Chemically mediated multitrophic interactions in a plant-insect vector-phytoplasma system compared with a partially nonvector species

We elucidated the life cycles of two jumping plant lice species (Hemiptera: Psyllidae): Cacopsylla picta, a vector of the apple proliferation phytoplasma (Candidatus Phytoplasma mali), and Cacopsylla melanoneura, a nonvectoring species in Germany and some neighbouring countries, which may transmit the phytoplasma in one region in Italy. The adults of C. picta reproduce exclusively on apple and migrate soon after emergence (emigrants) to conifers in mountainous regions, and return to apple plants in early spring (remigrants). Cacopsylla melanoneura also uses conifers as overwintering host plants but prefers to reproduce on hawthorn, despite its ability to reproduce on apple. Both psyllid species used chemical cues for the identification of their alternate host plants during migration. Remigrants of C. melanoneura preferred the odour of their main reproduction host plant hawthorn to apple but preferred the odour of apple when experienced by feeding and oviposition. Although emigrants of C. picta reportedly prefer the odour of apple trees infected by Ca. P. mali, the remigrants of both species did not distinguish between the odours of infected or uninfected apple plants. Investigating the distribution of Ca. P. mali in plant species involved in psyllid life cycle revealed that the phytoplasma is specialized on apple. Infection of apple by Ca. P. mali increased mortality and resulted in decreased body size of C. picta offspring. Gravid females of C. picta preferred to oviposit on non-infected plants. It is concluded that Ca. P. mali indirectly promotes its acquisition from infected plants and transmission to non-infected plants by behavioural manipulation of its vector C. picta.