Influence of primary and secondary plant metabolites on the migration and feeding behavior of Cacopsylla pruni, the vector of European Stone Fruit Yellows

Abstract

The plum psyllid Cacopsylla pruni is a univoltine herbivore, specialized on Prunus and coniferous tree species. During their lifetime, plum psyllids are alternating twice between their deciduous and evergreen hosts. For reproduction, C. pruni adults migrate to stone fruit orchards in spring, where they lay their eggs exclusively on several Prunus species. Adults of the old generation die after mating and oviposition. Young adults emerge from egg to adults during April, May and June. After several days the young adults (called emigrants) emigrate to conifers in higher regions until they remigrate (remigrants) to Prunus orchards in the next spring. Plum psyllids transmit the Phytoplasma ‘Candidatus Phytoplasma prunorum’ and are therefore of significant importance for fruit growers. In host plants, the wall-less bacterium is restricted to the phloem and causes the European Stone Fruit Yellows (ESFY). Psyllids acquire the bacteria during feeding on the phloem of infected Prunus trees. After multiplication of the phytoplasma inside the vector, plum psyllids transmit the disease to non-infected Prunus trees by salivary excretion during feeding. C. pruni is distributed all over Europe and bordering areas. ESFY is one of the most serious plant diseases in European fruit production, causing severe plant damage leading to
a poor harvest and high economic losses. Peaches (Prunus persica), apricots (Prunus armeniaca) and Japanese plums (Prunus salicina) are worst affected by typical symptoms, such as reduced dormancy, chlorotic leaf roll and premature ripening of the fruits. Trees of these species suffer severely from the infections, decline and finally die. Commonly indigenous Prunus species, such as blackthorn (Prunus spinosa) und wild plums (Prunus cerasifera, Prunus insititia) show more tolerance towards ESFY infections. Likewise, most of the cultivated varieties of European plums (Prunus domestica) do not develop severe symptoms.
To date no effective control agents or cures for phytoplasma diseases are available. The control of vector insects is an alternative strategy. Psyllid behavior could be manipulated with infochemicals and prevent C. pruni from feeding and oviposition on stone fruit crops and thus reduce the number of new infections. In this thesis I investigated the impact of plant-borne volatiles and phloem chemistry on the behavior of C. pruni as yet barely anything is known about the biology and chemical ecology of plum psyllids. The field monitoring presented in this thesis proved a preference of C. pruni for some Prunus species over others. P. insititia was identified as a favored host of C. pruni, in contrast very low numbers of plum psyllids were detected on P. insititia trees, which was therefore categorized as a non-preferred host for C. pruni. In the studies of this thesis, I compared the volatile organic compounds and the phloem sap composition of these two Prunus species and Conifers, to identify signals that were important for host plant preference of C. pruni. I demonstrated the detection of volatile compounds characteristic for Prunus trees as well as characteristic coniferous host volatiles of female plum psyllid antenna by electroantennography. Olfactometer tests revealed that this preference is not only based on olfactory cues. Additionally, gustatory cues seem to play a major role in host acceptance and preference. C. pruni nymphs showed greater development success on preferred wild plum species (P. insititia) compared to nymphs on non-preferred peach trees (P. persica). Next to effects on psyllid development, I detected differences in the phloem composition of both plant species. My research on the feeding behavior of plum psyllids on coniferous diets revealed that although C. pruni nymphs showed feeding on conifer needles, they are not able to develop on conifers. In contrast, adult plum psyllids survived longer on spruce (Picea abies) and silver fir (Abies alba) than without food supply. I concluded that adult C. pruni need evergreen tree species as resource of water and nutrition during overwintering. Furthermore, I investigated the impact of ‘Ca. P. prunorum’ infections of Prunus trees on the interaction between vector insects and their host plants. For this purpose, I recorded the feeding behaviour of C. pruni nymphs on infected and non-infected P. insititia and P. persica trees by electropenetrography. Interestingly, the average duration each nymph spend with the ingestion of xylem was shorter on infected than on non-infected Prunus trees. I found no influence on the average duration of phloem phases per nymph due to the infection status of both Prunus species. Chemical analysis of the phloem centrifugates showed that the chemical composition of trees infected with ‘Ca. P. prunorum’ was indistinguishable from the composition of non-infected Prunus trees. In accordance, the development of C. pruni nymphs was not influenced by the infection of host plants.
The knowledge obtained in this thesis is essential for the development of innovative and selective control strategies against C. pruni based on semiochemicals, such as push-pull and attractand-kill strategies. Further breeding programs of resistant Prunus cultivars should take the findings of this work into account.