An ingenious pest

European corn borer – the name indicates the pest’s origins: the corn borer (Ostrinia nubilalis) originated in Europe and was introduced to the US around 1920.

In Germany the European corn borer is found mainly in the maize-growing regions of Southern Germany, but there are also some pockets of infestation in Oderbruch and south of Berlin. Since the European corn borer has few natural enemies, and living conditions in the maize fields are ideal for it, the little butterfly has been spreading further and further north since about 1930. In Mecklenburg-Western Pomerania it has now reached the Baltic coast and in 2006 infestations were identified in Lower Saxony for the first time. In this region the pest has already penetrated as far as Lüchow-Dannenberg. In North Rhine-Westphalia the boundary of infestation runs south of Munster and in the west reaches the district of Cleves near Emmerich.

Only Schleswig Holstein remains unaffected by the pest.

Since 2006 a new breed of the European corn borer that produces two generations a year has been spreading in Baden-Württemberg, southwest of Freiburg. The first generation of butterflies penetrates the fields already at the end of May, the second wave arrives between the middle of July and the middle of September. Through this second generation the maize cobs in particular are damaged by caterpillars. In 2009 the infested area was already estimated to be about 8000 hectares.

According to the calculations of the former Federal Agency for Agriculture and Forestry (now the Julius-Kühn Institute) the European corn borer causes a loss of 11–12 million Euro yearly.

Little effect, lots of effort

There are various strategies for combating the corn borer, but they all have the disadvantage of being insufficiently effective whilst requiring a lot of work on the part of the farmer.

• Mechanical control methods: To limit overwintering of the caterpillars in plant remains and roots, it is a good idea to shred the plant remains left on the field and to plough them under. This control method is used a lot, especially by farmers in Southern Germany – in 2002 it was used on an area of around 320,000 hectares (total cultivation area: around 380,000 hectares). By contrast, soil-preserving ploughless tilling methods which are desirable from an ecological point of view (e.g. mulch sowing) encourage the spread of the corn borer.

• Insecticides are effective in controlling the corn borer only within a short timeframe. They have to be sprayed in the 2-3 days that it takes the freshly hatched caterpillars to reach the security of the stems.
  Using insecticides on the flying butterflies before they lay eggs is not very effective.
  The most effective active substances are pyrethroids. However, these affect other insects as well as corn borers. If insecticides are sprayed at the right time, they can achieve an effectiveness of between 60 and 90 per cent.

• Biological methods: Parasitic wasps (Trichogramma ) are natural antagonists of the corn borer. They lay their eggs inside those of the corn borer (parasitize them ), so that wasp larvae grow inside the eggs instead of corn borer larvae.
  Again, for this approach to be successful, the wasp eggs have to be introduced at the right time. When the beneficial wasp larvae hatch from the eggs after two to three days, the corn borer eggs must have been laid on the maize leaves.

• Bt preparations: A classic approach, which is also allowed in organic farming, is the use of preparations with cultures of a certain soil bacterium (Bt, Bacillus thuringiensis ). The active ingredient is a toxic protein (Bt protein ), which is naturally produced by the bacteria. It destroys the intestinal wall of certain chewing insects.
  Again, Bt preparations work only if they catch the corn borers before they reach the stem.

Since there is no effective approach, many farmers do not use any targeted measures against the corn borers, even in infestation areas. They just put up with the yield losses.

After the corn borer comes the mould

The corn borer does not just cause losses for the farmer. Its boreholes provide openings for fungal spores, which find ideal development conditions inside the maize stalk. Mould fungi become established – including some that produce extremely toxic metabolites (mycotoxins ). Feed and food products from corn borer-infested maize plants contain greater levels of mycotoxins than those from corn borer-free crops.

Bt maize: Genetic engineering has opened up a completely new strategy against the European corn borer. It causes the toxin familiar from classic Bt preparations to be produced within the plant itself. This means that the corn borer larvae can be reached where they used to be safe: inside the stem.

Since 2005 various types of Bt Maize MON810 have been approved for cultivation in Germany. However, the approval for MON810, granted EU-wide in 1998, was suspended in Germany in April 2009. The area under cultivation with Bt maize had in fact continually risen between 2005 and 2008, reaching 3171 hectares in 2008. This, however, only represents about 0.15% of the total maize production. Bt maize reached an effectiveness of 99.9%.