Patterns of genetic variation and host adaptation in the bird cherry-oat aphid, Rhopalosiphum padi, in southern Australia

The bird cherry-oat aphid, Rhopalosiphum padi (L.), is thought to have been introduced in Australia about 100 years ago and is now an established pest of cereal crops. We used mitochondrial COI, cytochrome b, nuclear microsatellite and SCAR loci to characterise reproductive mode and genetic diversity in invasive R. padi. Asexual reproduction was detected and genetic diversity was low with only 27 genotypes found. Two genotypes, Rp1 and Rp2, dominated all populations. No geographic or host association was detected between the dominant genotypes on cereals but Rp2 was not found on sweet corn crops. The fitness of the predominant R. padi genotypes, Rp1 and Rp2, and an uncommon genotype, Rp26, were investigated on different host types (wheat, barley, triticale and sweet corn) at two temperatures. While R. padi genotypes Rp1 and Rp2 performed similarly on cereals, the rare Rp26 performed poorly on all hosts - patterns that might explain field distributions and abundance of these clones. The results suggest that a high level of clonal diversity is not required for successful invasion and that host associations may develop despite low diversity.