Abstract
The psyllids Cacopsylla melanoneura and Cacopsylla picta reproduce on apple (Malus x domestica) and transmit the bacterium 'Candidatus Phytoplasma mali', the causative agent of apple proliferation. Adult psyllids were collected by the beating-tray method from lower and upper parts of the apple tree canopy in the morning and in the afternoon. There was a trend of catching more emigrant adults of C. melanoneura in the morning and in the lower part of the canopy. For C. melanoneura remigrants, no differences were observed. The findings regarding the distribution of adults were reflected by the number of nymphs collected by wash-down sampling. The density of C. picta was too low for a statistical analysis. The vector monitoring and how it is commonly performed, is suitable for estimating densities of C. melanoneura. Nevertheless, above a certain temperature threshold, prediction of C. melanoneura density might be skewed. No evidence was found that other relatively abundant psyllid species in the orchard, viz. Baeopelma colorata, Cacopsylla breviantennata, Cacopsylla brunneipennis, Cacopsylla pruni and Trioza urticae, were involved in 'Candidatus Phytoplasma mali' transmission. The results of our study contribute to an advanced understanding of insect vector behavior and thus have a practical impact for an improved field monitoring.