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Bt maize and non-target organisms

Bt maize pollen poses no risk to green lacewings

Scientists at the Swiss research institute Agroscope Reckenholz-Tänikon (ART) have carried out the first research on adult green lacewings to see whether they are harmed by ingesting Bt maize pollen.

During the maize-flowering period, lacewings ingest maize pollen, which they use as a source of protein. If genetically modified Bt maize is cultivated, these important natural predators therefore come into direct contact with the Bt toxin, which is also produced in the pollen of the maize plant. By contrast, lacewing larvae feed mainly on aphids. These in turn suck the phloem sap from the plants, which does not contain any Bt toxin. This means that in nature, lacewing larvae hardly come into contact with Bt toxin at all. So far, however, it is only the larvae that have been studied.

The adult lacewings have delicate, lacy wings. They feed mainly on honeydew, nectar and pollen. Their light green colour changes to yellow-brown in the autumn. The females lay up to 700 eggs over the course of their life on the undersides of leaves.

Photo: ©Jenny Ziegler/PIXELIO

acewing larvae are important beneficial organisms. They feed mainly on aphids (in German they are also known as ‘aphid lions’). The aphids do not ingest any Bt toxin from the maize plants since they feed on the plant sap, which does not contain Bt toxin.

Nevertheless, the same Swiss research institute has previously carried out various studies on lacewing larvae – with varying results.

In this first study on adult lacewings, Bt maize pollen was combined with a sucrose solution and fed to the insects over 28 days. Maize usually flowers for five to eight days, but in exceptional cases the flowering period can last 14 days. This time period was doubled to 28 days.

For the experiments, maize varieties were selected that produce two different Bt toxins: Cry1Ab, to which the European corn borer, a moth, is susceptible, and Cry3Bb1, which is effective against the Western corn rootworm, a beetle.

Bt176 was selected as the Cry1Ab-expressing maize variety. Bt176 is no longer grown today but has a particularly high concentration of Bt toxin in the pollen. Other Bt maize varieties that are resistant to the European corn borer, such as Bt11 and Mon810, which is authorised for cultivation in Europe, have much lower Bt toxin concentrations in the pollen. The second variety chosen was Mon88017, which expresses Cry3Bb1.

As a control, pollen from the isogenic maize varieties, i.e. the unmodified conventional parent varieties, was fed to the insects.

In a second series of experiments, the lacewings were offered an artificial diet containing pure Bt toxin. The toxin concentrations here were between eight and ten times higher than those found in maize pollen. A substance from snowdrops (GNA, Galanthus nivalis agglutinin) was given to the lacewings as a positive control. Lacewings are known to be sensitive to this substance.

In both feeding experiments the researchers studied the mortality, length of time to egg laying, the number of eggs laid, the fertility and dry weight of the lacewings on completion of the test.

During the tests, the stability and bioactivity of the cry proteins and the uptake of the toxin by the lacewings were checked and confirmed. Interestingly, when fed with maize pollen, the toxin uptake was 25 times lower for the male insects than for the females. A possible explanation, according to the authors of the study, is that the males need only carbohydrates to survive and therefore concentrated primarily on the sugar solution, whereas the females need proteins for reproduction and this is provided by the pollen.

No differences were found between the Bt and conventional diet for any of the parameters studied in either test. By contrast, with the GNA, the length of time to egg laying, the number of eggs laid, the fertility and dry weight clearly suffered.

The authors draw the following conclusions:

  • Lacewings are not harmed by Bt maize pollen. Nor are they sensitive to levels of toxin that are much higher than those found in the pollen.
  • Uptake of Bt maize pollen is therefore regarded as a negligible risk to lacewings.