Development of resistance to Bt maize among Western corn rootworm

Larval mobility: The mobility of the larvae was tested in two different soil types (soil and sand mixture). This involved planting a maize plant at one end of a long, narrow trough (1m x 10 cm x 10 cm). Five days later larvae were released at the other end. After two, four and eight hours, the larvae were extracted from soil samples at intervals of 10 cm. In this way it was possible to measure the distance covered by individual larvae.

Bioassay: In a bioassay, various plants (grass weeds frequently found in and around maize fields and various cereals) were tested for their suitability as alternative host plants for the Western corn rootworm. Roots of these plants were fed to the larvae and then their weight gain and grazing capacity were measured.

Effect of different age classes of Bt maize plants: In order to investigate the effect of roots of different ages on L2 larvae, 120 Western corn rootworm eggs ready to hatch were placed on two and a half, four, six and eight-week-old Bt maize and isogenic plants. After twenty days, the larvae were extracted from the root balls. The larval stage, head capsule width and dry weight were recorded.

The experiment was repeated ten times for each plant age class.

Container experiments: In container experiments (micro habitats) larvae from the Western corn rootworm are offered different combinations of maize plants (transgenic, isogenic) and potential host plants deemed suitable from the bioassay at different row and plant intervals. The development of the larvae is assessed.

In a prior experiment, between 90% and 100% of all larvae were found in the top 20 cm of soil. The soil samples are therefore taken in the form of soil cores (10 x 10 cm, depth 20 cm).

Effect of different age classes of Bt maize plants: The largest numbers of larvae were found on the older plants. As expected, many more larvae were found on the plants of the isogenic line than on the Bt maize plants. Some surviving larvae were found on the Bt maize. Most of these were in the second larval stage on eight-week-old plants. By contrast, on eight-week-old plants of the isogenic line, most of the larvae were already at the third larval stage.

Container experiments (preliminary results): In the container experiments there were significant differences between the variants. Initially, the alternative host plants were placed in the middle between two rows of maize. Significantly more larvae were extracted from the containers with alternative host plants than from the containers with only Bt maize. The largest numbers of larvae were found in a mixed planting with rye, followed by foxtail millet and winter wheat. However, most larvae were not found on the roots of alternative host plants, but on the root balls of the Bt maize plants. This fits in with the hypothesis that freshly hatched L1 larvae feed on alternative host plants to start with and then switch to maize. In these experiments too, a delay in larval development was observed. In the experiments with the isogenic line, the majority of the larvae were already at the L3 stage, while those on the Bt maize were mainly L2 larvae.

Experiments are currently being conducted/evaluated in which the alternative host plants are planted in the maize rows, in order to simulate a closer proximity between the roots for the larvae.