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).