Breaking the enemy within: disrupting mutualisms as a novel target-specific and environmentally-friendly control strategy for the citrus mealybug

The citrus mealybug (Homoptera: Pseudococcidae: Planococcus citri) has been introduced to most areas of the world and causes major damage to a wide variety of outdoor and greenhouse horticultural crops. The adult females suck sap from plants, causing leaves to wilt, fruit to drop off, stunting of plant growth and plant death. In addition, the mealybugs excrete large amounts of honeydew that enables the growth of sooty mould which blocks photosynthesis and reduces the market value of plants. The conventional insecticides available for its control are non-target specific and relatively harsh, killing many beneficial insects and contaminating food material. Insecticides are also often ineffective because the citrus mealybugs are protected against topical applications by a wax coating and form colonies in which many individuals are hard to reach. The biological control agents available include parasitoid wasps and predatory beetles, but these are also often of limited effectiveness. There is therefore a strong need for the development of novel, smarter and more environmentally friendly methods of managing populations of citrus mealybugs, both in greenhouses and in outdoor crops. Sap-feeding insects are typically highly reliant for their survival on a complex of mutalisms because of the limited nutritional value of plant phloem sap. Mealybugs host a primary, beta-proteobacterial endosymbiont which is essential for their nutrition and is vertically transmitted. Mealybugs also host a secondary, gamma-proteobacterial endosymbiont which is located within the cells of the beta-proteobacteria, making it the only known example of prokaryote-prokaryote endocellular symbiosis. The much better studied pea aphid harbours at least eight secondary symbionts and it seems likely that mealybugs too may have additional secondary endosymbionts that are yet to be discovered. The significance of the secondary symbionts in mealybugs is unknown, but in aphids they protect against pathogens and heat stress, and can even influence the ability of the insect to exploit particular plant species. Importantly, there is also evidence that these secondary symbionts may be horizontally transmitted, possibly via the host plant. While vertical transmission selects strongly for symbionts to have mutualistic benefits for their host, horizontal transmission can select for a more parasitic relationship and this may be exacerbated if multiple symbionts are competing within the host. In addition to the endosymbionts, mealybugs, including P. citri, often also have a mutualistic relationship with ants which feed on the honeydew excreta produced by the mealybugs. The ants protect their mealybug herds against predators and parasitoids, making biological control more difficult, and may play a role in endosymbiont dynamics. This project will investigate the potential for controlling citrus mealybug populations by disrupting their mutalistic relationships with endosymbionts and ants. It will use molecular methods to characterise the endosymbiont community and use quantitative PCR to determine the within-host dynamics of the endosymbionts. It will use experimental manipulations to determine the effects of the endosymbionts and ants on the fitness of P. citri and its impact on its host plants. It will also establish the importance of the colonial lifestyle for P. citri, and whether there is a population threshold required for the establishment and maintenance on host plants. The project will then utilise this information to explore the potential for controlling P. citri by disrupting the mutualistic relationships, using targeted, environmentally-benign insecticide to control ants, and controlled infections or curing to disrupt endosymbionts dynamics. The results will help inform the development of novel control strategies for P. citri, potentially including paratransgenics, which may also be applicable to other sap-feeding insect pests.