Field testing of new biocontrol strategies to decrease the population density of Melolontha hippocastani, an important scarab species in Germany

The current outbreak of the forest cockchafer, Melolontha hippocastani, starting in the eighties in Germany, particularly in the Southern Federal States of Hessen, Rheinland-Pfalz and Baden-Württemberg, has reached an enormous extent (a total of approx. 7.500 ha are damaged). Therefore, different application methods of Beauveria brongniartii were tested in the laboratory and in the field during the main flight period in 2002. First, barley kernel products were used to protect young seedlings. In one trial, the fungus kernels were applied into the planting hole (20g/plant) whereas in another trial, the kernels were distributed in rows (dosage of 50 kg and 100 kg ha-1), mechanically pre-treated by a rotary tiller. Pine trees have been planted into these rows in spring 2003. Data analysis of these trials still is in progress. Second, an experimental product was tested in different formulations in the laboratory. The efficacy after application of either 10 µL onto the pronotum of single specimens (5x108 spores mL-1) (n=10) or 50 mL onto groups of 10 (n=5), was 57, 78 and 87% or 75, 80 and 68% respectively for two oil/lecithin emulsions and one ProNet-Alfa suspension, respectively. Mid-May, B. brongniartii was applied as 0.1% ProNet-Alfa suspension (conc. of 5x1013 spores ha-1), on the crowns of red oak trees on 2000 m2. Cockchafers collected from the treated area and from an untreated one were kept in cages until death. Mycosis by B. brongniartii was recorded in 41% of the treated, but only in 1% of the untreated cockchafers. Third, a combination of funnel traps baited with a recently developed cockchafer attractant and equipped with B. brongniartii spores was used to investigate the possibility to smuggle B. brongniartii in the cockchafer population by this means. It was shown that captured males having passed the traps transmit the pathogen to females; four weeks after death, 26% of the specimens (17% females) were mycosed. Further studies for optimisation of this method will be conducted this year.