Hemlock Woolly Adelgid

By Julie Barnes 

This past month, District VIII gave an informative presentation on the Woolly adelgid. After spotting this pest on my favorite Hemlock, I decided to share this past article I had written to reiterate the ongoing problem occurring with our

native Eastern (Canada) hemlock (Tsuga canadensis) and the ornamental Carolina hemlock (Tsuga caroliniana).

Devastating these particular species is the hemlock woolly adelgid (HWA) (Adelges tsugae Annand), a tiny insect native to Asia and closely related to the aphid. The hemlock woolly adelgid does not affect pine, spruce, fir or other conifers, but develops and reproduces on all species of hemlock. The Eastern and Carolina hemlocks are most vulnerable to their attack while the native plants, mountain hemlock (Tsuga mertensiana) and the western hemlock (Tsuga heterophylla) tend to have more resistance. However, all species may succumb to this pest

when subjected to stressors, such as drought, poor site conditions, and infestations by additional insects or diseases. Our beloved state tree, the Eastern hemlock is planted in many of our landscapes, while populating woodlands throughout the state. Identified here in Western Pennsylvania in recent years, the Adelgid was first recognized in the early '50s in the Eastern United States near Richmond, Virginia.

Hemlock Woolly adelgids cause damage through piercing-sucking mouthparts that enable them to significantly sip tree sap rich in critical growth starches in a manner similar to drinking through a straw. HWA additionally inject a toxin that will eventually destroy the hemlocks vascular system, which is in far contrast to their aphid relatives, who would feed solely off the nutrients in the outer tree sap. Infested trees then lose vigor, drop needles prematurely, followed by dieback of major limbs. Severe infestations can kill a mature tree within four years. Once HWA is found on a hemlock, treatment is essential. A cold-tolerant nature makes HWA quite distinctive from other insects. After settling onto the twigs as crawlers, they persist in a dormant state throughout the hot summer months. Then in October, Adelgids

become active feeding and developing through winter. The most obvious sign of HWA presence are woolly masses on the undersides of hemlock tree branch tips to protect them from natural enemies, or otherwise drying out. Fortunately, this pest is somewhat easy to control in our landscapes starting with awareness and monitoring for signs of HWA on a regular basis. Hemlock woolly adelgids are spread by wind, birds, squirrels, deer and other forest-dwelling mammals that come in contact with the sticky wooly masses, and from people moving infested nursery stock. Therefore, placing bird feeders in, or near hemlocks should be avoided to discourage birds from landing on non-infested trees. During hot, dry weather notorious shallow rooted hemlocks need tended to in order to avert drought stress. They should receive at least one inch of water weekly. With infested trees the temptation to "help" by fertilizing should be resisted until you get this problem under control. High nitrogen content may actually make hemlocks more appealing to the adelgids. If chemical control is needed, then September through October is the best time to apply the necessary treatment needed to penetrate the HWA which would have been totally ineffective during their summer dormancy. You also have the option of hiring a knowledgeable professional to diagnose and treat your hemlocks, which may be susceptible to other problems as well.