On the trail of the European corn borer

Early October 2002. A wonderful late summer day with blue sky and sunshine. Even from a distance there’s obviously a lot going on by the maize field today. Lots of people are busy writing notes, unpacking and distributing folding rules and knives, and a large set of scales is being assembled. Today the team is recording to what extent the maize has been infested by its main pest, the European corn borer . This is called corn borer scoring .

Since our last visit, the maize has hardly grown. It has just become older and more mature. The cobs are fully formed and can be harvested.

There is a loud rustling and a first group of ten people, each with five maize plants in their arms, comes out of the field, most of them encased in rainproof clothes with wellies and hats. "The field is still wet from the dew," says project leader Achim Gathmann, commenting on the wrapped up figures, and someone adds: "The maize leaves are very hard and can cut. Another reason why a cap is a good idea."

Weighing, measuring, cutting open

A sample of 50 plants is taken from each of the 24 plots on the two trial fields – five plants each from ten different rows.

In pairs, the assistants now start processing the first bundles of five plants. They enter on a form the plot from which the plants were taken, whether they are Bt maize plants (BT), from the isogenic parent variety (ISO) or the parent variety sprayed with insecticide (INS).

Each of the 1200 plants is measured and weighed and those who are proficient take a knife to cut open the stalks and look for corn borer larvae.

Data from each plant is recorded in detail. The weight, height and number of cobs are noted. The biggest plants weigh around 1000 grams and are often over two metres tall. If a stem has snapped as a result of feeding damage by the corn borer, it is measured only as far as the break.

The team notes whether the plants are infested with corn borers and if so, how many caterpillars were found in the plant. The corn borer larvae have already eaten their way right down the stem towards the ground and if the maize is then harvested they will overwinter in the stubble.

On their feeding journey down the stem, the larvae often bore their way out of the stem above a knot, bypass it on the outside and make a new borehole below the knot. It is usually possible to tell from these boreholes that a plant is infested before cutting it open to trace the feeding tunnel. Any caterpillars found are collected in a jar for later laboratory experiments.

The plants are also inspected to see whether there is any fungal infection inside the plant. This often occurs as a secondary infection, particularly if the plant has already been weakened by the corn borer, with its boreholes offering suitable attack sites.

Only moderate corn borer infestation

Corn borer infestation is not as high this year as last year, although the soil was not ploughed under, thereby providing good opportunities for the corn borer larvae to overwinter in the chaff. "Maybe the weather was too bad when the female butterflies flew off to lay their eggs on the underside of the maize leaves", Achim Gathmann conjectures.

The untreated isogenic parent variety is clearly the most severely affected and as well as corn borer infestation, also has the highest level of fungal infection.

The Bt maize has remained largely untouched by the corn borer. Last year too, only isolated plants were affected and investigations found that the Bt gene was not active in these plants, so that no Bt toxin was present.