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  • European corn production at risk from the “billion dollar beetle”

Thursday, July 17, 2008

European corn production at risk from the “billion dollar beetle”

Spread of the Western corn rootworm has significant consequences for agriculture

The main flying time for the western corn rootworm beetle, and hence the most important period for monitoring, is July, August and September.
The main flying time for the western corn rootworm beetle, and hence the most important period for monitoring, is July, August and September.

Monheim - The Western corn rootworm is considered one of the world’s most dangerous pests to corn. Though originally native to South America, the insect is now spreading at lightning speed across Europe and could soon be responsible for yield losses of up to 80 percent.

The Western corn rootworm (or Diabrotica virgifera to give it its scientific name) is five to seven millimeters long and, with its black and yellow striped wing covers and long antennae, looks harmless and even almost elegant. Its name, however, is an initial indication that this beetle is far from harmless. From early to mid June, the young larvae of the Western corn rootworm infest the hair roots of young corn seedlings. Later development stages bore right to the center of the young corn roots. The consequences for the sensitive plant are clearly apparent; its long-term supply of water and nutrients is permanently disrupted. The affected plant remains stunted and its reduced root mass gives it little resistance to lodging.

In single-crop systems, the yield loss is between ten and, in particularly severely affected regions, up to 80 percent
The larvae, shown here next to a corn kernel to illustrate their size, tunnel into the roots and destroy them.
The larvae, shown here next to a corn kernel to illustrate their size, tunnel into the roots and destroy them.
It is not just the larva, however, that poses a threat to corn, as the emerging beetle also feeds predominantly on the plant. It targets mainly the soft silks of the female flower as well as the milk-ripe cobs and the plants’ leaves. In single-crop systems, the yield loss is between ten and, in particularly severely affected regions, up to 80 percent.

As a result of the beetle’s affinity for the corn plant, the spread of the pest has considerable consequences for agriculture, as very recent history shows. Though originally native to Central America, the Western corn rootworm spread to North America as a result of agricultural globalization in the early 20th century. Nearly 90 years later, the corn rootworm is continuing its global spread in mainland Europe. The pest was presumably introduced from America into the former Yugoslavia in the early 90s with consignments of food for U.S. troops.

The economic toll in the USA is around US$ one billion each year

Since then, the corn rootworm has gradually spread throughout Europe. Permanent populations of the corn pest have become established in Austria, Switzerland and France and the pest has already emerged in the southern states of Germany. The Ortenau region of Baden-Württemberg is particularly badly affected. Experts are sure that the population will spread continuously. The Western corn rootworm can increase its radius by 40 to 80 kilometers per year through flight or as a result of being carried on the wind.

The corn rootworm therefore has the status of a quarantine pest and developments are being monitored by the European Commission’s Standing Committee on Plant Protection. Few figures are needed to show the risk potential. By 1997, the corn rootworm had already infested around 20 million hectares of corn worldwide. The economic toll in the USA is around US$ one billion each year. The crucial question from the point of view of agriculture and downstream production sectors is how can losses of these dimensions be prevented or at least limited in Germany and Europe?

Most long-lasting control with crop protection agents

Corn seed protected by dressing. A protective zone of dressing is formed around each seed in the soil.
Corn seed protected by dressing. A protective zone of dressing is formed around each seed in the soil.
In Europe, the corn rootworm has no direct natural enemies, with the exception of the weather. The adult beetles die as soon as the first frosts occur and eggs only survive provided that the temperature does not fall below minus 10 degrees Celsius. Temperatures this low are already no longer being recorded in many regions of Germany. A reduction in spread in terms of lasting control therefore cannot be guaranteed.

One theoretical alternative means of control is crop rotation: by suspending corn cultivation, the pest’s main food source is interrupted and its spread is slowed. Experiences in the United States where the corn rootworm has been prevalent since the 1960s have shown that this approach cannot guarantee long-term success, however. The pathogen has developed numerous resistances to this measure, including one-year egg dormancy. This means that the larvae halt their development for a year without food, in other words when another plant is being grown as a result of crop rotation. In the following year, when corn is being grown again, the larvae hatch again as before.

Work carried out in the United States as long ago as the 1960s and early 1970s identified almost 20 potential host plants for the larvae of the corn rootworm. The first field trials to investigate the host spectrum in Europe were carried out in Romania in 2004. These trials were continued in 2005 and 2006. A recent publication by the German Society for General and Applied Entomology (DGaaE) showed that five different weeds have been identified as possible host plants for the larvae of this pest. The results demonstrate that efforts to eradicate the corn rootworm must also focus on the weeds that grow in affected fields. It is precisely the types of weed that are found particularly commonly in corn fields that are used as an alternative source of food by the corn rootworm. Even though the pests found in fields containing crops other than corn are generally smaller and lighter than their corn-fed brethren, it must be assumed that they are fully capable of reproduction and will attack corn fields again the following year.

Another scientific study published recently by the DGaaE also showed that, in the longer term, the larvae of the corn rootworm will adapt to new host plants in Europe, which places a question-mark over the inclusion of crop rotation in integrated eradication programs. Since a tendency for other cereals to act as host plants has been demonstrated in the laboratory, the larvae could even destroy other crops. Studies carried out in Slovenia show that the pest is also happy in oil squash, and the possibility must be considered that it will spread in this crop, which is important in southern Europe, in the medium to long term.

It must also be kept in mind that the corn rootworm is the tourist of the insect pests. It generally spreads along transit and transport routes such as airports, rivers and motorways. Regular reinfestion from the main areas of infestion in southern Europe each year is therefore not just possible but certain.

Longest-lasting control is achieved with crop protection agents. Effective insecticides can either be sprayed onto the leaves of the corn plant or delivered with spot precision and highly effectively into the soil through seed dressing. The solution can therefore only lie in balanced control. Use of dressed seeds is probably the most ecofriendly way of using crop protection agents.


The western corn rootworm

The western corn rootworm (Diabrotica virgifera) originates from Central America, the botanical home of the corn plant. It migrated to the United States several decades ago, where it causes damage worth a billion dollars every year. The rootworm beetles can increase their radius by between 40 and 80 kilometers every year either actively or by being carried on the wind.

Heavy infestation with the western corn rootworm can destroy up to 80 percent of the corn harvest.
Heavy infestation with the western corn rootworm can destroy up to 80 percent of the corn harvest.
The pest
The rootworm beetle is about 5 – 7 mm long and has a dark head, a yellow plate on the back of its thorax and yellow and black-striped wing covers. The thin, elongated larvae have three pairs of legs, a whitish-brown head and a brown plate of chitin at the end of their abdomen; they are about 10 – 18 mm long. The first larvae generally appear in May and June. The larvae develop through three stages over a period of 3 – 4 weeks. Pupation lasts only a few days. After she has mated in the summer, the female lays up to a thousand eggs in the soil around the roots of the corn plant. The adult rootworm beetles only survive until the first frost. The larvae hatch the following spring as long as the temperature does not go below minus 10 degrees Celsius.

Damage
The rootworms eat mainly pollen, corn silk and sometimes milk-bearing kernels. Their feeding activity impairs fertilization and thus the development of the kernels. They start feeding on the leaves (producing a window-pane pattern) as soon as the corn buds appear.

The larvae cause far more damage than the adult rootworm beetles. The young larvae first eat the hair roots; later instars eat the larger roots. They tunnel into the roots and eat all the root tissue, thus impairing water uptake and the transport of nutrients to the parts of the plant above ground. This predisposes the plants to secondary infections and causes the stems to lean severely to one side, a phenomenon known as lodging.
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[ last update: Monday, August 4, 2008 ]