Under control or under influence? Wolbachia invasions, cytoplasmic incompatibility, and Drosophila suzukii population dynamics

Wolbachia pipientis is a bacterium that infects numerous arthropods and nematodes. It is located in the cytoplasm of its host cells and is transmitted from mother to offspring. Wolbachia manipulate host reproduction in favor of their own transmission, the most common mechanism being cytoplasmic incompatibility (CI). In its simplest form, CI results in sterility of crosses between infected males and uninfected females. In this context, Wolbachia could be used in programs aimed at controlling pest species. In this thesis, we present initial experimental advancements for such a control program targeting the fruit fly Drosophila suzukii.Theoretically, an incompatible strain of Wolbachia should invade the host population into which it is introduced, causing an increase in sterile crosses and thus a decline of the population. Such a strategy, combined with other destabilizing mechanisms such as the Allee effect, could eradicate the pest population.To test this theory, we conducted experimental research on D. suzukii and three Wolbachia strains: wSuz, a wild strain that does not induce CI, and wTei and wHa, two artificially transfected strains that induce CI. For large insect populations at carrying capacity, we did not observe the predicted decline in numbers. We therefore hypothesized a mechanism neglected by theoretical models: density-dependence compensation. By reducing the proportion of viable eggs, CI could trigger a relaxation of competition among larvae, leading to similar number of adults as in the absence of CI. We tested this new hypothesis and obtained two major results. First, in D. suzukii populations subjected to a factorial manipulation of density-dependent competition and CI, a relaxation of competition leads to a strong effect of CI on reproductive rate. Second, in growing populations, CI causes a decrease in fly numbers that persists as long as competition is moderate. Thus, we show that D. suzukii populations are under the influence of a density-dependent compensation that interferes with a control strategy based on cytoplasmic incompatibility. In parallel, experiments at low numbers shows an exponential increase of small populations, with no destabilizing Allee effect. Our research therefore suggests that the development of new biocontrol methods that bypass fundamental research into the complex processes that determine the eco-evolutionary dynamics of populations run a high risk of producing misunderstood failures.