I stumbled across a book published in 1970 entitled The Fertilizer Treatment of Forest Trees. Published in Germany originally. The book is mainly about forestry and timber but contains a great deal of fascinating information that could apply to orchard culture as well. One section I found very interesting was on the effect of fertilization on pest infestations, I’ve excerpted and paraphrased a few paragraphs from the book:
Schwenke (1961) found that the pupal density of the pine looper moth (Bupalus piniarius L.) on a soil of pure sand was reduced 54 per cent by a fertilizer treatment.
Ohnesorge (1957) by an application of quick lime a 50 dz/ha obtained a reduction of the cocoon density of the Small Spruce Saw-fly (Pristiphora abientina) by 75 percent compared with the untreated plots.
Merker (1958a) depressed the cocoon density of Small Spruce Saw-fly by 50 per cent by the application of calcium ammonium nitrate at 15 dz/ha.
Merker (1963) it is shown that the nitrogen effect on the Small Spruce Saw-fly persisted only 5 years. Should be applied during the first half of April till the region under attack is cured.
Schwenke (1960) A reduction of the Common Pine Saw-fly (Diprion pini) cocoon density by 29 percent was attained by a combination of an NPK-treatment with liming the forest district of Schwabach in Bavaria.
Only one third of Gypsy Moth caterpillars (Lymantria dispar = Listrig dispar) survived when their food had been enriched with nitrogen, phosphorus, calcium and trace elements (Buttner 1961 cited in Merker 1964).
Merker (1960) In the case of bark beetles it was found that adequate moisture and fertilizer treatment made it difficult for the beetles to colonize their host plants.
Schwenke (1961) Water shortage evokes a disturbance not only in the water economy of the tree but also the nitrogen economy. Both disturbances result in the displacement of the protein-carbohydrate metabolism in favor of sugar. Thus the food value of the needles or leaves in increased for certain pests.
(Buttner 1961, Merker 1961, 1962a, Merker and Berwig 1962) The way in which the fertilizer treatments act on the activity of the needle and leaf eating pests can be direct or indirect. It is accepted as direct when the composition of the food is changed in such a way that it become unfavorable for the pests or evokes a disturbance in the physiology of these insects, for example by an accumulation of calcium in the Malpighian vessels, and of phosporus in the skin and middle intestines.
Indirect actions include strengthening of the cell walls, or changes in the growth and maturation of the plant resulting in an un-synchronizing of the life cycles of plant and insect pest. By these mechanisms, such as more rapid lignification of tissues problems in digestion can arise for the pests. More energy must also be expended in eating due to thicker cell walls. Buttner (1961) confirmed this by feeding of needle-pulp and uncomminuted needles.
Merker (1965) Establishes insuperable difficulties for the development of pests are caused by changing the osmotic pressure of plant sap by the addition of water or application fertilizer.
Merker (1961) Spruces treated with Urea were “lousy” within a short time from Cinaropsis pruinosa and C. ficieornis while caterpillars with a similar food affected by inorganic nitrogen had a 50 per cent die off.
I can’t go study the original referenced studies because they’re all in German. I also don’t know how such methods would affect pest pressure on fruit production. Does anyone have such experience?