Temporal Dynamics of ‘Ca. Phytoplasma mali’ Load in the Insect Vector Cacopsylla melanoneura

SIMPLE SUMMARY: Cacopsylla melanoneura is a psyllid vector of the phytoplasma ‘Candidatus Phytoplasma mali’, the causal agent of apple proliferation. It spends part of the year in apple orchards, then moves to conifers for aestivation and overwintering. The aim of this work was to analyze the quantity of phytoplasma inside the insect body throughout the year, during different life stages, in order to assess the relative acquisition efficiency and the vector infective risk. The phytoplasma was detected in all the life stages. The phytoplasma load significantly increased during the period spent on conifers, but in many cases also nymphs and newly emerged adults contained a sufficient quantity of phytoplasma to consider these stages infective. The obtained results allow us to optimize control strategies against the vector and thus against the spread of the disease. ABSTRACT: The transmission of phytoplasmas is the result of an intricate interplay involving pathogens, insect vectors and host plants. Knowledge of the vector’s competence during its lifespan allows us to define more sustainable well-timed control strategies targeted towards the most worrisome life stages. We investigated the temporal dynamics of ‘Candidatus Phytoplasma mali’ load in Cacopsylla melanoneura in the different developmental stages in Northwest Italy. The phytoplasma load in the vector was evaluated in overwintering adults, nymphs and newly emerged adults after different acquisition access periods. Moreover, we followed the multiplication of the phytoplasma during the aestivation and the overwintering period on conifers. Our results confirmed the ability of remigrants to retain the phytoplasma until the end of winter. We also highlighted the high acquisition efficiency and vector competence, based on phytoplasma load, of nymphs and newly emerged adults. Therefore, particular attention should be paid to the management of overwintered C. melanoneura as soon as they return to the orchards, but also to newly emerged adults, particularly in orchards with a high infection rate and when the migration to conifers is delayed.