Bee death: In search of the cause
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A dramatic spate of bee deaths is being observed in the USA. In many regions almost 90 percent of the population has been affected. In Germany and Switzerland too, colony numbers have virtually halved in the last 15 years. But what are the causes? Suspects include insecticides, old and new pathogens and more recently, radiation from mobile telephone masts. Agricultural monocultures or too little genetic diversity within the bee populations have also been cited as possible causes, as have genetically modified plants: especially Bt maize and Bt cotton, which are grown on a large scale in the USA.
Mystifying bee deaths. Entire bee colonies are disappearing in the USA and also in Europe. Speculation is growing, since the causes are not yet known. The long list of suspects includes genetically modified plants.Photograph: Pixelquelle
On the west coast of America, almost 60 percent of the bee colonies have collapsed; on the east coast and in Texas, more than 70 percent. More than half of all US federal states have been affected, as well as parts of Canada. Significant bee colony losses have occurred before in certain years, but this time the symptoms are different: instead of individual dead bees being found, the bees are simply leaving the bee hives and not coming back. In the USA this phenomenon is known as "Colony Collapse Disorder" (CCD).
So far no underlying causes for bee death have been clearly identified. Known bee diseases and parasites, such as the Varroa mite, have been ruled out as the sole cause. The affected bees evidently show signs of a weakening of the immune system. For this reason bee death is referred to as "bee AIDS" in the USA
Beekeepers in Germany, Austria, Spain, Poland and Switzerland have also recorded heavy losses. Manfred Hederer, president of the Deutscher Beruf und Erwerb Imkerbund (German professional beekeepers’ association), reports a 25 percent decline in bee colonies in Germany, rising to 80 percent in certain regions. Hederer speculates that "a particular toxin, some kind of unknown substance" is killing the bees. In Switzerland, annual losses of around 25 percent were recorded between 2003 and 2006.
Beekeepers are not alone in suffering financial losses as a result of bee death. Bees are essential for the pollination of over 90 different cultivated fruits and vegetables worldwide. It has been estimated that the economic value of the pollinating activity of honey bees in the USA alone is $14.6 billion annually. The bees are also of ecological value, since many wild plants rely on pollination by bees.
US working group in search of clues
A working group consisting of researchers and officials has come together in USA to examine the causes of the mysterious bee deaths. After conducting a preliminary analysis of the situation, the working group is now focusing on three different hypotheses:
new pathogens
chemicals, such as insecticides
a combination of different factors, e.g. Varroa mite infestation, diseases and nutritional problems
Bee inbreeding and associated genetic impoverishment could also explain an increased susceptibility to disease and parasites. Some experts are therefore suggesting that increased efforts should be made to breed resistant strains.
Bt maize under suspicion - but virtually no substantiated evidence
Environmental organisations suspect that genetically modified plants may also account for the baffling bee deaths. For this reason the Sierra Club, one of the oldest environmental organisations in the USA, wrote to the US Senate calling for a detailed scientific examination of the correlation between genetic engineering and bee deaths. Insect-resistant maize and cotton plants, which contain a toxin gene from the soil bacterium Bacillus thuringiensis (Bt), are the prime suspects.
Various expert committees in the USA and Europe have considered this Bt hypothesis and sifted through the available scientific research. They concluded that existing findings contain no evidence to suggest that Bt plants are harmful to bees.
The National Research Council points out that although negative effects caused by Bt pollen have been found in individual studies, these effects were sublethal in all cases; in other words they did not result in bee death. The Council stresses that in no cases were transgenic plants shown to have had a negative impact on honey bee populations. In a summary the Mid-Atlantic Apiculture Research and Extension Consortium also emphasises that, according to the existing body of data, this new type of bee death is not caused by Bt plants currently under cultivation.
The European Food Safety Authority (EFSA) looked at one study which the Greek government had cited as proof of the potential negative effects of Bt maize on bees and had used to attempt to justify a ban on Bt maize cultivation in Greece. In the study, published in 2005, the observed reduction in food intake by the bees was presented as a consequence of feeding them the Bt toxin. The EFSA expert committee, however, criticised technical shortcomings in the way the trial had been conducted, as a result of which they were unable to make any general conclusions. In particular, the trial had been conducted without independent control experiments and without repetitions on only one colony. EFSA pointed out that the authors of the study themselves were unable to rule out that the bees’ reduced food intake might in fact be attributed to seasonal effects.
A trial at Jena University conducted in 2004 indicated that bees fed with Bt pollen could have increased susceptibility to a certain parasite. However, it has not yet been possible to repeat the trial to verify these findings. Further trials conducted by this research group also indicated that Bt maize pollen has no chronic toxic effect on healthy honey bee populations. Feeding the bees large quantities of Bt toxin neither shortened their lifespan nor modified their food intake. In addition, there is no indication of a negative impact on larval development. These findings are further substantiated by the fact that honey bees living in the wild gather only small amounts of maize pollen even in large maize-growing regions, if other plants are available as pollen sources.
A range of further indications suggest that a correlation between current bee deaths and the cultivation of genetically modified plants is unlikely. Bee death has yet to occur in the US states of Illinois and Indiana, which are among the main Bt maize growing regions in the USA. By contrast, it does occur in regions where maize cultivation plays only a minor role.
Bee death has also been observed in Switzerland, despite the fact that no genetically modified plants are currently grown there. A similar picture emerges in Germany, where Bt maize is grown only on 0.06 percent of the maize-growing area. In addition, this maize is found almost exclusively in the East German states, whereas bee death is a phenomenon that occurs throughout Germany.
