THE EFFECT OF PRUNING ON WILLOW GROWTH AND SAWFLY POPULATION-DENSITIES

Interactions between different kinds of herbivores sharing a common fo od resource may potentially be important in natural systems. We studie d the effect of experimental pruning and natural ringbarking by rabbit s on growth characters of the willow Salix lasiolepis and on the susce ptibility of the willow to attack from an insect herbivore, the stem g aller Euura lasiolepis. Pruning of pelted willows and ringbarking by r abbits on clones in the field resulted in willows having longer but fe wer shoots. The pruned willows also produced slightly, but not signifi cantly, more total shoot mass than controls. However, the plants were unable to compensate for biomass loss due to pruning. Ringbarking resu lted in increased densities of E. lasiolepis and on pruned willows bot h the densities and total number of E. lasiolepis increased. There was also an indication of higher levels of leaf damage on pruned willows. We also found a strong correlation between gall densities and shoot l ength but no indications that pruning induced other changes in shoot q uality to E. lasiolepis than those associated with shoot length. Our r esults indicate that senescence (physiological ageing) plays an import ant role in increasing plant resistance to herbivores with age. Furthe r, that interaction between browsers and insect herbivores, for exampl e sawflies, might be important in natural systems. Insect herbivores m ight in some cases be dependent of browsers to reach and maintain high population densities.