Woolly Adelgid

Sonia Uyterhoeven is Gardener for Public Education at The New York Botanical Garden.

Who’s afraid of the hemlock woolly adelgid? Certainly, many gardeners in the Northeast. Like many pests that plague us, these little creatures were introduced from abroad. In their homeland (Japan) they are kept in check by a type of ladybug that is a natural predator. Here in the United States, though, the woolly adelgid has free range. The only natural challenge it faces is extreme temperatures—once the thermometer drops below 23°F or -5°C, the mortality rate of these fluffy little critters rises.

So what is hemlock woolly adelgid and why should we be concerned? This pest spends most of its life protected under its telltale fluffy white secretion. You probably have seen it cover the branches of hemlocks. It plants itself at the base of the needles and slowly sucks the sap—and life—out of a tree. This can take anywhere from 1 to 15 years, depending on the tree and environmental situations. While many pests target unhealthy and, therefore, susceptible trees, this pest prefers healthy ones.

For those of you who have grown up on the East coast, as I have, you will appreciate the importance of the Eastern hemlock (Tsuga canadensis) in the landscape. For the homeowner, it is one of the few conifers that make an outstanding addition to a shady border. It has delicate foliage, it can withstand hard pruning, and its lower branches often reach down to the ground, creating a nice, full specimen. It is an important screening and landscape plant.

What you may not have thought about is the important role that it plays in the ecosystem. This shade-tolerant tree grows slowly into giants in the forest, creating cool, shaded areas by stream and river beds that allow many fish and aquatic invertebrates to thrive.

To learn what you can do to help your hemlocks fight the woolly adelgid…

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