Aug 21, 2009

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Urgent need for effective pest control methods

Western corn rootworm causes massive harvest losses in Northern Italy

In Lombardy, Italy’s main maize-growing region, the Western corn rootworm has already destroyed 30 per cent of this year’s harvest. The pest is now spreading through other European countries as well, including Germany. Maize farmers need appropriate control strategies soon. For the first time, there appears to be a possibility of controlling the Western corn rootworm using a biological method, with the help of a natural predator. The development of this biological control method could be speeded up with the help of genetic engineering methods.

The Western corn rootworm is the target of the highest number of insecticide applications around the world. In the USA the cost of the damage it causes and the cost of controlling it add up to around a billion US dollars each year.

Most of the damage is caused by the beetle larvae, which eat the plant during the various larval stages - first the fine roots, then the primary root and then the stem. A severe infestation can lead to up to 80 per cent of the plant stems snapping in half.

Since the 1990s the Western corn rootworm has been spreading across Europe: first through Eastern Europe and then, since 1998, across Western Europe as well.

Roundworms or nematodes are one of the phyla with the largest number of species in the animal kingdom. Over 20,000 species have already been recorded. Some of them feed on the larvae of the Western corn rootworm.
Photo: Sebastian Höss

The Western corn rootworm (Diabrotica virgifera virgifera), is a beetle and is one of the primary maize pests. It originated in Central America, but has been established in North America with its vast areas of maize monoculture for several decades. It reached Europe in the 1990s thanks to modern means of transport, spreading first through Eastern Europe. Italy was the first country of Western Europe in which the pest was found: near Venice airport in 1998, and near a goods station in Lombardy in 2000. More beetles were found in France, the UK, Belgium, the Netherlands and Switzerland and, in July 2007, in Germany. Wherever the beetle was found, people tried to eradicate it by setting up quarantine zones, using insecticides and introducing crop rotation. In October 2003 the EU Commission passed a standard list of minimum protection measures.

The eradication measures have met with different levels of success in the different countries affected. Whereas it was possible to prevent Diabrotica from spreading in Belgium and the Netherlands, the infestation in Lombardy was not spotted until it was relatively far advanced, so the measures introduced there were too late. Nine years after the first beetle was found, Lombardy is reporting harvest losses of around one million tonnes, which represents around 30 per cent of this year’s production in the Po Valley, according to the farmers’ associations. Representatives of the agricultural association Confoagricultura are therefore calling for Lombardy to be officially designated a disaster area entitled to compensation from the state.

In Germany the beetle has so far been found only in Bavaria and Baden-Württemberg. However, it is spreading and it is probably no longer possible to eradicate it. Experts are assuming that the natural spread of the Western corn rootworm can no longer be prevented in Europe - only delayed. This means that maize farmers will soon need appropriate control strategies.

Introducing crop rotation, instead of growing maize every year, can, however, only lower the pressure from the pest and reduce harvest losses. Some of the beetle eggs can survive for up to two years in the soil, and some are laid on other plants.

For some years now, farmers in the USA have been growing genetically modified maize varieties that produce a Bt protein, especially in their roots, which targets the Western corn rootworm. In Europe, authorisation for Bt maize with Diabrotica resistance has been applied for, but as yet there is no sign of it being approved. In Germany, a research project funded by the Federal Ministry of Education and Research from 2005 to 2008 investigated whether cultivation of this Bt maize variant could have harmful effects on the environment and on biodiversity.

As long as such genetically modified varieties are not authorised for cultivation in Europe, farmers will continue to use insecticides - either applying them to the soil or using them to treat the seeds. However, there were some problems in 2008 with the treatment of maize seed: The pesticide did not adhere sufficiently to the maize grains and landed on the flowers, leading to widespread bee deaths in the Upper Rhine area. Since then the approval of treatments for maize seed has been suspended.

One possible new method for controlling the Western corn rootworm is the use of a natural predator, a nematode that goes after the beetle larvae. The research in this area is still at a very early stage. Scientists have discovered that some maize varieties produce an active substance that attracts these nematodes. This trait has been lost in the high-yielding varieties grown today, as a result of breeding. The gene for this active substance can be reintroduced into the genome of the maize varieties grown today using conventional breeding methods or genetic engineering methods. The genetic methods would be faster. A group of international researchers has now tested one such genetically modified maize variety – with extremely positive results.