Valeria Trivellone, Kerstin Krueger

Leafhoppers (Hemiptera: Cicadellidae) have long been known to transmit a number of plant pathogens, although the elucidation of the vector-host plant-pathogen relationships are far from well-defined and irrefutable. Due to their small size, the phloem-limited bacterial pathogens in the taxon ‘Candidatus Phytoplasma’ were only visualized some 50 years ago. They are difficult to culture, hence their relationships with both their insect vectors and host plants present an ongoing scientific struggle. Precise phylogenetic knowledge of the vector and bacteria may eventually allow the prediction of potential vector-phytoplasma associations. As leafhoppers are poikilothermic, abiotic factors figure strongly in the development of both the insect host and the bacteria within, which in turn affects pathogen transmission. As a group, their life cycle is varied from univoltine to multivoltine and monophagous to polyphagous, and their phytoplasma-associations are equally varied. Furthermore, adult leafhoppers are strong flying insects and some have been documented to move thousands of kilometers. When aided by human conveyance, both the vectors and the pathogens have been transported among continents. In this chapter all these interactions are explored.