Influence of cropping practices on corn rootworm in Canada as a basis for assessment of the potential impact of Diabrotica virgifera in Germany

Monitoring of populations of adult corn rootworms ( Diabrotica virgifera and D. barberi ) in Ontario (CA), and surveys of insecticide use against rootworms, showed that populations declined in the last 10 years and that the proportion of the maize area treated with rootworm insecticide has fallen to <7%. This proportion was directly correlated with the percentage of cropping with continuous maize. The area of insecticide‐treated maize could be further reduced by educating livestock producers about the merits of crop rotation and the possibilities of alternative high‐energy crops, such as winter barley. Populations of rootworm adults and area of continuous maize are currently so low in Ontario that there is a good probability of success for an adult monitoring and insurance programme to replace the use of corn rootworm insecticides. In Germany (where D. virgifera is now absent but may potentially be introduced from the outbreak in the lower Danube basin), the four Federal Länder of Niedersachsen, Nordrhein‐Westfalen, Bayern and Baden‐Württemberg maintain a high frequency of continuous maize. If areas with more than 50% of arable land with maize are considered to be at high risk of rootworm infestation, about 348 000 ha of maize fall into this category. This is a third of the maize production area of the above‐mentioned Federal Länder and more than a fifth of the entire maize production area of Germany in 1995. On the basis of long‐term experience of insecticide use in Ontario, there is a potential for treatment of up to 111 000 ha with rootworm insecticide (terbufos) annually in Germany, at an added insecticide cost of 13 million EUR per annum. Without crop rotation or treatment, rootworm infestations could cost German maize producers 25 million EUR in crop losses each year. The experience in Ontario suggests that Germany should follow a strategy of changing crop rotation away from continuous maize, so as to reduce the potential for establishment of D. virgifera , the potential for economic loss and the potential for increased reliance on insecticides.