Nov 16, 2011
Ecological corn borer control
Ploughing, chemicals or Bt maize?
The European corn borer (Ostrinia nubilalis) is a serious problem for maize farming in Germany. Maize is now grown on 2.5 million hectares and is still on the increase. A severe corn borer infestation can lead to harvest losses of up to 50 per cent. It is difficult to combat the pest using crop rotation or chemical and biological methods, and the corn borer continues to spread to other parts of Germany every year. The losses it causes each year are now valued at between 11 and 12 million euros. What alternative control methods can be used to halt the corn borer and how environmentally friendly are they? GMO Safety spoke to Bernd Hommel from the Julius Kühn Institute in Kleinmachnow.
Bernd Hommel from the Institute for Strategies and Technology Assessment at the Julius Kühn Institute in Kleinmachnow. He is Coordinator of the National Action Plan on the Sustainable Use of Pesticides and is responsible for assessing GM plants to see whether they meet the objectives of integrated plant protection.
Soil erosion on a maize field. An average of 8 to 9 tonnes of soil are lost per hectare in Germany every year. If this trend continues, the harvest losses on the most affected areas could be as high as 15 per cent. Photo: Hessisches Landesamt für Umwelt und Geologie (HLUG)
GMO Safety: Bt maize is regarded as an effective method for controlling the European corn borer, yet critics of genetic engineering say it is completely unnecessary. They claim that by ploughing in the maize stubble after harvesting, the corn borer larvae are effectively killed, thereby solving the corn borer problem.
Bernd Hommel: For year now, the clean ploughing-in of maize stubble has indeed been one of the most important and most effective preventative measures for keeping European corn borer populations low. But it only works if all the farmers in an infested region do the same. It only takes one to step out of line and he can produce enough corn borers for all his neighbours. Fields are increasingly being left unploughed across Germany, and some German states are even promoting this trend. Maize is now grown on over two million hectares. New varieties and climate change are making it possible to grow grain maize in the north. This means more European corn borer problems.
GMO Safety: Why are some farmers not ploughing their fields?
Bernd Hommel: Conservation or non-inversion tillage is a necessary measure for conserving soil fertility by reducing soil erosion and for conserving resources generally, like water and fuel. Today a quarter of fields are affected. Soil erosion on ploughed maize fields can be particularly high. The maize is sown in the spring and develops relatively slowly, so the soil is left largely uncovered during the spring and early summer. This means that the soil can easily be carried away by wind and rain.
GMO Safety: What are the consequences of soil erosion?
Bernd Hommel: The loss of the nutrient- and humus-rich top layer has a lasting effect on soil fertility, and reduces agricultural yields over the long term. In addition, nutrients and pesticides end up in nearby rivers and lakes, with negative consequences for drinking water quality and the biodiversity of these habitats.
GMO Safety: According to this argument, conservation tillage is important for sustainable agriculture and for protecting biodiversity?
Bernd Hommel: Absolutely, and there are other good reasons for this type of soil cultivation. The ploughing-in of plant residues leads to an increase in microbial breakdown processes in the soil, which in turn leads to humus breakdown, which releases more carbon dioxide into the atmosphere. The decline in the level of organic substance in the soil also has a negative impact on the soil structure and its water storage capacity. Ploughless or non-inversion tillage maintains these processes and the soil remains covered by plants and plant residues (a mulch layer). This provides a much better habitat for animals and adds to species diversity. Soil organisms like earthworms, soil mites and insects also benefit. They create cavities in the soil, which improves the soil structure and water absorption. Another side-effect of conservation tillage is that farmers use much less fuel, because ploughing is energy-intensive.
The negative impacts are problems with pests like mice and snakes and, frequently, more intense applications of plant protection products. The weed-removal effect of ploughing has to be compensated for by the use of herbicides. Plant residues on the soil promote the development of diseases. And we have already mentioned the corn borer problem. We need to weigh up competing sustainability objectives: conserving soil fertility versus reducing the use of plant protection products.
GMO Safety: Are there other methods for controlling the European corn borer apart from Bt maize and ploughing?
Bernd Hommel: Yes, but in my view none that can effectively halt the further spread of the corn borer. There are chemical insecticides that can be used, but that is not easy. The corn borer larvae have to be controlled in June or July, by which time the maize plants are already very tall. Farmers have to use special high-clearance vehicles on the maize fields so as not to damage the maize. In addition, such measures are effective only if the timing is right. If insecticides are applied too late, the larvae will have already bored their way into the maize, and treatment with chemical insecticides will be ineffective.
Biological control methods are also used. Parasitic wasps (Trichogramma) are released into the maize field two or three times either by hand or by machine. These insects lay their eggs inside the corn borer eggs, which the wasp larvae then eat from the inside. This method is effective, but is generally used only on smaller farms because it is time-consuming and expensive. It is also often only found in places where the farmers receive financial support for it – up to EUR 50 per hectare – and where crop protection agencies specify the first application date.
It is also important to note that the European corn borer cannot be controlled through crop rotation. In this respect it is very different from the Western corn rootworm, an equally important maize pest.
GMO Safety: What is your conclusion?
Bernd Hommel: We need to get our priorities right. Preventative ploughing to control the European corn borer is, in my view, outdated for the reasons I have mentioned. Problems such as soil erosion will increase as a result of climate change and the higher rainfall levels it brings. We must protect soil fertility over the long term.
GMO Safety: But surely we do not want to see an expansion of chemical insecticides.
Bernd Hommel: Plant protection is not an end in itself. It secures yields and the quality of the harvested produce and plays a vital role in efficient use of resources. Plant protection today consists of a wide range of preventative and direct methods. The use of chemical products is always a last resort. Research has been going on for many years to reduce our dependence on chemical plant protection products. In the 1990s large programmes were run by the German Research Foundation and the German national and regional governments to promote the use of biological plant protection. However, the results are sobering. We have the parasitic wasp method for maize and have made considerable progress in biological pest control for greenhouse cultivation, but there are still too few practicable non-chemical methods available for arable farming and horticulture.
GMO Safety: Integrated plant protection is due to be made law in the European Union in 2014. An important objective is to reduce chemical plant protection risks for human health and the environment, including the loss of biodiversity. What is the planned approach?
Bernd Hommel: In Germany we started a national action plan back in 2004 to further reduce the risks associated with plant protection. As well as developing more effective biological plant protection methods, one important measure is the development and cultivation of resistant varieties. The breeding and cultivation of e.g. pest-resistant maize is what can help us in this situation.
GMO Safety: What do you think is the possible role of genetically modified Bt maize?
Bernd Hommel: Varietal resistance, like crop rotation and non-chemical control methods, is at the centre of integrated and ecological crop protection strategies. Increasingly, genetic engineering methods are being used in plant breeding worldwide alongside conventional methods. GM varieties are being authorised on the basis of high safety standards and have been cultivated on several million hectares for years. There is no conventional breeding approach for making maize resistant to the European corn borer. The results of biological safety research have shown that Bt maize does not have any negative environmental effects, particularly when compared with chemical methods. We cannot emphasize the importance of resistance breeding in one breath and rule out an important resistance breeding method in the next.
GMO Safety: So do you see Bt maize as an important part of integrated crop protection in maize farming?
Bernd Hommel: Yes, a resistant variety should always be given preference over the chemical alternative. We have relatively few active substances available for direct chemical control of the European corn borer, and the approval of new pesticides is becoming increasingly difficult. If we keep treating maize with the same products, we increase the risk that resistant pests will develop quickly. Eventually, farmers will not have any effective methods for controlling them. Resistant varieties and non-chemical alternatives also play in important role in effective resistance avoidance strategies, ensuring the continued availability of effective chemical products.
GMO Safety: If that happened, farmers would have to start ploughing their fields again to control the European corn borer.
Bernd Hommel: Yes, and don’t forget that nearly all the farmers in a region would have to be persuaded to plough their fields. Otherwise there is still a risk of harvest and quality losses. You see, even without genetic engineering, coexistence has a role to play in agriculture. The best strategy is to use different plant protection methods alternately. For instance, cultivating pest-resistant Bt maize one year, and growing conventional maize with direct control methods the following year. This means that resistant varieties will alternate with biological or chemical control methods and it will be easier to prevent the pests developing resistance both to Bt maize and to the chemical products. And there will be no need for ploughing.
GMO Safety: Mr Hommel, thank you for talking to us.
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European corn borer infestation
- European corn borer larva in the stem of a maize plant.
- Extreme European corn borer infestation
- Maize plant snapped after being chewed by larvae
- Maize plant snapped in half
- Signs of feeding damage by a corn borer larva in the stem of a maize plant
- Signs of corn borer larva feeding damage in the stem of a maize plant
- European corn borer larva in the stem of a maize plant
- Corn borer larva in the stem of a maize plant
- European corn borer larva in a maize cob. Sometimes the corn borer larvae even eat their way into the cobs.
- Infested ears