Wanted: Resistant European corn borers!

The European corn borers are unable to tolerate the Bt toxin produced in Bt maize and so when they eat the plants, they die. A few insects, however, manage to survive. They could be carriers of a resistance to this substance. If, in the future, Bt maize were to be grown on a large scale, many of the sensitive insect pests would die, but a few insects could be resistant and could then rapidly multiply in the short or long term. This would result in an ever-increasing army of resistant European corn borers, against which the built-in insecticide in the Bt maize would be virtually ineffective. For this reason the BBA project aims to record the occurrence of resistant European corn borers at an early stage, to enable countermeasures to be taken in good time. In the USA, for example, there are special regulations for crop growing, which are designed to counteract the emergence of resistant European corn borer populations.

"The rude awakening"

The search for resistant European corn borers should really be quite straightforward. You go into a Bt maize field and look for live European corn borers. They are still alive because they are resistant to the Bt toxin. But in reality it’s actually quite different. The Bt maize plants in the field do not all contain the same amount of Bt toxin. The Bt toxin content in some plants can be very low, depending on their age and condition. Furthermore, not all plants in a Bt maize field are actually Bt maize. For production reasons, the seed from plant breeders still contains up to two percent of non-Bt maize seed. Therefore a surviving larva may not necessarily be resistant. These are the results from last year: In 2002 144 live larvae collected from Bt maize plants the previous year were examined. None of them were actually Bt resistant.

A lengthy process: Waiting to test the next-but-one generation in the laboratory

The insects are developing particularly well this year”. Project leader Renate Kaiser-Alexnat is delighted with the large collection of European corn borer larvae, which, thanks to the mild weather, have nearly all survived the winter. The older caterpillars collected in the field cannot be tested for resistance now, because they are scarcely taking in any more food. It is their offspring that will be tested in the laboratory. Almost all of the 600 or so caterpillars collected last year, which have been overwintering at the institute, are metamorphosising into butterflies at the same time, which means a lot of work in a short space of time. When looking for resistant insects, the life of the European corn borers spent in the laboratory and also that of subsequent generations must be carefully steered along well-ordered pathways.

Arranged marriage

However, scientists assume that a Bt resistance in European corn borers is most likely to be passed on by so-called recessive genes. The characteristic inherited from these genes only takes effect when two of these genes are present at the same time in a single organism.

However, since it is statistically highly probable that the very rare resistant butterflies will breed with non-resistant butterflies in the glass jars, the researchers are not anticipating homozygous resistant insects in the next generation. This is because the insects will have at most one resistance gene as a result of cross breeding. Resistant insects carrying two resistance genes can only occur when these offspring in turn breed with one another.

Rare resistance genes

However, scientists assume that a Bt resistance in European corn borers is most likely to be passed on by so-called recessive genes. The characteristic inherited from these genes only takes effect when two of these genes are present at the same time in a single organism.

But this is extremely time-consuming: If 100 pairs from the first generation are investigated for the presence of a resistance gene, 10,000 larvae from the next-but-one generation must undergo the so-called biotest, which is used to detect resistant insects. At least the test can be performed this year, since there are only a few weeks between one European corn borer generation and the next. In August the larvae of the second generation of progeny, which are to be tested, will have already hatched.

The moment of truth: The biotest

Every single one of the 10,000 or so tiny larvae is then placed in a depression in the biotest trays using a small brush and the escape route is blocked with a piece of plastic film. This requires a great deal of patience and the slightest draft can blow the miniscule insects away. If you’re not quick enough, the tiny creatures can abseil from the brush on a gossamer thread and escape. Each depression in the test tray contains a nutritive substance, the surface of which has been treated with the Bt toxin. Only resistant larvae can survive the next few days here and they are counted. As already mentioned, in the two previous years all the insects died; none of them were resistant.

This year, however, a slightly different approach will be taken to increase the chances of finding resistant European corn borers. Preference will be given to testing the insects which were found on maize plants with a particularly high Bt toxin content. Altogether around 600 caterpillars from the previous year are ready to examine, which is almost six times more than last year. Everyone at the institute agrees about one thing: "We certainly won’t be bored".