Information Center

  • Garden Knowledge

Hemlock Woolly Adelgid

The hemlock woolly adelgid is a small, aphid-like insect which attacks both the Carolina and eastern hemlock and is capable of severely weakening and killing its host plants. It was introduced into Massachusetts in 1988 from an already existing infestation in Connecticut.

Identification and Life Cycle
These small insects are brownish-red in color, oval in shape and about 0.8 mm in length. They are inactive for much of the growing season and very active throughout the winter. In mid-October, they resume feeding; adults with new egg-masses appear beginning in March resembling small, white cotton balls lined up at the base of tree needles.

Description of Damage
Adelgids damage trees by sucking sap from the twigs. The tree loses vigor and prematurely drops needles to the point of defoliation, which can lead to death. If left uncontrolled, the adelgids can kill a tree in a single year.

Application of horticultural oils is a recommended treatment for adelgids. It can be applied as either a dormant oil spray and/or as a summer spray. Complete coverage of the tree is essential. Oil may need to be reapplied later in the growing season. Bayer Tree & Shrub liquid or granules applied in late March/early April can also be helpful. It is important to kill all of the adelgids on a tree or they will soon reappear.