Pest conquers Europe

The pest and the problem

The Western corn rootworm (diabrotica virgifera) is a small beetle, no more than five to eight millimetres long. It produces only one generation per year. After mating, the females lay up to 1000 eggs in the soil around the maize roots in high to late summer. If the temperature does not fall below minus ten degrees, the larvae hatch out in spring and feed on the maize roots, causing the majority of the damage. The first two larval stages graze on the fine roots; the third larval stage penetrates the main roots and stem, making the plant unstable. In severe cases up to eighty percent of plants collapse. Secondary infections in the roots then cause further problems. Adult beetles emerge in summer from July to September and survive until the onset of frost. They feed on the plant plants above ground, especially the pollen and silks. During heavy infestation, grazing damage to the silks can impair fertilisation resulting in fewer kernels on the cob.

The Western corn rootworm causes immense damage. Yield losses and spraying costs in the USA are estimated to be in the region of one billion dollars, which has given the beetle the nickname "the billion-dollar bug". The projected costs in Europe are half a billion euros.

The jet-setting beetle

The Western corn rootworm is native to Central America, but in recent decades it has made a new home for itself in North America. The vast maize monocultures provide it with the perfect habitat and limitless potential to spread. Corn rootworm populations have literally exploded as a result.

Thanks to modern methods of transport, the pest has managed to secure a foothold in Europe in just ten years. The beetle first appeared in 1992 near Belgrade airport and then spread to other parts of Eastern Europe in subsequent years. The beetle's natural range covers distances of up to 100 kilometres in areas of intensive maize cultivation.

Italy was the beetle's first port of call in Western Europe. In 1998 it was sighted near Venice airport and then near a loading station in Lombardy in 2000. In 2002 it reached Paris, and in 2003 Belgium, the Netherlands and Great Britain. In autumn 2003 the beetle was discovered in Basel-Mulhouse in Alsace, triggering a "beetle alert" for the first time in Germany.

German authorities had been setting traps since 1997 in anticipation of the beetle’s arrival, so as to ensure its prompt detection and at least slow down its spread by means of appropriate containment measures. Visible damage can be expected just five years after its first appearance, and areas of intensive maize-on-maize cultivation in particular can expect to see an enormous proliferation of the beetle. In Germany this amounts to around thirty percent of maize growing areas.

The strategy: crop rotation

The western and northern corn rootworms have been causing problems in the USA for decades. Up until recently, however, Diabrotica damage was restricted to maize monocultures. The larvae hatch out in spring and rely on maize roots to continue their development. If a different crop is grown, they can find nothing to eat and so starve. For 30 years farmers in the USA have rotated maize with soya as a successful control strategy. Until the mid-nineties, that is, when a "rotation tolerant" variant of Diabrotica came on the scene. Over time crop rotation has exerted such a strong selection pressure on the beetle that a new biotype has managed to evolve. The females now lay their eggs in soya fields as well and when the larvae hatch out the following year, the maize is waiting for them.