The branches of my hemlock trees are covered with some kind of small, white cottony dust, for lack of a better word. I don’t know what it is and if that’s a problem for the trees.
—Jim in Pennsylvania
Those are hemlock woolly adelgid (Adelges tsugae), and yes they are a problem. The pest has been around for many decades but for some reason has become worse over the last 10 years, especially in the eastern U.S.
The organic control for the pest is to spray it with horticultural oil, which isn’t a problem for small trees, but very tall specimens will need application by an ISA certified arborist who can safely get to the top of the hemlock.
One note about the pest: it doesn’t do well in bitter cold. The winter of 2013-14 knocked back infestations dramatically in the East. In my own landscape, I’ve seen lots of wooly adelgid this season after coming out of a mild winter, and I’m spraying with the horticultural oil whenever I see the pest.