Changes in the frequency of facultative endosymbionts in insect populations: overview and applications

Many insect endosymbionts are facultative from the host perspective, and their population frequencies across time and space will depend on their transmission fidelity and effects on host fitness. These effects and transmission rates in turn depend on the environmental and host genetic contexts where the endosymbionts occur. Endosymbionts like <i>Wolbachia</i> and <i>Cardinium</i> affect host reproduction to produce transient or persistent presence/absence polymorphisms, while other endosymbionts like <i>Regiella</i> and <i>Hamiltonella</i> persist through providing host fitness benefits and transmitting horizontally. Evolutionary changes in hosts and endosymbionts affect these impacts and endosymbiont polymorphisms in host populations and host sexes. We review this diversity of endosymbiont-host interactions and their influence on the usefulness of endosymbionts for applied strategies. Current strategies focus on endosymbionts driving useful traits to fixation (particularly <i>Wolbachia</i> suppression of arbovirus transmission by mosquitoes) or endosymbionts suppressing populations due to infected males sterilising females. Transinfected endosymbionts sourced from one species and microinjected into another have proven effective in these <i>Wolbachia</i>-mosquito strategies. Novel strategies involving transinfected <i>Rickettsiella, Regiella</i> and <i>Wolbachia</i> may decrease the impacts of pest invertebrates by suppressing pest numbers, reducing the capacity of vector hosts to transmit plant viral diseases or bolstering the effectiveness of natural enemies. Because many endosymbionts are already present in the environment, their applied use raises fewer safety concerns when compared to genetic modification, as supported by more than 13 years of field experiences with <i>Wolbachia</i> in mosquitoes that have not raised major concerns.