The effect of pruning on willow growth and sawfly population densities

Interactions between different kinds of herbivores sharing a common food resource may potentially be important in natural systems. We studied the effect of experimental pruning and natural ringbarking by rabbits on growth characters of the willow Salix lasiolepis and on the susceptibility of the willow to attack from an insect herbivore, the stem galler Euura lasiolepis. Pruning of pelted willows and ringbarking by rabbits on clones in the field resulted in willows having longer but fewer shoots. The pruned willows also produced slightly, but not significantly, more total shoot mass than controls. However, the plants were unable to compensate for biomass loss due to pruning. Ringbarking resulted in increased densities of E. lasiolepis and on pruned willows both 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 length but no indications that pruning induced other changes in shoot quality to E. lasiolepis than those associated with shoot length. Our results indicate that senescence (physiological ageing) plays an important role in increasing plant resistance to herbivores with age. Further, that interaction between browsers and insect herbivores, for example sawflies, might be important in natural systems. Insect herbivores might in some cases be dependent of browsers to reach and maintain high population densities.