Insect population dynamics underWolbachia-induced cytoplasmic incompatibility: puzzle more than buzz inDrosophila suzukii

Abstract Wolbachia pipientis is an intracellular bacterium that infects many insects and other arthropods. Cytoplasmic incompatibility (CI) is one of several effects that Wolbachia has on its hosts. In its simplest form, CI causes host reproductive failure when an uninfected female mates with an infected male. Theoretically, this has two consequences. First, when an incompatible Wolbachia strain is introduced into a recipient host population, uninfected females are selected against, triggering Wolbachia invasion. Second, reproductive failures during Wolbachia invasion lead to a transient decrease in population growth rate. Modelling studies combining Wolbachia invasion and host population dynamics show that these two processes could interact to cause population decline and even extinction. However, these effects could be sensitive to density dependence, with competition reducing the demographic impact of CI and Allee effect increasing the probability of extinction. We tested these predictions with laboratory experiments in the fruit fly Drosophila suzukii and the exogenous Wolbachia strain w Tei. Surprisingly, the introduction of w Tei into D. suzukii populations at carrying capacity did not result in the expected invasion of w Tei and transient population decline. In parallel, we found strong negative density dependence, but no Allee effect. From these results, we propose that negative density dependence is an important but underappreciated driver of the eco-evolutionary feedback underlying Wolbachia invasion and insect population dynamics. If future models and data support this hypothesis, pest management strategies using Wolbachia -induced CI should target early-season populations with negligible competition but a possible Allee effect.