Short rotation plantations of aspen and balsam poplar on former arable land in Germany: Defoliating insects and leaf constituents

Damage by herbivorous insects to fast-growing clones of aspen (Populus tremula x P. tremuloides, clone Astria) and balsam poplars (Populus trichocarpa, clone Muhle Larsen) was estimated on a short rotation plantation which was established on former arable land near Regensburg (Germany). During the first rotation period of 5 years insect pests could not be observed. The present study was carried out in the second year of the second rotation period in which defoliators became obvious. The leaf roller Byctiscus populi (Curculionidae: Coleoptera) and leaf beetles of the genus Chrysomela, especially C. tremulae, and Phratora vitellinae (Chrysomelidae: Coleoptera) were found to be the dominant herbivorous insects. In total, the damages were significantly higher with the aspen clone Astria. In the clone Astria about 50% of the leaves and 37% of the side shoots were damaged by B. populi. Corresponding values with the clone Muhle Larsen amounted to 21% and 20%. Fertilized plots (N or N, P, K, Mg) of the clone Muhle Larsen were less attacked by B. populi. Almost all the remaining leaves not damaged by B. populi were injured by leaf beetles. Chrysomelid-caused damages of the leaf area were classified as follows: class 1 (0% damaged leaf area), 2 (1-5%), 3 (6-25%), 4 (26-50), 5 (51-75), 6 (>75%). About 80% of the investigated leaves of the clone Astria were classified in groups 4-6, while 80% of the clone Muhle Larsen belonged to the classes 1-3. Fertilization resulted in a slight reduction of chrysomelid damage. Biochemical analysis of the leaves revealed that the quantities of proteins and soluble carbohydrates could not explain the increased susceptibility of aspen, but both the clones differed significantly in the amounts of phenolic compounds, indicating a good protection against herbivory by high concentrations of tannins, especially procyanidins, in the clone Muhle Larsen.