Abstract
Scarab beetles (Scarabaeidae) within the order Coleoptera include several species causing major damage in agriculture, horticulture, and forestry. In central Europe, the most problematic, native scarabs are the cockchafer (Melolontha melolontha) and the garden chafer (Phyllopertha horticola). Additionally, the Japanese beetle (Popillia japonica) invaded mainland Europe recently and is spreading in northern Italy and southern Switzerland despite quarantine measures. The larvae of these species feed on plant roots, causing substantial economic losses in crops, forage plants, and recreational areas. While adult of the two native species rarely cause economic losses, Japanese beetle adults feed on the flowers, fruits, and foliage of a various crops and ornamental plants, resulting in major damage in the outbreak areas. To manage these scarab pests sustainably, there's a growing need for environmentally friendly alternatives to synthetic insecticides, such as entomopathogenic fungi (EPF) which are among the most important biocontrol agents globally. Especially, the control of cockchafer larvae with its specific pathogen Beauveria brongniartii is one of the most successful inundative biological control approaches. This thesis centers around the adaptation of the successful cockchafer control to the invasive Japanese beetle using the commercially available EPF Metarhizium brunneum BIPESCO5 (Bip5) and contributes to the general understanding of the biological control of scarab beetles with EPF. Using the cockchafer and B. brongniartii BIPESCO2 (Bip2) as role model enabled to contextualize the outcome of laboratory virulence tests conducted with Japanese beetle adults and larvae and offered valuable insights into factors influencing the virulence measured in bioassays. Field experiments on the control of the Japanese beetle with Bip5, along with accompanying field and pot experiments with the garden chafer, known as susceptible host, increase the knowledge in this understudied area. The study revealed that Japanese beetle adults can be readily infected by the applied EPF strains in laboratory bioassays, while larvae exhibit resistance. However, caution is needed in interpreting these virulence tests, as cockchafer larvae, known to be susceptible to Bip2, could not be efficiently infected using the same methods. The outcomes emphasize the variability of bioassay results, which may depend on the application method, spore type and the presence of fungal secondary metabolites. This underscores the necessity for more elaborate studies for successful biocontrol product development under different experimental conditions, considering host and pathogen specific characteristics. In various field experiments, Bip5 and M. brunneum ART212 could be applied efficiently to larval habitats of the garden chafer and the Japanese beetle. Additionally, Bip5 was successfully applied to a soybean field to target adult Japanese beetles. Furthermore, we could show that the applied fungal strains infected garden chafer larvae as well as Japanese beetle adults but not Japanese beetle larvae. However, the fungal applications did not effectively reduce any pest insect populations. Additionally, naturally occurring EPF infecting garden chafer larvae were identified, while Japanese beetle larvae remained mostly unaffected. This suggests that naturally occurring EPF might not be well adapted to the invasive Japanese beetle, contributing to its uncontrolled propagation. In contrast, the indigenous garden chafer larvae are controlled by a consortium of natural enemies and control interventions might only be justified on areas which have a lack of natural enemies due to management practices. For the control of the Japanese beetle larvae with EPF, we suggest screening for native fungal strains adapted to the conditions in the outbreak with improved bioassay methods. Regarding the more susceptible adult Japanese beetles, more efficient spraying techniques, or alternative spore dissemination such as attract and infect strategies may contribute to the control of this pest insect. In summary, this thesis investigates various aspects of scarab beetle control using the EPF M. brunneum and B. brongniartii, integrating results from bioassays, pot experiments, and large-scale field applications and thus, contributes to the understanding of the host-pathogen relationship of scarabs and EPF.
Graf, Tanja