Effects of Conventional Pruning Operations on the Leaf Damage of Viburnum awabuki K. KOCH Caused by the Viburnum Leaf Beetle, Pyrrhalta humeralis (CHEN) (Coleoptera

The number of overwintering egg-masses of the viburnum leaf beetle, Pyrrhalta humeralis (CHEN), and subsequent feeding damage by larvae were investigated following three different experimental treatments (summer pruning, winter pruning and no pruning) on a hedge of host plant trees, Viburnum awabuki K. KOCH. Conventional pruning practices were conducted in late July, when the elongation of the shoots stopped, and in mid-March, just before the eggs started to hatch, for the summer and winter pruning operations, respectively. The number of egg-masses left on the tree was observed in late March, and the leaf damage caused by the larvae was estimated in the following mid-June. Results showed that significantly fewer egg-masses and lighter leaf damage were observed under both pruning operations on the north side of the hedge, where shoot growth was extensive. On the south side, shoot growth was minimal and thus the effects of the pruning operations were not significant although lighter leaf damage was observed during the winter pruning operation.