Dinotefuran products, such as Safari 20SG, when applied properly and according to the label, may be able to get into the tree’s system in a matter of a few weeks. Imidacloprid products take longer, several months even up to a year, to build up in the tree tissue and begin killing hemlock woolly adelgid.
That being said, the time it takes for insecticide to start killing the adelgid will vary based on the individual tree and environmental conditions at the time of application, which influence how quickly the tree is transpiring and moving fluids (and the insecticide) through its vasculature. The insecticides will be transported through the vasculature and distributed to the tree’s “storage cells” where they will persist for a period of time. Dinotefuran typically protects the tree for 1-2 year. Imidacloprid can protect the tree for 5-7 years.
However, even after the insecticide gets into the tree, you should not expect to see change right away. Hemlock woolly adelgid is mostly dormant in the summer; they go through a summer dormancy period called aestivation where they are not actively feeding. Once the adelgid nymphs “wake up” and start feeding on the starches stored in the tree tissue, they should also take up the insecticide, and consequently die. They will not begin feeding until temperatures begin to cool off in October-November. Please also be aware that even after the adelgids die, they will remain attached to the branches for several more months. Even the white “wool” that protected the previous generations’ eggs may persist on the branch for a number of months.
After control has been achieved, it can take several years for the tree to begin to look visibly different. After all, hemlocks are slow-growers, but eventually the foliage on treated trees should become full and thick in areas where it had thinned out but the branches were still alive. Dead branches will not produce new needles.