Jan 11, 2008

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New meta-analysis from the US

Bt plants: no evidence that they are harmful to bees

Genetically modified Bt plants have no adverse effect on the survival of honey bees – this is the conclusion of a group of US ecologists who evaluated 25 independent laboratory studies using standard criteria. The new meta-analysis, published in January 2008 in the multidisciplinary online journal PloS ONE, confirms and increases the statistical significance of previous individual studies.

Mysterious bee deaths in the US in 2007 triggered speculation that Bt plants could be involved. Evaluation of 25 individual studies found no evidence that Bt proteins have an adverse effect on bees.
Photo: Bees infested with varroa mites USDA, Wikimedia

The meta-analysis is particularly interesting against the background of a dramatic decline in bee populations in the US, which provoked worldwide concern in spring 2007. There has also been public speculation in Europe that genetically modified Bt maize could be behind the bee deaths. However, research groups in various countries which have since looked into this issue have so far been unable to find any significant effects of Bt plants on honey bees. The problem however is that the statistical power of many studies is limited because the monitoring periods were too short and too few repetitions were carried out.

To make the studies comparable and to increase the statistical power of the individual studies, Jian Duan of the Ecological Technology Center of the agribusiness Monsanto, together with researchers from Santa Clara University in California, the University of Maryland and Michigan State University, evaluated the results using standard criteria. According to the publication, “meta-analysis can reveal effects even when each of the individual studies failed to do so due to low replication”. The meta-analysis is based on 25 individual studies, which looked at the effects of Bt proteins or Bt plant parts on the survival rates/mortality of honey bee larvae or adult bees under laboratory conditions. Bt plants contain genes from the soil bacterium Bacillus thuringiensis (Bt), which enable them to produce a protein that is toxic to specific chewing insects.

To be included in the meta-analysis, the individual studies had to involve Bt variants which act specifically to control butterflies (Lepidoptera, Cry1, Cry2, Cry9) and beetles (Coleoptera, Cry3) and which are used mainly in Bt maize and cotton. Since bees belong to the order Hymenoptera and are not target organisms of the Cry protein, most experts believe it is unlikely that they will be harmed by Bt plants. The results from the new meta-analysis confirm this appraisal and are also consistent with prior studies: No statistically significant effects on the survival rates of honey bees could be found for any of the Cry proteins investigated, even when all the individual results were combined.

The US meta-analysis confined itself to evaluating laboratory studies. Since the concentration of Bt proteins that the bees in the laboratory are exposed to is usually more than ten times that found in the field, the authors suggest that Bt protein is unlikely to have a direct toxic effect on honey bees in the field. The need for additional field studies could reveal whether stress factors such as heat or disease alter the susceptibility of the bees to the Bt toxin.