Bt maize has almost no effect on earthworms
Earthworms burrow through the soil and feed on plant remains. If Bt maize is cultivated they will eat its stems, leaves and roots as well, thereby ingesting the Bt toxin. Various studies have investigated whether Bt maize could harm earthworms.
Earthworms are everywhere in the soil – one cubic metre of soil can contain up to four hundred of them. Since they ingest soil material when burrowing, they are regarded as "living indicators": if the earthworms are happy, the soil quality must be good too.
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Some biosafety projects have now researched in greater detail whether the cultivation of Bt maize could harm earthworms and therefore the soil quality. In the summer of 2006 a new study by a Danish working group led by Martin Holmstrup was published. For the first time, it looks at the effect of Bt maize on the most common species of earthworm in Germany, Aporrectodea caliginosa. This species accounts for way over half of all earthworms. Most of the other studies were carried out using Lumbricus terrestris (dew worm), the second most common species in Germany. The first study on this topic, published in 1995, focused on Eisenia fetida, as does a recent study published in August 2006. This species of earthworm, also called brandling, is mostly found on manure or compost heaps. It plays no role in agricultural soil, although it can occur in meadows. |
Bt maize leaves in the soil: the Danish study
In its experiments, the Danish team placed earthworms in soil containing ground up leaves from various different maize varieties. Young worms were also placed in pots with maize plants and observed. In each case, a Bt maize line resistant to the European corn borer (Mon810 varieties) was compared with two isogenic lines largely identical to the Bt line but without the Bt toxin , one conventional line and, as a control, a line treated with a commercial fungicide (Benomyl). This spray is toxic to micro-organisms but also to earthworms.
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Effect on mortality.
In the Danish study, as in all the other studies, the Bt maize was not found to have any effect on mortality. -
Effect on weight gain.
Effect on weight gainIn its recent study, the Danish team did not find any effect of Bt toxin on the weight of the earthworms. Almost all previous studies had produced similar results.
By contrast, in one study conducted by the University of Bern using Lumbricus terrestris, weight loss was observed after 200 days. However, it was not possible to determine for certain whether the Bt toxin was actually a possible cause. This is currently being further investigated in a project by Prof. Nentwig’s team in Bern. First results are expected in spring 2007. -
Effect of pesticides.
The Danish scientists were able to demonstrated clearly that the fungicide used in the experiment had a negative effect on the growth of the earthworms. This result was in line with expectations, since the active fungicide ingredient is known to have a negative effect on earthworms. That was why this type of fungicide was used as a positive control.
By contrast, in research carried out by the State Research Centre for Agriculture in Freising/Bavaria (LfL) the insecticide used against the European corn borer (Baythroid) was found to have no effect on the earthworms. -
Effect on reproduction.
The Danish team was the first to investigate possible effects of Bt maize on the reproduction of earthworms. They investigated the development of the cocoon and the hatching of the young worms. High Bt maize leaf concentrations in the soil were found to have a small, but significant effect on hatching from the cocoons. The scientists point out that the Bt concentrations in the experiment were very high and way above the levels found in field conditions.
Generally, the experiments to investigate the effect of Bt maize on earthworms were carried out in the laboratory or under controlled experimental conditions. Now, for the first time, the Bavarian State Research Centre for Agriculture in Freising (LfL) has evaluated field trials with Bt maize in relation to earthworms. They found that site-specific factors – the soil quality or the type of soil cultivation – have a much greater effect on the incidence and species composition of earthworms (Lumbricidae) than the choice of maize line. Significant effects on earthworms were found only when comparing different sites, but not when comparing Bt maize and non-Bt maize.
More from GMO Safety
Bt maize and earthworms: results of various studies
| Study | Earthworm species | Trial | Result |
| Danish working group (Vercesi, Krogh, Holstrup 2006) | Aporrectodea caliginosa | The earthworms were fed on ground maize leaves (1-5 g/kg soil) over 28 days; 4 more weeks until cocoon developed | No increase in mortality with higher Bt concentration;small, but significant decrease in hatching from the cocoon |
| Growth experiment with ground leaves: Soil renewed every 28 days, total duration 14 weeks | No Bt effect | ||
| Young worms in pots with Bt maize plants (5 young worms introduced once the plant had germinated), Evaluation after 28 days | No Bt effect | ||
| Clark and Coats (2006) | Eisenia fetida (brandling) |
28 days ground maize leaves | No Bt effect on survival or weight gain;significant impact of protein content on weight gain |
| LfL, Bavaria (Lang et al. 2005) |
Lumbricidae community | Species spectrum, biomass, total number of earthworms in Bt and non-Bt maize. Field trial |
No Bt effects on species composition or numbers of individuals;significant site effect |
| University of Bern (Zwahlen et al. 2003) |
Lumbricius terestris (dew worm) |
Fed on Bt maize for 160 days and 200 days; "Semi-field experiments" with litterbags (which do not let worms through) | Mortality and growth largely unaffected at 160 days; significantly lower weight after 200 days |
| Saxena und Stotzky (2001) | Lumbricius terestris | 5-day study with pots in soil with Bt plant biomass | No Bt effects on survival or body weight |
| Ahl-Goy et al. (1995) | Eisenia fetida | 14-day acute test | No Bt effects on mortality or weight gain |
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