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Pesticide cocktails harm bumblebees

22 October 2012, by Tamera Jones

Exposure to combinations of common pesticides can severely affect individual bees, and whole nests, say researchers.

Bumblebee Bombus pascuorum.

Bumblebee Bombus pascuorum.

Scientists have already shown that pesticides can kill bees. It seems these chemicals affect bees' ability to find their way home successfully, and they reduce the number of queens produced by colonies.

But bees are typically exposed to lots of different pesticides when collecting nectar and pollen from crops, not just one.

'The big question is when individuals are sub-lethally affected by pesticides, can whole colonies buffer those effects, or does it lead to a cumulative effect, resulting in reduced colony success or even failure?' says Dr Richard Gill from Royal Holloway, University of London, lead author of the study.

'The novelty of this study is that we show how the sub-lethal effects of pesticide exposure affect individual bee behaviour with serious knock-on consequences for the performance of the colony as a whole.'
Dr Richard Gill, Royal Holloway, University of London

To answer this question, he and colleagues from Royal Holloway, University of London decided to investigate the specific and combined effects of a neonicotinoid and a pyrethroid on individual bumblebees' foraging ability. They also looked at the chemicals' effects on the development and growth of bees' colonies, focussing on bumblebees rather than honeybees.

'Honeybees are smaller than bumblebees, so may be more susceptible to pesticides. But bumblebees have much smaller colonies than honeybees do; some nests hold fewer than 50 individual bees. This means they may be less able to buffer the negative effects of pesticides compared with honeybees,' explains Gill.

Pollinating insects are important for many crops, with bees contributing a massive 80 per cent towards pollination. Bumblebees pollinate wildflowers and crops such as courgettes, green beans and tomatoes. Honeybees pollinate different crops, as well as producing honey.

In the UK alone, pollination is worth an estimated £430 million to the national economy every year.

But pollinators are in big trouble. Global bee populations have crashed in recent years, with many US honeybees threatened by so-called Colony Collapse Disorder. Scientists have responded by upping their efforts to understand the problem.

Even so, this study, published in Nature is the first time scientists have studied the effects of exposure to a combination of pesticides under field realistic conditions.

They built an experimental set-up which meant bumblebees from 40 colonies were exposed to either a neonicotinoid called imidacloprid or a pyrethroid called lambda-cyhalothrin, a combination of the two, or no pesticide at all. Importantly, the bees were free to bypass the chemicals if they chose.

Exposure to both pesticides has a severe effect

The researchers found that bumblebee nests exposed to the neonicotinoid produced fewer adult workers. The chemical also damaged the foraging ability of these colonies' worker bees, and killed many before they managed to get back to the hive. The pyrethroid pesticide led to the deaths of worker bees.

But contact with both pesticides had the most severe effect on the insects, increasing the likelihood of entire colonies perishing.

'The novelty of this study is that we show how the sub-lethal effects of pesticide exposure affect individual bee behaviour with serious knock-on consequences for the performance of the colony as a whole,' says Gill.

Worryingly, the findings also show that the bees either aren't detecting the pesticides or they're choosing not to avoid them.

Farmers use neonicotinoids and pyrethroids on crops like oil seed rape and tomatoes, and gardeners use them to protect their highly-prized plants from aphids.

'Now we know about the negative effects of these pesticides, we need to use this information to get the balance right between maximising the killing of crop pests but limiting their harmful effect to bees,' Gill adds.

Pesticides are not the only problem that bees face. The varroa mite, a bloodsucking parasite that lives on them, has spread rapidly around the world in the last 50 years. It increases the transmission of a deadly virus to levels capable of wiping out whole colonies.

This threat is amplified by the reduction of habitat. The UK has lost most of its wildflower meadows, dramatically reducing the food sources available to bees, butterflies and moths. Add the threat of behaviour-altering pesticides to both kept and wild bee populations, and it's clear that pollinators are in for a rough time.

The study was supported by the Insect Pollinators Initiative, which is joint-funded by the Biotechnology and Biological Sciences Research Council, the Department for Environment, Food and Rural Affairs (Defra), the Natural Environment Research Council, the Scottish Government and the Wellcome Trust. It is funded under the auspices of the Living With Environmental Change programme.


Richard J. Gill, Oscar Ramos-Rodriguez & Nigel E. Raine, Combined pesticide exposure severely affects individual- and colony-level traits in bees, Nature, published 21 October 2012, doi:10.1038/nature11585


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