Psyllids are sap-feeding insects belonging to the hemipteran suborder Sternorrhyncha. The critically endangered Cumberland Plain Woodland of Western Sydney has recently experienced massive infestations by a psyllid belonging to the genus Cardiaspina (Hemiptera: Psyllidae). The psyllid attacks only one host plant in the region, Eucalyptus moluccana (Grey Box). Cardiaspina psyllids have not been reported previously from Grey Box, making our study the first description of this host-herbivore relationship. The first aim of our study was to determine the placement of this Cardiaspina sp. within the genus using genetic markers.

Psyllids are also known for their intimate interactions with bacterial endosymbionts. Many bacterial taxa have evolved obligate endosymbiotic associations with animal hosts, often characterised by vertical transmission of bacteria to host offspring. Candidatus Carsonella ruddii is the primary obligate endosymbiont of psyllids, while Arsenophonus is a genus of bacteria described in a range of arthropods and can be either vertically or horizontally transmitted. This study aimed to test the phylogenetic congruence of both bacteria with the phylogeny of their psyllid hosts, by using mitochondrial and nuclear host genes and at least two bacterial genes.

When looking at a number of species, we found that the Grey Box psyllid is very closely related to Cardiaspina species that feed on other eucalypt species in the Box group, supporting strong conservation of host plant associations. In fact, very low levels of genetic differentiation suggest that they could all be one Cardiaspina species, and this finding creates the need for a more comprehensive phylogenetic analysis of this genus. Candidatus Carsonella ruddii has been found with 100% prevalence in all species of the Cardiaspina genus tested and appears to have a phylogeny congruent to that of its host supporting the idea of long-term cospeciation with the host. Unexpectedly, Arsenophonus also has 100% prevalence in almost all Cardiaspina species screened. This makes the genus Cardiaspina an ideal system in which to compare phylogenetic congruence with the insects of a vertically transmitted primary symbiont and a secondary symbiont shown in other insect groups to be transmitted both vertically and horizontally. For the first time, we have tested such coevolutionary relationships at the lowest taxonomic level (between species) in the Psylloidea superfamily.