Tiny woolly adelgid big problem for hemlocks
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Q. I have several large hemlocks that really make my yard a haven. I have been hearing about an insect -- hemlock adelgid, I think -- and am concerned about my trees. Can you tell me more about it and what I can do to protect them?
A. Hemlock woolly adelgid (Adelges tsugae) is an insect pest introduced from Asia that has been a problem in southeastern Pennsylvania since the mid-1960s. They have spread westward in spite of the prevailing winds and now cover about two-thirds of our state. Adelgids have devastated Canadian hemlocks throughout the northeast and mid-Atlantic states since they were first identified in the early '50s.
This destructive insect has now been identified in various parts of Western Pennsylvania, including Pittsburgh, Fox Chapel, Mt. Lebanon, Marshall and Ligonier. It is quite likely that other infestations are present, although they have not been identified yet. In areas where they are prevalent, Canadian hemlocks (Tsuga canadensis) and Carolina hemlocks (T. caroliniana) are no longer recommended for planting.
Hemlock woolly adelgids cause damage by sucking sap from host trees. They are equipped with piercing-sucking mouthparts that enable them to sip tree sap much the way you drink through a straw. They inject a toxin as they feed, adding insult to injury. Infested trees lose vigor and drop needles prematurely. This leads to reduced growth and dieback of major limbs. Severe infestations can kill a mature tree in about four years. Fortunately, this pest is reasonably easy to control in the landscape. The real devastation of our beloved state tree occurs in the woods, where it is much more difficult to control.
Adults are small aphid-like insects, less than 1/16 inch long. Slate gray in color, they are all female and able to reproduce asexually, a phenomenon known as parthenogenesis. Two generations of this pest mature annually in Pennsylvania. Adelgids overwinter as mature females and begin to lay eggs in late March.
Immature nymphs known as crawlers begin to hatch sometime in mid-April. Crawlers are reddish-brown and extremely tiny, less than .3 millimeter. They may leave their original host plant and move to another hemlock, or they may stay where they hatched. In either case, they soon settle on twigs near the base of the needles and begin feeding. They remain in place for life once they settle. (Some individuals require a specific alternate host to complete their life cycles. This alternate host is a variety of spruce not found in the United States, and those individuals die.)
First Published May 15, 2010 12:00 am
