Evaluation of Natural Antagonists for Biological Control of Drosophila suzukii

Abstract

The polyphagous insect Drosophila suzukii (eponymous: spotted wing drosophila, SWD) is indigenous to Southeast Asia and has been spreading rapidly in North American and European countries since 2008. It became primarily a pest insect of berries, grapes and stone fruits as it lays eggs in ripening fruits leading to rapid fruit collapse and causing high economic losses. Because of its life cycle, chemical control of SWD is very complicated and often impossible. Hence, the evaluation and development of biological control measures are of great importance for conventional and organic fruit growers. As a first approach (Chapter II), biological control agents (BCA) that are already available for Diptera have been tested for their efficacy against SWD. For this purpose, various products of the gram-positive subspecies Bacillus thuringiensis serovar. israelensis (B.t.i.), which is specific for the Diptera, such as mosquitos and blackflies, were applied to SWD larvae and adults. It could be demonstrated in laboratory experiments, that the examined products showed neither an increased mortality after exposure to SWD larvae or adults nor a repellent effect on the oviposition behaviour after application to host fruits. Thus, B.t.i. products could be excluded as possible candidates for SWD control. As shown in Chapter III, Neem oil, an extract from the seeds of the Neem tree Azadirachta indica, was also inefficient against SWD. The extract is mainly applied to larvae of leaf-sucking insects, on which the active ingredient Azadirachtin A has a lethal effect and inhibits ecdysis (moulting). These effects also appeared on SWD larvae, but only at concentrations ten times higher than suggested for target organisms. In addition, no repellent effect to SWD was noted. Another approach to find host-specific antagonists suitable as BCA is to examine natural populations for pathogens that are already associated to the respective species. In Chapter IV, such a pathogen belonging to the phylum Microsporidia is described. Based on comprehensive light and electron microscopic studies as well as molecular analysis of rDNA sequences and phylogenetic studies revealed a new microsporidian species, which was eventually named Tubulinosema suzukii sp. nov. T. suzukii was further tested for its competence as an antagonist of SWD (Chapter V). The median lethal spore concentration (LC50: 6900 spores/μl) and concentration-dependent mortality after exposure of larvae showed moderate to high virulence of T. suzukii to early developmental stages of SWD. Molecular examination of the infection process via RT-qPCR showed that replication of T. suzukii increased especially during the larval and pupal stages of SWD. This resulted in restricted or delayed development of adult SWD. In addition, population-reducing effects were evaluated by experiments on survival rates and on the ability to lay eggs (oviposition). After inoculation of SWD larvae with T. suzukii, hatching rates were significantly reduced (over 70%), and the survival rates of hatched adult flies as well as their reproductive ability (up to 70% less progeny) were severely impaired. These effects were less pronounced when adult SWD were exposed to T. suzukii. Considering the clear effects on viability and egg deposition as well as generally chronic and sublethal impact of microsporidia on its infected hosts, a long-term effect of T. suzukii infection affecting SWD populations seems to be likely. The results of this thesis indicate that application of B.t.i. products or Neem oil do not offer sufficient control options for SWD since they are either ineffective or would require an excessive and therefore uneconomical application rate. On the other hand, a new microsporidian species, T. suzukii, was isolated and characterized. It showed highly promising effects on larval stages of SWD and its finding encourages further evaluations in semi-field trials.