Repellent strategy to avoid transmission of ‘Candidatus Phytoplasma mali’ by its vector Cacopsylla picta

Cacopsylla picta overwinters on conifers and remigrates in the early spring to apple. These remigrants are highly infectious and readily transmit ‘Candidatus Phytoplasma mali’, the agent associated with apple proliferation, to apple. Insecticide treatments before and during blossom are not everywhere applicable and shall be avoided for environmental reasons and in addition, they rarely prevent the transmission of the pathogen. Trials to establish a repellent strategy against C. picta which reduces the probe feeding and transmission of the phytoplasma were carried out. It was hypothised that the volatiles emitted by conifers have a repellent effect during the remigration phase of the psyllids. Hot water and EtOH extracts of spruce were sprayed on small ex vitro apple plants. Remigrant C. picta could chose in a double choice cage experiment conducted in the greenhouse between treated and non-treated plants. Conifer extracts significantly reduced the presence of the psyllids population on the treated plants. Oviposition and larval development was also recorded less on treated plants. However, this effect was not sufficient to reduce the number of transmission events of the phytoplasma and improvement of the technique is needed for possible field application.