Symbionts in excess? No correlation between symbiont density and the ability of mealybug hosts to exploit plant species or tolerate insecticide stress

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Abstract The acquisition of obligate, nutritional, vertically‐transmitted bacteria has been pivotal to the evolution and diversification of many insect taxa. Sap‐feeding citrus mealybugs Planococcus citri are an interesting example of obligate nutritional symbiosis, harbouring a pair of symbionts: Tremblaya princeps and Moranella endobia. Hosts can often vary in the densities of their symbionts and symbiont cells will inevitably carry some cost to the hosts, and so it would be predicted that a higher symbiont density will have some form of benefit to the host to outweigh this cost. In the present study, we examine whether populations of citrus mealybugs, with heritably different symbiont densities, differed in their ability to exploit multiple plant species or to tolerate the stress from insecticide exposure. Plant species were found to significantly impact mealybug fitness, but higher symbiont densities did not hold any evident compensation for reduced host‐plant suitability and did not correlate with mealybug susceptibility to insecticide treatment. Planococcus citri harbour variable symbiont densities but this did not correlate with the fitness of the host. This apparently sub‐optimum symbiont density regulation in an otherwise intricate and tightly‐knit tripartite symbiosis could be an evolutionary artefact of previous conflicts of interest.