Development of resistance to Bt maize among Western corn rootworm

(2005 – 2008) University of Göttingen, Institute of Plant Pathology and Plant Protection, Göttingen

Bt maize with resistance to the Western corn rootworm (Diabrotica v. virgifera) primarily attacks the pest in its first larval stage. Unlike the European corn borer, the Western corn rootworm can also feed on other plants found in maize fields. If the newly hatched larvae initially eat these alternative host plants and then switch to maize, partially resistant pests could emerge over time, leading to the development of resistance over several generations.
Alternative host plants increase the probability of survival of the Western corn rootworm larvae. Important factors are the plant species and the positions of the plants within the crop. A comparison of various different arrangements of refuges showed that the ‘refuge in a bag’ strategy, in which 20 per cent conventional seed is mixed in with the GM seed, encourages the development of partial resistance rather than preventing it. If Bt and conventional maize are grown in adjacent plots, a separation distance of at least two rows needs to be left between them to prevent the larvae, which are very mobile in the soil, from moving between them.
The presence of the larvae led to more serious colonisation by a fungus that produces a mycotoxin (F. verticillioides).

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Research into the activation of Bt proteins in the Western corn rootworm

(2005 – 2008) Federal Biological Research Centre for Agriculture and Forestry (BBA) (since 2008 Julius Kühn Institute (JKI)), Institute for Biological Control, Darmstadt

Bt Protein (in this project Cry3Bb1, which targets the Western corn rootworm) is broken down by certain enzymes (proteases) in the intestinal juice of insects.
The intestinal juice of the Western corn rootworm was found to contain a whole range of different proteases.
In individual tests on the types of proteases found in the intestinal juice, some commercially available proteases broke down the Cry3Bb1 protein. In tests with intestinal juice from the Western corn rootworm, however, Cry3Bb1 and two other Bt toxins (Cry34Ab1 and Cry35Ab1) were not broken down. The probability of protease-related resistance to the Bt proteins studied is therefore judged to be slight.
During these experiments the team characterised the specific binding sites for the Bt toxins in the intestinal wall.

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Investigations for the early detection of resistance development in the European corn borer and for understanding the resistance mechanisms

(2001 – 2004) Federal Biological Research Centre for Agriculture and Forestry (BBA), Institute for Biological Control; Darmstadt

European corn borer larvae were collected in Bt maize fields over a period of three years and tested for Bt susceptibility/resistance. No resistant insects were found, even though sensitivity tests were carried out on a number of subsequent generations with the Cry1Ab Bt protein.
Enzymes in the gut were identified and their role in protein activation assessed.

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Ecological impacts of insect-resistant Bt maize on various insects and the European corn borer

(1999 – 2002) RWTH Aachen University, Chair of Biology V, Aachen; Federal Biological Research Centre for Agriculture and Forestry (BBA)(since 2008 Julius Kühn Institute (JKI)), Institute for Biological Control, Darmstadt

The choice of maize type – Bt maize or a conventional isogenic reference variety – had no recognisable effect on the incidence of arthropods, aphids and their antagonists.
Caterpillars: Pollen from the transgenic maize line Bt176 had a negative effect on the caterpillars of the diamond-back moth and cabbage white butterfly. A species of surface caterpillar (caterpillars that pupate in the soil) was not noticeably affected.
Developing resistance. None of the larvae collected in the Bt maize fields were resistant to the Bt toxin.

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