Parasitoid-host associations are amongst the most intimate biological interactions because most parasitoids have specialized morphological and physiological traits that allow them to survive in, or on, their specific hosts. Two hypotheses have been proposed to explain the long-term (evolutionary) associations between parasitoids and their hosts. The cospeciation hypothesis proposes that the obligatory interactions between parasitoids and their hosts result in divergence of the host and lead to reciprocal divergence in the parasitoid. Over time, the phylogeny of the parasitoid will match that of its host. Alternatively, the ecological attributes hypothesis might best explain host associations. Under this hypothesis, host shifts of parasitoids could occur frequently among hosts that share similar ecological attributes, such as body size or geographic distribution. The Cameronella (parasitic wasp) – Apiomorpha (scale insect host) – Eucalyptus (plant host of scale insect) system is a good candidate for studying host-parasitoid associations.
This study investigates the biology of Cameronella by collecting and rearing larvae from their hosts, and documenting life history and host use. A one-time observation of oviposition behaviour by an adult female of Cameronella was videoed. Morphological and genetic analyses were conducted in order to estimate the phylogeny of Cameronella, and this was compared with that of its host Apiomorpha and its plant host. Four distinct clades of Cameronella were found from phylogenetic analyses of four DNA regions from two independent genomic regions (mitochondrial and nuclear).
Using the phylogenies, and ecological attributes of the interacting partners, two hypotheses were contrasted: cospeciation and ecological attributes. Strict cospeciation was rejected because of different divergence times between Cameronella and its host Apiomorpha, and because there is not a strict one-to-one relationship between the wasp and its host – some separate clades of wasp use the same hosts, and some wasps within a clade use phylogenetically distant hosts. Alternatively, no single ecological attribute could fully differentiate clades in Cameronella. However, differences in body size and ovipositor length of females from the four clades are hypothesized to allow differential host resource use in sympatry. Furthermore, use of Apiomorpha from different host plant subgenera separates one clade from the other three, and the two clades of Cameronella in which females are the same size and share hosts are allopatric.
Species limits in Cameronella were investigated using morphology represented by two type specimens. However, the other five type specimens in overseas institutions were not able to be examined, and thus species of Cameronella are not yet fully resolved. Additional data and observations are needed for a complete systematic revision of Cameronella.