Endosymbiosis has been a major driver of evolutionary diversification of eukaryotes. However, symbiosis can create conflict between partners and symbiont density is often tightly regulated within hosts to ensure optimal functioning of the holobiont. The horticultural pest insects, citrus mealybugs, make an intriguing and potentially-powerful case study for endosymbiosis, harbouring two obligate, nutritional, vertically transmitted bacteria: Tremblaya princeps and Moranella endobia, in a nested mutualism. In this thesis, I examine the variation in the density of each of these obligate symbionts in citrus mealybugs under controlled environmental conditions, using qPCR, as well as the diversity of facultative symbionts that infect the mealybugs using next-generation sequencing and conventional targeted PCR. Citrus mealybugs were found to harbour Wolbachia, Spiroplasma, Cardinium and Rickettsia, which have been found to impact the fitness of their hosts in other insect species, whereas long-tailed mealybugs were not found to harbour any of these bacteria, but the symbiont communities in both species were found to be dominated by their obligate symbionts. The density of the two obligate symbionts varied by up to six-fold between different populations kept under identical environmental conditions and a hybridisation experiment indicated that M. endobia and T. princeps density may be controlled by symbiont and host genotype respectively. However, symbiont density was not found to correlate with life-history traits in the laboratory, the ability of mealybugs to exploit different plant species, or the susceptibility of the mealybugs to insecticide and artificial reduction of symbiont density by heat-stress also had no effect on host fitness. Citrus mealybugs harbour seemingly superfluous symbionts with no clear fitness costs or benefits.