Adelgids are sap-feeding hemipteran insects resembling aphids. The name comes from the Greek adelos ‘unseen’.
Hemlock Woolly Adelgids are attacking Hemlock trees in the USA. They suck the sap from the trees and kill them over the course of several years.
The preventative is to add insecticide at the base of the tree, which the tree takes up through its vascular system. But when you have a lot of trees, such as in the national forests, that’s an large task. USDA has identified 250 sites with about sixty trees in each location across the northern end of the Cherokee National Forest in Tennessee and North Carolina that they will treat with insecticide every few years. This is just a small percentage of the total hemlock population, but it is a core from which forests can regenerate.
Forest researches have tried breeding beetles that feed on the adelgids, but that is still a huge task. And then there are the possible unforeseen consequences of increasing the beetle numbers.
In The Apple Tree 