Short-term heat stress results in diminution of bacterial symbionts but has little effect on life history in adult female citrus mealybugs

Mealybugs are sap-feeding insect pests that pose a serious threat to horticulture. The citrus mealybug, Planococcus citri (Risso) (Hemiptera: Pseudococcidae), like most other mealybug species, harbours two obligate maternally transmitted bacterial endosymbionts, which are essential for nutrient acquisition and host survival. These are Candidatus Tremblaya princeps', a member of the -Proteobacteria, and Candidatus Moranella endobia', a member of the -Proteobacteria. The density of symbionts in the hosts is now understood to be dynamic, being influenced by the age and gender of the host and by environmental conditions during development. Here, we examine the impact of short-term heat stress treatment on the obligate symbionts and life-history parameters of P.citri, using qPCR to measure changes in symbiont density. Heat stress killed juveniles and adult males, and significantly reduced levels of Ca. Moranella endobia' and Ca. Tremblaya princeps' in adult females. However, adult females were resilient to this and it did not affect their fecundity or brood survival, although the sex ratio of their brood was slightly, but significantly, more female biased. Our results suggest that Ca. Tremblaya princeps' and Ca. Moranella endobia' are not as essential to the survival of adult mealybugs as they are to the survival of immature mealybugs and that sub-lethal heat treatment alone is unlikely to be effective as a disinfestation tactic.