Effects of fertilization on insect resistance of woody ornamental plants: Reassessing an entrenched paradigm

Woody plants are commonly fertilized in ornamental landscapes, based in part on the rationale that fertilization enhances pest resistance. However, a critical evaluation of evidence finds little to support this claim. Rather, many studies have found that fertilization decreased woody plant resistance to spider mites, sap sucking insects, mandibulate folivores, subcortical feeding insects, and browsing mammals by enhancing the nutritional quality of the plant and/or decreasing secondary metabolite concentrations. The growth/differentiation balance hypothesis (GDBH) postulates a physiological trade-off between growth and secondary metabolism, and predicts a parabolic response of secondary metabolism to variation in nutrient availability. Specifically, fertilization of moderately nutrient-deficient plants is predicted to decrease secondary metabolism if growth is increased but photosynthesis is not affected. However, fertilization of extremely nutrient-limited plants is predicted to increase secondary metabolism if photosynthesis is also increased. A number of studies have found fertilization to increase growth and decrease secondary metabolism. A few studies on extremely nutrient-deficient sites found fertilization to increase foliar secondary metabolism, but insect performance was not affected, possibly because increased foliar nitrogen counteracted effects of secondary metabolites on host quality. These studies, while consistent with the GDBH, do not represent adequate tests because none measured effects of fertilization on photosynthesis as well as growth. Only a few studies have addressed effects of fertilization on the ability of woody plants to tolerate herbivory, and all found fertilization to have no effect. The entrenched paradigm that fertilization enhances the insect resistance of woody plants in ornamental landscapes needs to be reassessed.