Control of the European corn borer - What possibilities are there and what is to be paid attention to?

Larva of the European corn borer

Larva of the European corn borer

Measures of sanitation and control regarding the most important pest in maize, the corn borer, established throughout Bavarian maize growing areas since 15 years. The European corn borer is the most important pest in maize and causes enormous problems to more and more farmers, especially in Southern Germany. In case of a massive infestation, substantial losses in yield and quality do occur. The most promising precaution is the deep ploughing of maize stubbles and maize straw. But success can be expected only, if these sanitary measures are carried out consequently by all farmers in the infested area over years. At locations that do not permit a clean, deep ploughing without residual stubbles, a direct control with insecticides or beneficial organisms is inevitable up to now. Presumably genetically modified maize cultivars will also be available in the future to reduce damage.

Development of the European corn borer infestation in Bavaria

Figure 1:Development of the European corn borer infestation in BavariaZoombild vorhanden

Figure 1:Development of the European corn borer infestation in Bavaria

The occurrence of the European corn borer (Ostrinia nubilalis) was initially limited to warmer regions (Mediterranean countries, Rhine rift). In the last decades, the insect has spread out more and more to regions with less favourable climatic conditions. By the end of the seventies the European corn borer occurred in Bavaria for the first time. Since then it has widely spread and since 2005 it can be found in almost every maize plot in Bavaria (Figure 1). According to estimations of the World Food Organization (FAO), four percent of the maize harvest are destroyed worldwide by the borer's caterpillars annually. This corresponds to the food requirements of 60 million people.

Multiple damage

Maize tassels folded over or broken off are distinct signs for the larvae's feeding activity. On closer examination, cavities show at the breaking points, with material produced by boring into the stem and excrements. Both are found in the leaf axils situated beneath. The bored tunnels inside the stem may reach from the tassel inside the top of the stalk to the stem base. Often bore holes and feeding tunnels can also be found in the ear. Bored tunnels and holes along the stem being produced by the repeated boring in and out impair the mechanical stability of the maize plant. As long as only the upper parts of the stem bend, the damage can be neglected. But especially in years with frequent thunderstorms plants break off in the lower section. Under such conditions, a great part of the ears can not be seized, not even with row-independent harvesting methods. Besides, the larvae's maturation feeding hinders water and nutrients supply as well as transport of assimilates. With two to three caterpillars per plant yield reduction of 10 to 30 percent can be expected in the long-term average. In addition, the larvae promote fungal infections by their boring activity. Primarily important in this context are Fusarium spp. They cause stalk and ear rot, and under certain conditions they produce mycotoxins such as deoxynivalenol or nivalenol, which reduce forage quality considerably.

Development of the corn borer

From June on, fist moths fly from last-year’s maize fields into the new crops. In most cases, migration reaches its peak in July, but continues well into August. Female moths fly in the evening and night hours and deposit their whitish eggs in clusters on the underside of maize leaves. Larger leaves in the middle of the plant are preferred for egg deposition. The average clutch size is about 10 to 20 eggs. One to two weeks later, yellowish-coloured little caterpillars eclose. Eggs as well as young larvae are very sensitive to humid and cool weather. Then the borer larvae spread to neighbouring plants and bore into the upper part of the maize plant. In the course of the growing season, they feed downwards inside the stalk, partially interrupted by boring in and out anew. The older the larvae are, the further they advance downwards and the greater is the damage caused. That is why control measures must be set in a way that the first batches of eggs or larvae - depending on the control method - are seized definitely. Hibernation takes place in maize stubbles. The pest pupates the following May.

Best control measure: deep ploughing under of maize stubbles

In silage maize production chopping off maize plants very low can reduce a population of corn borer for the following year. Crushing of stubbles and straw by rotary tiller or disc harrow is also suitable for killing caterpillars. But experience has shown, that these measures on their own are not sufficient to keep the European corn borer population below the damage threshold. Only a deep (25 cm), clean ploughing-under of maize stubbles and straw is a control method promising success in the long run. Thus, hatching moths are hindered in leaving the soil. An additional attachment for the plough, to put the straw well into the furrow, is necessary. If the deep turning soil cultivation is practised in the entire infestation area, the European corn borer has hardly any possibilities for further development. In most cases further controlling measures are not necessary. The ploughing-under of maize residues reduces the Fusarium toxin risk for the following crop wheat at the same time.

Chemical and biological control methods and genetic engineering