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Genetically modified maize and moving water

Bt maize in rivers and streams – ecological implications uncertain

Whether Bt protein from GM maize can end up in rivers and streams and harm the insects that live there has been a subject of debate for some years. A new study has been published in the USA, where Bt maize is now cultivated on around two-thirds of the maize-growing area.

Jennifer Tank (University of Notre Dame; left), Emma Rosi-Marshall (Loyola University, Chicago; right) Photo: G. Lamberti

Caddis-flies are related to moths and butterflies and live in water. If the water contains remains of Bt maize plants, caddis-flies could ingest Bt proteins. However, there is evidence that the amounts ingested are too small to cause any harmful effects. Photo: Wikimedia

The latest study by Jennifer Tank and Emma Rosi-Marshall assessed waterbodies in the US state of Indiana. Indiana is situated within the American Corn Belt, where maize is grown intensively. The maize-growing areas in the Corn Belt are criss-crossed by a large number of drainage ditches and underground drainage channels. During the maize harvest, it is usually only the cobs that are harvested. The chopped remains of the maize plants are left on the field and are not ploughed under. This means that maize remains regularly end up in the surrounding waterbodies, from where they can be carried further afield.

In May 2007, samples of maize remains (where present) and water were taken from 217 aquatic locations. An analysis of the GIS (Geographical Information System) data revealed that in 2006, 99.9 per cent of the 630 water kilometres in the survey area were no more than 500 metres away from a maize field. Remains of maize plants were found at 86 per cent of the sites investigated. Bt protein Cry1Ab was found in maize remains at 13 per cent of the sites, and in the water at 23 per cent of sites.

In the view of the two biologists, not enough research has been conducted into the impacts of Bt maize remains being washed into aquatic ecosystems. Maize remains are rapidly colonised by aquatic invertebrates such as caddis-flies, earthworms and waterlice and used as a source of food. Tank and Rosi-Marshall admit that the amounts of Bt protein they detected are small in relation to the level of Bt protein in fresh maize plants. They claim, however, that we do not know whether the Bt concentrations found in the water and maize remains could have harmful effects on e.g. caddis-flies. Potential negative impacts on these non-target organisms would also depend on the amount ingested and the rate at which the maize remains break down. Since Bt protein was found at the survey sites six months after harvesting, it is, they claim, probable that non-target organisms would be exposed to it for months. All these points would need to be checked during further studies.

Preliminary results from another piece of research indicate that the ecological impacts could be limited. A study published in April by the University of Maryland showed that, just two weeks after harvest, Bt maize plant parts no longer have any impact on the European corn borer or on the larvae of two species of caddis-fly. Unlike the study by Rosi-Marshall and Tank, this project assessed not only whether Bt protein could be detected in the waterbodies, but whether it was still biologically active. Bt protein would appear to be broken down quickly under natural conditions to a point where it is no longer effective.

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