt bee behaviour and memory. These chemicals target nicotinic acetylcholine receptors, one of the major ways that signals are sent through the insect central nervous system. Scientists at Newcastle University recently showed that bees exposed to neonicotinoids were less able to form long-term memories associating a smell with a reward, an important behaviour when foraging for pollen and nectar in the wild. 

Researchers at the University of Stirling fed bumble bee colonies on pollen and sugar water laced with neonicotinoids for two weeks to simulate field-like exposure to flowering oil seed rape. When the colonies were placed into the field, those that had been fed the pesticides grew more slowly and produced 85% less queens compared with those fed on untreated pollen and nectar. The production of new queens is vital for bee survival because they start new colonies the next year. Studies in other bee species have found that only the largest colonies produce queens, so if neonicotinoids have even a small effect on colony size it may have a devastating effect on queen production.

So why does the government argue that there is not enough scientific evidence to support a ban on neonicotinoids?

Image by Kath Baldock
In 2012, the Food and Environment Research Agency set up a field trial using bumble bee colonies placed on sites growing either neonicotinoid-treated oil seed rape or untreated seeds. They found no significant difference between the amount of queens produced on each site, although the colonies near neonicotinoid-treated crops grew more slowly. The study also found that the levels of pesticide present in the crops was much lower than previously reported.

I personally think that both laboratory and field studies bring important information to the debate, however neither has the full answer. Whilst more realistic, the government’s field trial suffered from a lack of replication, variation in flowering times and various alternative food sources available to bees. Only 35% of pollen collected by the bees was from the oil seed rape plants, so where oil seed rape comprises the majority of flowering plants available to bees the effect on neonicotinoids may be more pronounced. The laboratory research can control more variables to establish a more clear picture, however the bees in these studies were often given only neonicotinoid-treated pollen and nectar to eat, which clearly is not the case in a rural landscape. Flies and beetles have been shown to avoid neonicotinoids, which could mean that bees would find alternative food sources where possible. This would have a major impact on crop pollination.

We desperately need well-designed field studies looking at the effect of neonicotinoids on bees and the environment in general. Despite an EU moratorium on growing neonicotinoid treated crops, an allowance should be made for scientists to set up controlled field trials to study the effect of these pesticides on bees during the two year ban. It could be our only chance to determine the danger these chemicals pose to vital pollinators and the wider environment

This blog is written by 
Sarah Jose, Biological Sciences, University of Bristol

Sarah Jose

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