There is a longstanding belief that healthy, vigorously growing trees and shrubs are better able to tolerate or repel plant-feeding arthropods (insects and mites). However, a critical review of the evidence finds little support for this claim. In fact, fertilization, particularly with nitrogen-based fertilizers, actually decreases woody plant resistance to piercing–sucking and wood-boring insects by increasing the nutritional quality of plants and decreasing the production of secondary metabolic compounds that are responsible for the production of chemical defenses against insect and mite pests. The fertilization of woody plants has been shown to increase susceptibility to a number of different insect and mite pests, including aphids, adelgids, scales, plant bugs, lace bugs, spider mites, caterpillars, sawflies, leaf beetles, leafminers, white pine weevil, and Nantucket pine tip moth.

Growth and reproduction of plant-feeding insects and mites is limited by the nutritional quality of the hosts that they are feeding on and generally increases as the plant’s nitrogen content increases. Thus, fertilization benefits plant-feeding insects and mites by in-creasing the nutritional quality of the host plants. In addition, fertilization may decrease the concentration of secondary metabolic compounds in plants. This is a result of a resource-based physiological tradeoff between the primary and secondary metabolic pathways. For example, fertilization of loblolly pine, Pinus taeda, increased growth and decreased concentrations of foliar phenolic compounds, which resulted in decreasing resistance to the Nantucket pine tip moth.

When fertilization stimulates growth, this may result in plant resources being diverted away from other processes such as secondary metabolism. In addition, rapidly growing plants may have lower levels of carbon available to support other processes. To avoid dealing with plant-feeding insects and mites during the season, apply only the proper amount of fertilizer. In addition, send a soil sample to a laboratory that can analyze the nutritional content of the soil in which trees and shrubs are growing and provide recommendations on what should be added and what should not be added to the soil.

Source: Herms, D.A. 2002. Effects of fertilization on insect resistance of woody ornamental plants: reassessing an entrenched paradigm. Environmental Entomology 31(6): 923–933.