The hemlock is one of my favorite trees, growing cathedral-like in moist forest coves, and providing heavy shade to keep mountain streams cool. They are also popular landscape trees. Unfortunately, the Hemlock Wooly Adelgid moved into our area several years ago and threatens this important tree. This pest was accidentally introduced from Asia around 1924, and moved our way from the New England states. Many hemlocks in local parks are dead or dying, threatening pristine mountain stream habitats where trout hang out.
The Hemlock Woolly Adelgid (called HWA for short) is a tiny reddish-purple aphid-like insect almost too small to see. You can tell you have them (especially in the spring) by the white fuzzy sacs that appear on the needles. This is a waxy secretion that protects the insect and its eggs from predators. HWA feeds on hemlock trees during all but the hottest months, with the greatest damage occurring during the spring. They suck juices from fresh hemlock needles, and may also inject toxins into the tree as it feeds, accelerating needle drop and branch dieback. The loss of new shoots and needles means a slow death for the hemlock. Trees can die within 4 years after attack, but often persist in a weakened state for as long as a decade. Hemlocks that have been affected by HWA often have a grayish-green appearance rather than their naturally shiny, dark green color.
The biology of this pest goes like this: All populations are females that reproduce asexually. In the spring, overwintering females lay 100-300 eggs in the wooly sacs beneath the branches. Mobile larvae, know as crawlers, hatch out in April or May and find suitable feeding sites. Wind, birds, and mammals often spread crawlers to nearby hemlocks. Once they settle in at the base of hemlock needles, crawlers become immobile nymphs, which feed and mature into adult females by early summer. These turn around and lay another batch of eggs; crawlers again emerge, settle onto feeding sites, but then go dormant until October or so. They then resume development over the winter into adults, feeding throughout the winter and early spring.
Control on trees in the wild is difficult and expensive, but local parks are trying out both chemical and biological controls on trees that are located along important stream habitats and high use areas. If you have hemlocks in your yard and want to treat them, you have 2 options. The adelgid is difficult to control because the fluffy white coating protects them and their eggs from pesticides. One control method for small trees is to hit them with insecticidal soap or horticultural oil before the crawlers form the waxy coating. Both are sold at garden centers under many different name brands. Application will be most effective from late march through April and also in late August to October. Horticulture oil should not be applied during the early growing season, as it could burn the needles.
The other control method that is best for large tree is to apply an insecticide to the soil that is taken up by the tree’s root system and poisons the adelgids as they feed. The active chemical is called imidacloprid, and is sold under names like Merit, Marathon, and Bayer Advanced. Closely follow label directions. Hopefully some natural controls will kick in and help reduce the damage this pest can do. Releasing predator beetles that eat adelgids is showing some promise in places.