INSTYTUT DENDROLOGII

Influence of native and alien Prunus species and light conditions on performance of the leaf beetle Gonioctena quinquepunctata

Shrubs and understory trees are intrinsic components of mixed forests. Among many other beneficial functions, the forest understory constitutes a rich food source for insect herbivores . In European forests, species of the family Rosaceae, including the genus Prunus, play a main role in the species composition of the understory. Data presented in this study were performed on both species, the native Prunus padus L. (European bird cherry) and the invasive alien Prunus serotina Ehrh. (black cherry) and the broad-shouldered leaf beetle Gonioctena quinquepunctata([Fabricius, 1787; Coleoptera: Chrysomelidae) ,dominant folivore on these Prunus species and causes the greatest damage to leaves in both of the above-mentioned host species. The level of leaf perforations produced by beetle feeding differs in the two Prunus species, and is largely dependent on light conditions. While the herbivory of shaded leaves is extensive in both species, insect feeding on sun leaves is significantly greater in P. padus as compared to P. serotina.

Based on these results, the major objective of the present study was to determine the effect of the species of bird cherry and light conditions on the growth and development of G. quinquepunctata. The results should contribute to a better understanding of the factors determining the food preferences of G. quinquepunctata and answer to questions which of these two Prunus species is a better source of food for this insect. The research was carried out both under controlled conditions in the laboratory and in the field in conditions of competition between insects and the presence of their natural enemies – predators. The studies also determined in leaves concentration of the defense compounds (the sum of soluble phenolics and condensed tannins), as well as the level of nutrients (sugars unstructured - soluble and starch, carbon and nitrogen.), which determine the attractiveness of leaves as food for folivores. One of the strategies of plant cells resistance in response to insect feeding can be a modification of the anatomy and the quantitative and qualitative changes in the distribution of tannins and other phenolics. Therefore, the effect of the leaf structure as well as differences in the location of the defensive compounds (histochemical study) were determined. In addition, the aim of the study was to know some aspects of the biology G. quinquepunctata e.g. study assessed the effect of time of occurrence (insect age) on body mass in both sexes and on the sex ratio F/M depending on the host species and light conditions.

Based on the laboratory experiment, we concluded that leaves of the alien P. serotina were an equally good food source for larvae of G. quinquepunctata as leaves of the native P. padus. Although shade leaves of both host species had a higher of N content and lower leaf toughness, higher food quality of these leaves had no effect on final insect mass, and duration of insect development. The greater amount of consumed food compensate for lower food quality of sunlit leaves, whereas the time to development was not extended significantly. Under natural conditions, insects are influenced by a combination of factors (e.g., availability of food of good quality, presence of natural enemies, abiotic factors, phenological differences, and the structure and chemical composition of leaves). This may explain why, in the present case, the food preferences of the insects could not be explained by the structure and chemical composition of the leaves alone. In summary of anatomical and histochemical results, the localization of defense compounds (phenolic compounds and catechol tannins) in specific tissues, rather than any anatomical parameters of the palisade and spongy mesophyll, appear to be beneficial to P. serotina as an invasive species growing outside of its native range. An important result of the study is also to show that the reproductive cycle, as well as different adaptations of males and females to their distinct reproductive roles, have a significant impact on the seasonal fluctuations in body mass and the F/M ratio of G. quinquepunctata.

The results of the present study contribute to the clarification of food preferences of this species and furthermore a better understanding of the interaction: polyphagous beetle - native and invasive alien host species.