Bt in the soil

April 2011

Soil fungi 

Bt insecticidal toxins generated by GM plants will enter the soil as root exudates, as decomposing litter and as ploughed-in stubble. The toxins are known to be able to accumulate in soil where they can remain active for several months.

In 2008, an Italian study looked at the effects of Bt-toxin on soil health, with particular reference to the soil fungi which play a vital role in soil fertility and plant nutrition.

The research identified two strains of Bt crop which had the ability to reduce fungal colonisation of root systems. The finding was supported by a previous study demonstrating that litter decomposition, which is dependent on fungal colonisation, was slower in Bt crop material.

A reduced activity of essential fungi in the soil has many ominous implications.

The biotech industry has been able to make much of the fact that Bt maize is consistently found to have significantly reduced levels of mycotoxins. Mycotoxins are produced by the fungus, Fusarium. There is routine assessment of the presence of mycotoxins in maize because some of them can pose real health problems for humans and livestock: they can be lethal, especially to horses and swine, they have been linked to birth defects and cancer, and some forms have even been considered for use in chemical warfare. The lower levels of mycotoxins in Bt maize is a plus for the GM crops in terms of consumer safety. However, the bigger picture may not be so rosy.

The lower mycotoxin levels in Bt maize have generally been attributed to reduced insect damage as a result of its systemic insecticide (a damaged plant will be particularly susceptible to fungal attack). However, the main aggravating factors for Fusarium colonisation do not seem to be insect damage, but rather the prevailing humidity and temperature due to weather and storage conditions. This fact, combined with the fungal inhibition noted in the Italian study, suggests that it is the Bt-toxin itself, or some GM-linked by-product, which is acting as a fungal inhibitor. Any reduction in mycotoxins as a result of reduced insect damage appears to be secondary. This is worrying because of its implications for the health of the soil in the long-term.

Like all organisms living in their own natural, varied ecosystem, Fusarium is held in check by other micro-organisms in its environment. If these controlling organisms are also being inhibited to differing extents by the presence of the novel Bt-toxin, several unwelcome events could unfold:

• the levels of Fusarium may explode due to lack of competition whenever the inhibiting Bt is reduced, resulting in higher mycotoxin levels in subsequent crops
• the levels of other harmful microbes may explode in the face of reduced competition
• mutant Fusarium or other pathogenic species resistant to Bt may evolve and boost mycotoxin levels many fold.

All of these could happen together.

Note that the pressure to evolve resistance is significant in Bt monocultures where Bt-toxin is pervasive in the crop and the soil. Microbial and fungal resistance also have every chance of being enhanced by horizontal gene transfer from Bt maize debris into soil microbes. Compare this scenario to conventional farming in which pesticides are applied as a one-off event after which the ecosystem can start to recover, or organic farming in which pests are controlled by encouraging a diversity of co-existing micro-organisms to keep each other in check.

The next generation of GM maize is emerging with stacked genes. These have several different kinds of Bt toxin, each with its own influence on soil fungi. The new crops are also stacked with genes for resistance to the herbicide, glyphosate. It's already been noted that crops grown in fields subsequent to glyphosate-resistant ones seem to experience higher than average levels of Fusarium, infestation. The soil ecology resulting from such crops is likely to be a very distorted one indeed, and by no means healthy nor able to support healthy food crops.

Sources

• Adapted from 'Bt is a TOXIN' which first appeared on GM-free Scotland in September 2008
• Turrini et al., Experimental Systems to Monitor the Impact of Transgenic Corn on Keystone Soil Micro-organisms, IFOAM Organic World Congress, Modena, Itlad 16-20 June, 2008, http://orgprints.org/

Earthworms

In August 2005, a study was published which examined the effects of Bt corn residues and living Bt corn plants on a species of earthworm common in agricultural soils. The Bt protein present in both crops was Cry1Ab. No deleterious effects on the worms' survival, growth, development or reproduction were found. However, cocoon hatching success was reduced from 95% to 75% at one of the highest concentrations of Bt litter. This was described as a 'slight' negative effect. The reason for it could not be explained.

Earth worms such as the one studied feed by ingesting large amounts of soil, and play an important role in recycling soil nutrients and in increasing drainage and aeration of the soil,especially where reduced tillage practices are used.

The authors also reviewed previous published studies dealing with the effects of a Bt-protein on earthworms. It didn't take long: there were only two. Both measured only acute toxicity (14-days and 45-days). Neither found any effect on survival or body weight. However, one of these studies was based on a species of earthworm not relevant for agricultural soils, while the other looked at a species not abundant, and therefore probably of minimal importance, in such soils. A longer-term examination of the latter worm species detected a lower weight in Bt-litter-fed adult worms.

Sources:

• Adapted from 'NON-TARGET EFFECTS'
• M. L. Vercesi et al. August 2005, Can Bacillus thuringiensis (Bt) corn residues and Bt-corn plants affect life-history traits in the earthworm Aporrectodea caliginosa?