Here is an excerpt from an article written by Professor Dan Herms, Ohio State University Extension, and published on the International Society of Arboriculture’s web site. An important point to keep in mind; when most “mainstream” institutions discuss “fertilization”, one might presume they are referring to conventional, synthetic fertilizers, that are relatively high in nitrogen, not natural, organic products. By their nature, organic fertilizers are fairly low in nitrogen. International Society of Arboriculture
The Resource Allocation Trade-Off (page 5) (by Professor Dan Herms, Ohio State University Extension)
Fertilization and Tree Resistance to Insects
Fertilization commonly is considered to enhance the insect resistance of trees. However, there is little evidence to support this claim. To the contrary, the overwhelming majority of published studies strongly support the generalization that fertilization decreases tree resistance to insects. Fertilization has been shown to decrease tree resistance to sucking arthropods, including aphids, adelgids, scales, psyllids, plant bugs, lace bugs, and spider mites. Fertilization also has been shown to decrease tree resistance to chewing insects, including caterpillars, sawflies, leaf beetles, leafminers, and shoot and stem borers, as well as browsing mammals, including deer, moose, and hares.
The general pattern that fertilization decreases tree resistance to insects has emerged in spite of great variation among studies in their experimental methodology, which includes the use of deciduous and evergreen species; mature and immature trees; and great variation in formulation, rate, and timing of fertilizer applications. Furthermore, similar patterns have been documented in forests across naturally occurring gradients in soil fertility. These findings suggest that fertilization influences insect resistance via general responses of trees to increased nutrient uptake, rather than through specific effects of particular formulations or timing of application.
Fertilization decreases insect resistance by increasing the nutritional quality of the plant for the insect and by decreasing concentrations of defensive chemicals known as secondary metabolites, or allelochemicals. The nutritional quality of plants plays a central role in their resistance to insects. As an essential component of protein, nitrogen generally limits the growth and reproduction of insects, as it does in plants. The nitrogen content of insects ranges from 7 to 14 percent dry weight, while that of plants averages about 2 percent. Because of this discrepancy, the growth and reproduction of plant-feeding insects are almost always limited by the low nitrogen content of their hosts and almost always increases as the nitrogen content of the plant increases. Increased nitrogen content is virtually a universal response of plants to fertilization. Hence, fertilization almost always increases the nutritional quality of plants for insects.
To protect themselves from their natural enemies, plants produce toxic defensive compounds known as allelochemicals, or secondary metabolites. Thousands of such compounds have been isolated, including phenolic compounds such as tannins, terpenes (found in pine resins and many herbs), alkaloids (for example, nicotine and morphine), and cyanogenic compounds (cyanide-producing compounds) found in the foliage of cherries and other plants. The defensive role of these compounds against insects, pathogens, and mammals is well documented. Many studies have shown fertilization to decrease concentrations of allelochemicals in trees, and substantial evidence indicates that this decrease is the result of a trade-off between growth and defense. Slower-growing plants have been shown to have higher concentrations of defensive compounds and to be more resistant to insects.
Social Homes