Utah State University Extension
Chemicals are applied to trees for many reasons. Insecticides repel or kill damaging insects, fungicides treat or prevent fungal diseases, nutrients and plant growth regulators affect growth, and herbicides kill trees or prevent sprouting after tree removal. Spraying is the most typical way to apply these chemicals. It is fast, uses readily available equipment, and is understood. The down side of spraying is that much of the chemical being applied is wasted, either to drift, run off, or because it can not be applied precisely to where it is needed in the tree. Also, in many cases the chemical is more effective when placed inside the tree, and this is difficult with spraying.
Kuhns, Michael R., "Getting Chemicals into Trees without Spraying" (2011). All Current Publications. Paper 1188.