Transmission specificity and competition of multiple phytoplasmas in the insect vector

Phytoplasmas are transmitted by insects in the order Hemiptera. However, vector species are restricted in only a few families of the suborder Auchenorrhyncha: namely, Cercopidae, Cixiidae, Derbidae, Delphacidae, Cicadellidae and Psyllidae (Weintraub and Beanland, 2006). Within a family, some species are known to be phytoplasma vectors, while others are not. ‘Transmission specifi city’ defi nes the fact that only selected species can act as vectors of a pathogen. Transmission of phytoplasmas by insects involves, at several levels, elements of host–pathogen specifi city. Insect vectors can acquire more than one phytoplasma species/strain, either by feeding on multiple-infected source plants or by feeding sequentially on different plants infected by different phytoplasmas. The acquisition of multiple phytoplasmas leads to their interaction in the vector insects. This chapter analyses the concepts of phytoplasma transmission specifi city and competition in the insect vectors.