The influence of light conditions on insect grazing on the leaves of undergrowth shrubs
The main purpose of this study was to determine the cause of the differing herbivorous grazing rates of leaves of undergrowth shrubs, species and light-growing conditions. The study involved six species of undergrowth shrubs: European elder (Sambucus nigra L.), black cherry (Prunus serotina Ehrh.), bird cherry (Prunus padus L.) common dogwood (Cornus sanguinea L.), glossy buckthorn (Frangula alnus Mill.) and common hazel (Corylus avellana L.). Shrubs growing in full light and full shade were selected for this study. Leaves were collected throughout the growing season from the shrubs and the content of defensive compounds, such as total phenols (TPh), tannins, lignins and nutrient enrichment (nitrogen and non-structural carbohydrates) was analyzed. The purpose of these studies was to demonstrate whether differences in the degree of grazed leaves between species and light conditions arise from the content of defensive compounds in the leaves, or the nutrient enrichment for growth and development of insects. Herbivorous insects and their predators in the shrub crowns were also collected from the selected shrubs. Therefore, dominant species of herbivores responsible for leaf grazing were determined. The analysis of grazed leaves conducted throughout the growing season showed significant differences in the surface of grazed leaves in all species of shrubs. Common dogwood was least grazed and bird cherry was most grazed. The leaves of most species were more heavily grazed when growing in shade than in full sunlight. The exception is hazel, in which an inverse trend was observed. Differences in the degree of grazing, between shrub species cannot be fully explained by the leaf contents of defensive compounds and nutrients. However, they can be used to explain the differences of grazed leaves between light conditions, in all species except hazel. Shrubs growing in high light conditions had a higher content of TPh and condensed tannins and less nitrogen. A different relationship between the degree of leaf grazing and light conditions in hazel was due to higher carbohydrate (TNC) content in high light leaves with a dominant presence on the hazel beetle, Altica brevicollis. It is the only insect species sampled that prefers illuminated leaves over shaded ones. Another important species of herbivore occurring on analyzed shrubs was Gonioctena quinqepunctata. This insect commonly occurred on all the bushes in both light conditions. However, it dominated on bird cherry and black cherry and is responsible for most leaf grazing in those shrub species.