Herbivore Outbreaks on Only Young Trees: Testing Hypotheses about Aging and Induced Resistance

Populations of spider mites (Oligonychus spp.) occasionally become high on young Monterey pines (Pinus radiata) in natural stands and plantations but remain small on individuals. I investigated two hypotheses to explain this age-related pattern. 1) Resistance of trees to mites increases as they age, independent of herbivory. 2) Young trees attacked by mites become permanently resistant to subsequent infestations. Results did not support either of these hypotheses. Cuttings that were developmentally older tended to support more mites, not fewer. Mite populations were reduced on trees that had been damaged by mites during the previous year but this difference did not persist for three years. Other explanations for these observations are suggested.