The vector of apple proliferation behaves more adapted to phytoplasma infection than the other vectors.
By analysing VOCs (volatile organic components) emitted by the leaves of apple trees, it was shown that ‘Ca . P. mali’ changed the VOCs composition of infected trees compared to healthy ones by inducing the sesquiterpene β-caryophyllene (Mayer et al. , 2008a,b). The main vector of ‘Ca . P. mali’, the apple psyllid Cacopsylla picta , reproduces on apple and overwinters on conifers. The adults of the new generation (emigrants) are attracted by β-caryophyllene and lured to infected apple trees (Mayer et al. , 2008b), before migrating to their overwintering host. This behaviour increases the number of psyllids, which are able to acquire ‘Ca . P. mali’. By returning in early spring to apple trees, they prefer healthy apple trees for oviposition, in order to avoid detrimental effects of the phytoplasma on the offspring development (Mayer et al., 2011). As they need to feed before oviposition, they transmit the phytoplasma to healthy apples. This is a perfectly balanced transmitting system, which improves the spread of the phytoplasma without negatively impacting the vector. A similar adaptation of C. pruni , the migrating vector of ‘Ca . P. prunorum’ is unknown yet. This species did not distinguish its host plants by odour but phloem constitution (Gallinger et al., 2019; Gallinger & Gross, 2020). As the development on P. persica infected by ‘Ca . P. prunorum’ had no detrimental effects on the vector (Gallinger & Gross, 2020), there was no selection pressure on distinguishing between infected and uninfected host plants as observed in apple (Mayer et al., 2011). It has to be understood that the two main vectors of ‘Ca . P. pyri’, C. pyri and C. pyricolado not migrate between different host plant species, therefore volatile signals might be less important in host choice of this psyllid species (Jarausch et al. , 2019a).