News:Bt maize: No risk to woodlice
Woodlice are more likely to be found underneath damp stones in the garden than on farmland. These decomposers are not widely found in maize fields, but do occur in large numbers at the edges of piles of maize litter. Nevertheless, woodlice are regarded as model decomposers and have also been the subject of a few studies looking at possible effects of Bt maize.
Most studies on the effects of Bt maize on non‑target organisms have focussed on earthworms, springtails and microbes from the decomposer group. But woodlice also play an important role as decomposers in soil ecology. Their main task is the primary decomposition of dead plant material, which then becomes food for other decomposers such as mites, springtails and microbes.
A study published in August 2006 looked at the effect of Bt maize on two species of woodlouse, Trachelipus rathkii and Armadillidium nasatum (pill woodlouse). These two species of woodlouse make particularly suitable test organisms for the USA, since they are widely found in the maize-growing regions there.
In this laboratory study, leaf material from two Bt11 maize varieties, two Mon810 maize varieties and the corresponding isogenic lines were fed to the woodlice and their survival and growth was measured over an eight-week period. Both the plants and the woodlice were also examined for changes in nutrient content.
In 2000 and 2002, studies were conducted at the University of Bern with another species of woodlouse Porcellio scaber, also using Bt maize varieties that produce the Bt toxin Cry1Ab (Bt11 and Bt 176), as well as several conventional maize varieties. No Bt effect was observed, but there was a significant varietal effect, which the authors of the study attributed to variations in nutritional quality of the different varieties.
Mortality: In the current study no effects on mortality were observed for the two woodlouse species T. rathkii and A. nasatum. However, increased mortality was found in T. rathkii in additional tests using pure Bt toxin at the highest dosage. According to the authors, this could be an indication of chronic toxicity, which would have to be studied in more detail. But they also point out that this dosage is significantly higher than that which could arise in the field.
Weight increase and growth: No effect on weight increase or growth was observed for A. nasatum.
For T. rathkii the weight increase with all maize variants – irrespective of whether they were with or without Bt - was generally significantly lower than for the controls, where the woodlice were fed on guinea pig food that contains around 10 times more protein than the maize fodder provided.
No statistically significant difference between the Bt and corresponding isogenic lines was found. More significant differences were found between the different varieties. However, since a (statistically insignificant) tendency for more rapid weight increase and more rapid growth was observed for T. rathkii with the isogenic lines, further studies were conducted using pure Bt toxin. No Bt effect was demonstrated, but an effect on mortality was observed at the highest dosage (see above).
Moulting: No effect on moulting was found in the two woodlouse species T. rathkii and A. nasatum.
Einfluss der Nahrungsqualität von Mais auf Zersetzer
Effect of the nutritional quality of maize on decomposersIn this study too, significant varietal effects were found, but no significant Bt effects. The nutritional composition of the plants was therefore also examined. Different varieties have different nutritional qualities. For example, some Bt varieties are of higher nutritional quality whilst others are less suitable as a food source. In addition, individual species react differently to the food provided, as the differences in the two woodlouse species A. nasatum and T. rathkii show.
Significantly less protein was found in all maize variants compared with the control feed. Some Bt maize lines have a higher percentage protein content than their isogenic parent lines. However the protein content had no effect on the growth or weight increase of A. nasatum. With Trachelipus rathkii on the other hand, a marked positive correlation was observed between the protein content of the feed and weight increase and growth. The total protein content in the woodlice themselves, however, proved to be largely unrelated to the feed variants.
The authors conclude that differences in the nutritional composition between Bt lines and isogenic lines can lead to differences in the effects on non-target organisms that are not attributable to the Bt toxin itself. However, they stress that in addition to a control with the isogenic line, further varietal pairs and in particular an optimum food source should also be used as a control in order to be certain whether an observed effect is actually a Bt effect.
