Transovarial transmission in insect vectors.

Phytoplasma ability to infect a new generation of insects by transovarial transmission was demonstrated in some insect vector/plant host combinations mainly by molecular evidence coupled with biological assays. Scaphoideus titanus was the first one in which phytoplasma detection in eggs, newly hatched nymphs and adults (reared on phytoplasma-free Vicia faba seedlings) was demonstrated. This kind of transmission was proved also for mulberry dwarf phytoplasmas and for the agent of white leaf disease of sugarcane, transmitted respectively by Hishimonoides sellatiformis and Matsumuratettix hiroglyphicus. Recently Cacopsylla pruni, vector of ‘Candidatus Phytoplasma prunorum’ and Cacopsylla picta one of the main insect vectors of ‘Candidatus Phytoplasma mali’ were also shown to have phytoplasma transovarial transmission, therefore this kind of transmission should be taken into consideration when epidemiological studies are performed on phytoplasma-associated diseases. The fact that the insect is not only the vector, but also a reservoir of the phytoplasma has implications for disease management, increasing the difficulty of disease control. Up to now only a few phytoplasma ribosomal groups such as 16SrI, 16SrX and 16SrXI have been demonstrated to be transferred transovarially in their insect vectors, very likely those capable of better adaptation to both plant and insect environments. Therefore, it can be speculated that only strains of phytoplasmas with specific genetic characteristics have become transovarially transmissible and probably only after a long host–parasite relationship