technique to introduce substances directly into the xylem of a tree to treat or prevent
trunk injection (Wikipedia)
Tree injection—also known as trunk injection or stem injection,—is a method of targeting a precise application of pesticides, plant resistance activators, or fertilizers into the xylem vascular tissue of a tree with the purpose of protecting the tree from pests, or to inject nutrients to correct for nutrient deficiencies. This method largely relies on harnessing the tree's vascular system to translocate and distribute the active compounds into the wood, canopy and roots where protection or nutrition is needed.
Tree injection is currently the most popular method for control of damaging insects, pathogens, and nematodes in landscape tree care.