Phytophagous insects and their hosts are a major component of the diversity of life. Although the host associations of many insect groups have been studied, coccids or soft scales (Hemiptera: Coccidae) have received little attention. With over 1100 species described to date, coccids are the third most species-rich family of scale insects (Coccoidea) and occur on more than 200 families of host-plants. This study aims to investigate the relationships between coccid species richness and host-plant diversity. However, the current identification of species and above-genus level classifications of Coccidae are morphologically-driven and poorly resolved. This study also used the data from multiple resources to reconstruct robust phylogenies of Coccidae in order to test species boundaries in widely-distributed species, and to test the monophyly of higher level classifications.
There have been few attempts to synthesise the available information of the associations between scale insects and their host-plants. Chapter II investigates the relationships between coccid and their host-plant diversity by analysing host-use data from the database ScaleNet. By comparing host-plant data of coccids with species richness estimates of host-plant families, about 64% of examined coccid species were restricted to a single plant family. This is lower when compared with host-specificity measures for most other insect groups. The number of coccid species was positively correlated with host-plant family species richness (P < 0.0001), that is, more coccid species were found on plant families that were species rich. Species richness of clades (genera) of coccids was also positively correlated with host-plant family species richness (P < 0.0001). Although these results suggest that coccid host-use is randomly distributed across families according to plant family species richness, randomisation tests indicate that host-use by coccids is not totally random. This supports the hypothesis that some coccids have radiated in proportion to host-plant richness, and the idea that plant diversity partially drives the species richness of herbivorous insects.
The delimitation and definition of a genus is somewhat arbitrary. However, the use of Latin binomials is integral to nomenclature and requires the naming of genera. Chapter III provides objective criteria to assess the taxonomic status of genera using the monotypic coccid genus Taiwansaissetia Tao, Wong & Chang. On the basis of phylogenetic analyses of DNA sequence data and morphological examination, Taiwansaissetia is synonymised (syn. nov.) with Coccus Linnaeus. The analyses showed that T. formicarii (Green) is more closely related to C. hesperidum Linnaeus, the type species of Coccus, than C. hesperidum is to some current congeneric species. Explicit criteria, including monophyly, diagnosability, sister taxa being of equal rank, and the level of genetic divergence between T. formicarii and C. hesperidum relative to within-genus divergence of other scale insects, were used to assess the taxonomic status of Taiwansaissetia and to support synonymy with Coccus.
Some morphologically recognised species of scale insect are being found to comprise cryptic species complexes. This is of particular relevance to quarantine because scale insects are some of the worst globally invasive pests. Parasaissetia nigra (Nietner) (Coccidae) is a cosmopolitan, parthenogenetic and polyphagous pest feeding on more than 80 plant families. Previous work on adult female morphology for this species suggested that it might be a cryptic species complex. Chapter IV uses a coalescent species model approach to assesses the species status of P. nigra using mitochondrial and nuclear gene regions, and multiple analyses including: maximum parsimony, neighbour-joining, Bayesian inference, BEAST and *BEAST. Because it reproduces parthenogenetically, this precludes the use of the biological species concept, which is why the coalescent approach was used to delineate the five sampled species of P. nigra. The results have important implications for how this pest species is treated and managed by quarantine practices.
The current widely used classification of coccids was introduced by Hodgson in 1994 and he subdivided Coccidae into ten subfamilies. Saissetiini is one of the four recognised tribes in Coccinae and includes 12 genera. The 119 saissetiine species are found mainly in the Afrotropical and Neotropical regions. The tribal classification in Coccinae is currently based primarily on the morphology of adult females and, to date, there has been no rigorous testing of the monophyly of any of the tribes. Chapter V tests the monophyly of Saissetiini using DNA sequence data of five gene regions and different phylogenetic analyses. Multiple representatives from other tribes and subfamilies of Coccidae were also included. Saissetiini could not be recovered as monophyletic in any of the analyses; however, neither could monophyly be rejected because of the lack of the information from the type species of some genera, i.e., Hemilecanium Newstead and Stictolecanium Cockerell. On the other hand, the non-monophyly of Paralecaniini was supported by the position of Neosaissetia tropicalis Tao & Wong. No strong evidence was found for the non-monophyly of the two other tribes Coccini and Pulvinariini. A better sampling, including type species from most genera is necessary before a robust molecular phylogeny of Coccinae is available.