Bt maize residues in rivers and streams

Caddis-flies at risk?

A study by American scientists suggests that genetically modified Bt maize growing near bodies of water could be harmful to aquatic life such as caddis-flies. In laboratory trials the insects showed reduced growth rates when fed with Bt maize leaves. Whether the results found in the laboratory also transfer to natural field conditions, however, remains doubtful.

Bt maize residues in rivers and streams

In an initial stage, the scientists conducted field experiments to find out how much plant material from adjacent maize fields is found in water courses and how far it is then carried.

The authors conducted their investigations in 2005 and 2006 in twelve rivers and streams which had maize growing along the banks. The rivers varied in terms of width, flow, speed of flow, distance from the field and bank slope.

• Collection sieves were placed in the rivers, emptied monthly and the contents sorted and weighed. Plant material ranging from 0.1 to 7.0 grams of ash-free dry mass (AFDM) per square metre of river bed per year was found. During flowering time pollen traps were placed above the surface of the water and emptied daily. Pollen was collected at all locations, but in very varying amounts, ranging from 0.1 to 1 gram per square metre of river bed per year.

• Colour-marked leaves, cobs and pollen were used to calculate how far the plant material was transported. Leaves and cobs were transported over distances ranging from 0.38 to 180 metres, and pollen between 20 and 60 metres.

• The degradation rate of Bt and non-Bt leaves was calculated using litterbags containing plant material, which were suspended in the water. No differences were found in the degradation rate for Bt and non-Bt maize.

• During the 2005 pollen season, the intestines of caddis-flies (Hydropsyche ssp.) were examined for the presence of maize pollen. Pollen was found in the intestines of 50 percent of the net-building caddis-flies.

Caddis-fly feeding study

Having shown that harvest residues definitely enter streams via surface water, in a second stage of the research the authors conducted feeding studies in the laboratory with two different species of caddis-fly. This involved feeding them pollen and leaf material from Bt maize and comparing them with caddis-flies fed on conventional maize. However, instead of using the isogenic strains as a comparison, as is customary, the scientists used maize varieties with a similar lignin content and carbon/nitrogen ratio. This, they explained, was because Bt maize has a higher lignin content and therefore a different nutritional quality compared with the conventional parent variety.

• The caddis-fly Helicopsyche borealis was fed with algae (its typical food source) and pollen. The pollen concentrations used were the average amount found in the field trials, and two to three times the maximum amount found. The insects came from a river which had no maize growing alongside it. Six repetitions of 10 insects in each case were observed for a period of 18 days. The insects fed with Bt pollen at a concentration of two to three times the maximum showed an increased mortality rate (43 compared with 18 percent).

• Leaf-shredding caddis-flies (Lepidostoma liba) were fed with Bt and non-Bt leaves in small aquariums, and their growth and mortality rate were monitored. The feeding trials ran for 29 days. Leaves were added as required. Lepidostoma liba showed a reduced growth rate of more than 50 percent when fed with Bt maize leaves compared with non-Bt leaves, but no difference was found in the mortality rate.

Are the laboratory findings also relevant in the field?

The authors of the study believe that the feeding study findings indicate potential negative effects of Bt harvest residues on aquatic ecosystems. They therefore propose that assessments of genetically modified plants should include their impact on relevant aquatic organisms such as caddis-flies.

However, it remains doubtful whether the reduced growth rates found in the laboratory are transferable to the field. The authors did not measure the Bt levels in Bt maize pollen and Bt plant residues in the rivers and streams. It is still unclear how the Bt protein is broken down in the water and the extent to which caddis-flies are actually exposed to Bt toxin in natural conditions.

In the feeding trials the authors have given no indication of the origin of the Bt pollen or the Bt leaf material. Bt levels in the various plant parts can vary considerably from one transgenic event to another. There is no information about the concentration of Bt protein in the pollen and leaves, so no statements can be made about the effect of a specific dose of Bt protein.

Moreover, since leaf material and pollen from isogenic strains were not used for comparison as is customary, the varieties used could differ from Bt plants with regard to other plant substances as well. The observed effects cannot therefore be attributed conclusively to the Bt protein.

Subsequent field study: No influence from Bt pollen

The authors have themselves conducted further research in the field to check their hypothesis about possible harm to caddis-flies from Bt maize. They presented their results at a conference organised by the North American Benthological Society (NABS) in June 2007. In July 2006 the scientists installed flow chambers in three streams next to Bt maize fields and three next to conventional maize fields in order to observe the growth rates of two different species of caddis-fly. They placed 40 to 60 insects in these chambers suspended in the water and followed their growth over seven to nine days. They found growth rates of between 0.024 and 0.059 per day and an average mortality rate of 33 per cent. Bt maize pollen had no effect on the insects’ growth or mortality. The results of this research therefore provide no indication that caddis-flies are affected by Bt maize pollen.