The Invasive Insect That Can Wreak Havoc On Your Hemlock Tree

Hemlocks are one of the most elegant and visually striking of all conifers. They look good in landscapes and have tremendous wildlife benefits, too. However, they are not exactly easy to grow. With hemlocks, you have to find the right site — they won't grow just anywhere. But even if you manage to find the perfect place for your hemlock, there is an insect that can wreak havoc on and even kill your trees — the invasive hemlock woolly adelgid (Adelges tsugae).

The hemlock woolly adelgid is an aphid-like bug that spreads like wildfire. It will hitch a ride on animals, sneak into logs or nursery plants, and travel wherever the host goes. It can even drift through the air to new places. Even worse? Most people never even realize that their hemlocks have been infested by woolly adelgid. And more often than not, by that time, it is already too late to save the tree.

These bugs are tiny, measuring only about 1/20th of an inch. Chances are, you will spot their eggs before you actually see them running about on your hemlocks. Early detection, though, is key to saving your tree. Woolly adelgid can reproduce explosively. A single female, in theory, can produce tens of thousands of offspring in a given year. Just like many other pests that lay eggs on garden plants, the woolly adelgid will also lay tiny, 1/16-inch egg masses on your hemlocks.

The eggs are often covered by cotton or wool-like waxy secretions produced by the bugs. These egg structures are mostly present below or at the base of the tree needles. When the eggs hatch, bugs settle right at the base and start sucking up the hemlock's stored food. The trees respond by cutting off the feeding site, blocking nutrient flow to the area.

Eventually, the affected shoots die. Over seasons, heavy feeding damage can cause severe needle loss in trees, reduced bud formation, and diminished growth. Left untreated, the decline is rapid, and the tree can die in about four to 10 years.

If you want to save your hemlock trees from the invasive woolly adelgid, start monitoring them at least twice a year. Gently bend a branch or twig and check the underside to see if there are any new white woolly clusters. The best time for these inspections is in late fall and early spring. If you find any such masses, contact your local forest services. They can help with the identification and advise on treatment. For valuable trees, systemic insecticides like imidacloprid or dinotefuran are applied. Application methods vary, but mostly they are soil drenches, soil injections, or basal bark sprays. 

The downsides of using systemic pest control insecticides are that they act slowly and often require multiple applications. Nevertheless, a professional can easily apply these insecticides. If you are doing it yourself, make sure you follow all label instructions and are familiar with the proper way to spray pesticides. Lastly, minimize contact between infested trees and other materials, such as wood logs, outdoor furniture, and nursery stock, as these bugs will literally hitch a ride on anything and spread.