Changes in needle nitrogen and resin acid concentrations in young Scots pine trees fertilized with ammonium nitrate were followed over 3 years. Sawfly larvae (Neodiprion sertifer) were reared on fertilized and control trees the year after fertilization. Both nitrogen and resin acid concentrations increased in fertilized trees. The fact that resin acid concentrations increased contradicts predictions of the carbon/nutrient balance hypothesis. We suggest that needle resin-acid concentrations are limited more by the size of the resin ducts than by the availability of substrate for resin acid synthesis, and that the formation of resin ducts is limited by the availability of nitrogen. A modification of the carbon/nutrient balance hypothesis, relating compartment formation to allelochemical synthesis, is discussed. Performance of sawfly larvae was not affected by fertilization treatment, probably because concentrations of nitrogen (positively affecting performance) and resin acids (adversely affecting performance) increased simultaneously in fertilized trees. Thus, the results of this study do not support the notion that fertilization increases the resistance of trees to needle-eating insects.
Keywords: Carbon/nutrient balance; Diterpenoids; Neodiprion sertifer; Nitrogen; Scots pine.