Effects of fertilising with nitrogen and phosphorus on growth and crown condition of Eucalyptus globulus Labill. experiencing insect defoliation

We undertook a study to test the hypothesis that fertiliser application would moderate the effects of insect herbivory on stem growth of plantation-grown Eucalyptus globulus Labill. Three-year-old E. globulus with damage from the Eucalypt weevil (Gonipterus scutellatus) ranging from 0–50% loss of total crown leaf area were fertilised with either nitrogen (N), phosphorus (P) or the combination of N + P. Stem growth and crown condition were monitored for 12 months. Applications of N or N + P increased diameter and height increment for both treatments, suggesting nutrient limitations at the site. In the absence of fertiliser, diameter and height increment were significantly reduced when defoliation was more than 10%. The application of N + P or N alone increased diameter increment in all defoliation classes to at least the level of unfertilised trees with minimal defoliation. Application of P did not. While there was an overall improvement in crown condition over the experimental period, trees in the N only treatment displayed a greater improvement in crown condition, and less trees per plot had poorer crown condition, than did trees in the other treatments. It was concluded that fertilising with N offers a management tool to eucalypt growers concerned with loss of productivity following insect defoliation.