Southern New England used to have large green expanses of hemlock forests … until the woolly adelgid turned green to brown. An accidental release, from Japan in 1953, of this piercing/sucking insect – an endearing term used to describe any plant pest that pokes a hole in leaves and stems to access a plant’s life-juices – has decimated acres and acres of woodland in Pennsylvania, New York, and many areas of New England. I’ve witnessed the devastation brought on by the woolly adelgid in nearby state parks. First you could see a few white, cotton-like clusters on the undersides of the hemlock branches then, a few months or a year later, the undersides of these trees were so infected that it looked like snow had settled on the undersides of the branches. Now, dead hemlock trunks stand waiting to topple like pick-up-sticks in the next heavy wind.
The story of the woolly adelgid gives insight into how invasives can destroy native plantings. This report from Eastern Connecticut Forest Landowners Association provides good photos and a history of the woolly adelgid saga. Years of research has yet to come up with a definitive solution, but scientists keep trying. With time – and a beetle bit of luck -woolly adelgids may find themselves controlled, particularly if University of Massachusetts Amherst scientists have success with their release of a type of predatory beetles, Laricobius nigrinus, found in the northern Rocky Mountains. They have been releasing these beetles in hemlock study plots and watching whether the beetles have slowed the woolly adelgid. Unfortunately, it may take another decade or two to truly know whether L. nigrinus will control woolly adelgids on hemlocks in eastern forests.
Home gardeners seeking to save small hemlocks from woolly adelgid attacks might consider spraying the trees with a 2% horticultural oil solution or insecticidal soap – both effective for piercing/sucking insect infestations. These can be applied at any time – of course by strictly following the instructions on the packaging – but spring applications are likely to be most effective. The difficult part comes with spraying the undersides of the entire tree – not an easy task for towering, mature hemlocks – but doable for less a mature, and shorter specimen.