Codling moth granulovirus: Variations in the susceptibility of local codling moth populations

Summary

This study is part of a BMELV (German Federal Ministry for, Food, Agriculture and Consumer Protection) project for prevention of codling moth damage by long-term population control in large areas. Specimens from local codling moth populations were collected in fall 2003 from three different orchards in the South of Germany; two of them having been treated with granulovirus of codling moth (CpGV Madex 2 and/or Gran-upom) over many years (Lake Constance II and South Baden) and one since two years (Lake Constance I). In autumn 2004, specimens from populations in four other orchards with serious CM problems were col-lected to investigate whether more populations were involved in that region. Furthermore, the population Lake Constance I and South Baden were tested again. During the season, the location South Baden was almost weekly treated with 100 ml/ha of Madex 2 to test whether the level of susceptibility would change after such heavy treatments.
The susceptibility of the offsprings of the overwintering generation to CpGV was investigated in the spring of the following year in bioassays on artificial diet and compared to a laboratory strain of the codling moth. The results indicated significant differences in sensitivity to the virus between the local codling moth populations. The LC50-values showed that in 2004, South Baden and Lake Constance II were more than thousand fold less susceptible than the populations Lake Constance I and the laboratory strain. The results from the bio-assays in 2005 confirmed the low susceptibility of South Baden and of the new locations in Saarland and from an orchard 100 km away from South Baden. The population Lake Constance I, on the other hand, maintained its high sensitivity to the virus. The slope of the dose-mortality-regression lines of the unsuscep-tible populations was significantly lower than those of the susceptible ones, including that of the laboratory strain. This indicates a high inhomogeneity in the individual response of the unsusceptible larvae against the virus. Actually, the problem of reduced sensitivity to the virus seems to be limited to a few orchards in Germany, the majority of orchards being not affected.