Pesky Pest: the Hemlock Wooly Adelgid

     The Hemlock Wooly Adelgid (HWA), Adelges tsugae, is a small aphid-like insect that is infesting and seriously harming the Eastern (Tsuga canadensis) and Carolina (T. caroliniana) hemlocks in the eastern US. It was accidentally introduced into the US in 1924 and the first reported case of infestation was in 1951 in Virginia. The HWA is native to Eastern Asia where it has natural predators that keep the population in check; it is also believed that the Asian species of hemlocks have resistance to the pests.
     The HWA damages the hemlocks when it feeds. It has a piercing-sucking mouthpart, which it uses to suck the sap out of tender new branches. These wounds that they leave are openings for disease, and on top of that, it’s thought that the HWA injects some sort of toxin that causes further damage! As thousands of adelgids can congregate on one tree, these are mortal wounds. As a result, infested and infected hemlocks will look a silvery gray-green instead of the lush dark green that you’re used to seeing, and succumb in 7-20 years after initial infestation- faster the further south they are.

     The situation is dire: there are currently 11 states from Georgia to Vermont that have HWA populations. 50% of the range of the hemlock is compromised. The HWA has moved into Georgia as far south as Ellijay. If you have hemlocks, keep an eye on them to catch an infestation early – once on the tree, the HWA is not very mobile, so infected branches can be removed to save the rest of the tree. Notify your county extension office that you have a possible HWA infestation, take them a sample branch if you can to confirm your suspicions.
     There is no 100% effective treatment for the HWA. On a small tree horticultural oil, insecticidal soap and other insecticides can be used, but obviously as the trees grow to enormous proportions it is not feasible to spray an entire tree, also most pesticides cannot be sprayed within close proximity to water... which in the mountains is very limiting. There has been some luck with a pathogenic fungus that specifically targets the HWA, as well as some predatory insects: a beetle and another winged insect with a name that I cannot pronounce.
     Scientists have been crossing the American species with the HWA-resistant Asian species and have had some success maintaining the look of our hemlock while hanging on to the resistance. Hopefully this will prove to be a viable variety that can be available soon in nurseries or for reforestation of the northeast, where the hemlock is the 3rd most populous tree. Quarantines have also been established: no hemlock from a compromised area can be shipped outside that area.
     The outlook is grim for the Canadian and Carolina hemlocks in the Eastern US. Hopefully there will be a breakthrough in time to save this beautiful tree.

Jessica Watters, GCLP           Horticulture Technician, Chattahoochee Technical College