Lack of sequential radiation in a parasitoid of a host-associated aphid

Sequential radiation occurs when novel biodiversity at low trophic levels 'cascades' up to high trophic levels generating further biodiversity. When putatively generalist herbivore species comprise host-plant specialized populations, sequential radiation can be tested for their parasites and predators. We tested for sequential radiation in Aphelinus perpallidus Gahan (Hymenoptera: Aphelinidae), a parasitoid wasp of the yellow pecan aphid, Monelliopsis pecanis Bissel (Hemiptera: Drepanosiphidae). This aphid has been shown to consist of at least two genetically distinct host-tree-associated populations, one feeding on pecan, Carya illinoinensis (Wangenh.) K. Koch (Juglandaceae), and one feeding on water hickory, Carya aquatica (Michx. f.) Nutt. We found that this wasp consists of three genetically distinct populations with unique molecular phenotypes, but these populations did not correspond to host-plant species of origin. Collecting site accounted for 20% of the molecular variance found. Sequential radiation may be less common in aphid parasitoids than in other parasitoid groups because of specific aspects of their biology, namely parasitization of externally feeding hosts, multivoltinism, and learned habitat preference.