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The Western Corn Rootworm: On its Way to European Maize Fields

Already a widespread pest on American maize fields, the western corn rootworm has recently been on the move, making its way to cropland throughout Europe. In 2003, the US approved MON 863, a genetically modified maize line that produces a substance making it resistant to the corn rootworm. In January 2006, the EU followed suit by granting authorisation for MON 863 for food and feed. Some experts believe MON 863 maize would be a good solution to the increasing western corn rootworm problem in Europe. The approval process, however, has sparked disagreement between European authorities, environmental groups, and Monsanto, the company responsible for developing the resistant cultivar. Nonetheless, the European Food Safety Authority (EFSA) came to the decision that the genetically modified maize is as safe as conventional maize.

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Rootworm larvae can heavily damage maize roots.


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Damaged roots (left). Roots of a healthy maize plant (right)


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Adult beetle

A dreaded pest

The western corn rootworm (Diabrotica virgifera) is a major problem for American maize growers. Rootworm larvae begin their lives in the soil and emerge to chew the aerial roots of maize stalks. Later in life, the larvae penetrate these damaged roots and gain access to the inner stalk. The root damage caused by larval entry makes maize stalks prone to tipping before harvest, which in severe cases can destroy 80 percent of the crop. In addition, the damaged roots can serve as points of entry for secondary infections. Adult beetles emerge in the summer between July and September and survive until the onset of frost. They feed on above-ground plant parts, especially on pollen and silk. During heavy infestations, damage to maize silk can reduce fertilisation efficiency resulting in fewer and smaller kernels.

Traditionally, the pest has been combated with intensive insecticide applications. The price tag on the western corn rootworm including spraying costs and yield losses comes up to 1 billion US dollars annually, hence the pest’s nickname, the billion dollar bug. If the pest becomes widespread in Europe, costs are projected at half a billion euro.

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Rootworm beetles feeding on a maize cob



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Leaf damage caused by rootworm beetles



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Spread of the western corn rootworm in Eastern Europe


Transatlantic flight

The western corn rootworm is native to Central America, but in recent decades it has made itself a new home in maize growing regions of North America. Sprawling maize monocultures made a perfect environment for the explosive spread of the rootworm across the continent. Although the pest was unknown in Europe until the 1980s, European farmers can no longer expect to be spared.

Thanks to modern mass-transportation, the western corn rootworm has managed to secure a foothold in Europe. Since the early nineties, the pest has been turning up on European soil, primarily in the vicinity of airports. The beetles may be lured into the illuminated cargo holds of airplanes or may just happen to fly into passenger cabins. In any case, the pest seems to have no trouble surviving a transatlantic voyage.

The western corn rootworm was first reported in Europe near Belgrade’s airport in 1992, and it subsequently extended its presence across parts of Eastern Europe. The pest can spread up to 100 kilometres per year in regions of intensive maize cultivation.

In the following years, the rootworm appears to have reached Europe on other transatlantic flights. The beetle was found near Venice’s airport in 1998, near a loading station in Lombardy, Italy in 2000, in Paris in 2002, and in Belgium, the Netherlands, and Great Britain in 2003. In the fall of 2003, beetles were discovered in the Alsace/Basel-Mulhouse region near the borders of France, Switzerland, and Germany.

In response to the appearance of beetles in the Basel-Mulhouse region, authorities implemented containment measures. Approximately 100 hectares of maize were destroyed, and the rest of the maize in the affected region was treated with insecticide.

As early as 1997, German authorities have made pre-emptive efforts to promptly detect the potential appearance of the western corn rootworm and to develop appropriate containment strategies.

Five years after the pest’s first appearance in Europe, losses are already becoming visible. Areas of Europe with intensive maize cultivation can expect to see the presence of the rootworm spread extremely rapidly. Because the beetle over-winters in maize fields, regions where maize is planted on the same field year after year are particularly susceptible. Additionally, the species has evolved to exhibit extended diapause (suspension of development) which allows it to over winter multiple years, causing crop rotation to be ineffective as a control measure.

Is there a way to stop the spread of the western corn rootworm?

In October 2003, the European Commission agreed on a set of measures in order to stop, or at least slow down the spread of the western corn rootworm.

To detect the pest as early as possible, traps were set up and baited with pheromones to attract female beetles. Each year, the findings of the trapping studies are submitted to the European Commission. Should the western corn rootworm be found, it must be reported immediately. A one kilometre radius surrounding the site of incidence is declared a zone of infestation, and a five kilometre radius is declared a security zone.

The primary control measure within these zones is crop rotation; the affected sites may not be planted with maize the following year. In addition, the sites must be treated with soil insecticides. The area of infestation is continually monitored using pheromone traps. When the pest is present, soil and plant parts are not allowed to leave the controlled zone. Only after a certain period of time is harvesting permitted.

No guarantee of success – Genetic engineering as an alternative?

Some experts believe that crop rotation isn’t enough to ward off western corn rootworm infestation in the long term. Reports are now coming in from North America that after thirty years of maize, soybean crop rotations, beetles have adapted by laying eggs and surviving in soybean fields.

European research projects have found that the western corn rootworm can feed on more than just maize. It was reported that beetles fed on 73 percent of the weeds growing in tests fields. Because maize dies off relatively early in the fall, it is thought that the beetles must find alternative food sources. This enables the pest to spread more easily, making containment a more difficult challenge. The larvae can also survive on wild plants, making crop rotation strategies unlikely to suffice in the long term.

The shortcomings of crop rotation as a management strategy are making farmers on both sides of the Atlantic open to newer, more innovate approaches. Stefan Vidal of Göttingen University, a coordinator for a EU project on the spread of the western corn rootworm, is quite optimistic about the potential effectiveness of transgenic Bt maize in Europe. Based on the assumption that the infestation rates in Europe could reach those currently seen in the United States, he views chemical control methods and transgenic maize as the only management strategies that can be effective in regions of intensive maize cultivation.

Western corn rootworm resistant Bt Maize: Already grown in the United States

The genetically modified maize line known as MON 863 (YieldGard Rootworm Corn) received approval in the US in 2003, giving producers a new way to combat the western corn rootworm. The new maize line, developed by the company Monsanto, was planted on two million hectares in 2005.

Similar to the well known Bt concept used to combat the European corn borer, this genetically modified maize line also produces a toxin (Bt toxin). The form of the Bt toxin expressed in MON 863 specifically targets the corn rootworm. The newly introduced gene responsible for producing this toxin comes from the bacterium Bacillus thuringiensis (ssp. Kurmamotensis), an organism commonly used to control pests on organic farms. MON 863 contains less Bt toxin than most Bt maize varieties, producing the toxin primarily in the roots, which is the site of entry for the western corn rootworm.

Approval of MON 863 in Europe

In January 2006, the European Commission approved corn rootworm resistant maize (MON 863) for food use. It also granted authorisation for the import and industrial use of a cross between MON 863 and MON 810, a Bt maize line resistant to the European corn borer.

The approval process has sparked considerable debate over the safety of MON 863. The controversy was set off by the findings of a feeding study conducted on rats. Based on the data provided in the approval application, which included the disputed feeding studies, EFSA declared that MON 863 is as safe as conventional maize for consumption. However, scientists from France’s commission on genetic engineering, CGB, expressed concerns that the MON 863 feeding tests may have revealed potentially pathological changes to internal organs and signs of inflammation. Greenpeace successfully petitioned the courts to mandate the public release of documentation pertaining to the feeding studies. The results of the experiment were reviewed once again by two independent experts, and in the end, both EFSA and the CGB scientists came to the conclusion that the differences observed between rats fed conventional maize and those fed genetically modified maize did not exceed the differences normally observed between any two individuals. In any case, any differences were not biologically relevant. EFSA substantiated its position in a second recommendation, which was then used as the basis for the European Commission’s approval decision.

European approval committees are reviewing additional approval applications for the import and feed use of various hybrids derived from crosses between MON 863 and other maize lines. Monsanto has not yet sought approval for growing MON 863 in Europe. Three more western corn rootworm resistant maize lines joined MON 863 and are currently awaiting approval: line MIR 604, line 59122, and line MON 88017. One of the lines, 59122, is awaiting approval for cultivation in Europe. In years to come, corn rootworm resistant maize could become a pest management option for European farmers.

Strategies for preventing resistance among pests: Successful to-date

Pests have the potential to adapt to virtually every control method, which sometimes makes methods that were at one time effective become completely useless. MON 863 is no exception to this biological dogma. This is why the American Environmental Protection Agency (EPA) developed what is known as resistance management, which is designed to delay the development of resistance and to detect resistant pests as early as possible. Farmers in the US are required to surround Bt maize fields with strips of non-Bt maize, setting up areas called refugia. These refugia allow non-resistant pests to survive. If resistant individuals happen to be present, they should still be able to find susceptible mating partners resulting in susceptible offspring.

It was expected that pests like the European corn borer and the cotton bollworm would quickly develop resistance to Bt crops. However, findings from a 2003 study led by Bruce Tabashnik at the University of Arizona (Tucson, Arizona, USA) and at Cornell University (Ithaca, New York, USA) suggest the opposite. The study could not report a single Bt resistant pest. Even Tabashnik, an entomologist, was surprised by his findings. According to Tabashnik, everyone expected at least a slight increase in resistance.
Studies in China report similar findings. Researchers have been observing cotton bollworm populations over five years of Bt cotton cultivation, and no signs of resistance have yet been observed. These findings conflict with several non-scientific reports of Bt resistant insects.

Some countries in Europe have already begun research on the ecological safety of corn rootworm resistant maize and on early detection methods for resistance among the pests. One of the goals of this research is to develop improved monitoring procedures to implement in parallel to cultivation. In Germany, for instance, researchers are investigating effects on non-target organisms such as beetles, spiders, and aphids. Equipped with these research findings, authorities will better be able to detect potential unexpected negative ecological consequences and to respond to them promptly.

Additional documents on the western corn rootworm and Bt maize are provided on the right-hand column of this page.



An EU Research Project

What are the risks of growing GM crops?

What are the benefits?

Numerous studies have addressed the potential impacts of genetically modified (GM) plants. Yet the existing evidence on the effects of GM plants is often contradictory and the quality of scientific research varies widely.

Therefore, the GRACE project will establish new tools for assessing the quality of existing studies and will conduct comprehensive reviews to identify health, environmental and socio-economic impacts of GM plants.

More information


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