Pests usually attack weak trees and are not the main cause of tree death. Invasive pests, on the other hand, are detrimental to native trees, because the native trees do not have resistance to the pest, and the pest usually has no natural predator.
The two invasive pests that motivated me to become a licensed pesticide applicator were Hemlock Woolly Adelgid and Emerald Ash Borer. Without treatment, hemlock trees and ash trees are under serious threat.
Emerald Ash Borer (EAB):
An extreme threat to ash trees, this beetle WILL KILL every green and white ash in our area that hasn't been protected with a pesticide. EAB is an invasive bug from Asia, and feeds on leaves and vascular tissues in the cambium of ash trees. Once infested, a tree will usually be damaged beyond repair in as little as a few seasons, and will die completely a year or so later. It is ESSENTIAL to protect your ash trees by giving them preventative pesticide treatments. Treating a tree that has already been infested can save it, but is not always successful.
Hemlock Woolly Adelgid (HWA):
A pervasive threat to eastern and canadian hemlocks, this aphid-like bug is too small for most people to see without a magnifying lense, but can easily be noticed by the "wool" it forms on the tree's needles. HWA, an invasive pest from Asia, has spread to nearly every untreated Hemlock in our area. Although a hemlock tree can survive an infestation of HWA for a considerable timeframe, the pest WILL weaken and often kills its host eventually. Treatment for HWA is usually very easy and inexpensive. It's not worth leaving your Hemlocks unprotected.
Want to learn how to treat your own hemlocks? Check out Hemlock Restoration Initiative!