Invader kills native trees

Unlike the tree it uses for sustenance, HWA is tiny (0.8 mm long) and has a short lifespan. To feed, this aphid-like insect settles at the base of newly formed hemlock needles and stabs the plant tissue with a feeding stylet, a hair-like feeding tube three times the insect’s length. It will feed on the cells that transfer and store nutrients for the hemlock until the needles desiccate.

“The hemlock woolly adelgid reproduces asexually,” Salom explains. “This is good news for us because it means the adelgid doesn’t have the genetic diversity needed for adequate resistance to a new predator. But this also means the insect can reproduce at an incredibly high rate.”

Like other adelgid species, A. tsugae has a complex life cycle. Each year, three generations develop on the hemlock. Two generations begin in April: a wingless, asexual form that remains on the hemlock and its winged, sexual counterpart that flies away in search of spruce. Having never fully adapted to life in North America, the sexual population dies quickly, never finding a suitable host and instead leaving the all-female, asexual generation to perpetuate the species.

In May and June, the adults that remain on the hemlocks lay 25 to 50 eggs under a white woolly material. When these hatch, the next generation begins. Young crawlers search for new hemlock needles and attach themselves to the base. These adelgids remain dormant until October, when they begin feeding and growing through the winter months. By March, HWA adults are present again, and the cycle repeats.

The hemlock woolly adelgid lays its eggs under this white woolly material it places on hemlock leaves. Photo by Josh Armstrong.