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.)
As settled crawlers mature, they begin to lay eggs, and a second generation of crawlers is active by mid-July. These cool-weather pests go dormant through the heat of summer. Their development is delayed until the onset of cooler weather in October. They mature and overwinter to begin the cycle anew the following spring.
What you are likely to notice is a profusion of white, cottony-looking masses on young hemlock twigs at the base of the needles. This is a waxy covering produced by the settled life stages to protect themselves and their eggs from predators and from drying out. The cottony masses persist on the trees even after the insects are dead, for up to a year. New infestations will have a very white color to the waxy covering, while old, dead ones will be grayish-white. Hemlock woolly adelgids are spread by the wind, birds, squirrels, deer and other animals, as well as by people moving infested nursery stock.
There are a number of options to control hemlock woolly adelgids and keep hemlock trees healthy. Avoid placing bird feeders in or near hemlocks to discourage birds landing on uninfested trees. If you visit natural areas in the eastern United States where hemlock woolly adelgid is prevalent, wash your vehicle and clean camping equipment thoroughly before returning home. This is most important from March through June, when eggs and crawlers are most abundant and likely to be blown onto your belongings. The white, waxy covering makes them stick to things.
Protect hemlocks from drought stress by watering them during hot, dry weather. An inch of water weekly should be sufficient. Avoid the temptation to "help" infested trees by fertilizing until the infestation is under control; high-nitrogen fertilizers actually make hemlocks more susceptible to adelgids. The nitrogen content is more nutritious for the insects, and can cause their reproductive rates to skyrocket. That said, once an infestation is under control, moderate fertilization can help a tree recover.
There are a number of chemical control options. If you have smaller trees that can be sprayed thoroughly, properly timed applications of horticultural oil and insecticidal soap can provide excellent control of crawlers. Timing is everything, because they are impervious to sprays during their summer dormancy. September through October is the best time to apply these materials. They can also be applied mid- to late June to reduce the number of developing crawlers.
For larger trees that are hard to spray thoroughly, Bayer Advanced Tree & Shrub Insect Control containing Merit insecticide (imidacloprid) provides season-long systemic control. It is applied as a soil drench and is translocated throughout the tree. Merit moves rather slowly, so the best time for application depends on the size of the tree. It should be applied in fall (late October through early December) for trees more than 8 inches in diameter at breast height (41/2 feet from the ground on the uphill side of a tree).
It should be applied in early spring (mid-March) for trees less than 8 inches in diameter. Make sure the soil is not frozen or waterlogged prior to application. Because imidacloprid is taken up by the roots, it is also important that there is adequate soil moisture. If that is not the case, be sure to irrigate the tree thoroughly prior to application.
You also have the option of hiring a certified arborist to make soil drench applications or trunk injections. If you are not sure you have an infestation, consider hiring a knowledgeable professional to diagnose and treat the problem. Although hemlock woolly adelgid is a major pest of hemlocks, it is not the only problem they have.
Forest entomologists have been researching biological control agents to preserve hemlocks in our forests. Two insects in particular, a mite and a ladybird beetle imported from the adelgid's indigenous range, show promise, and the research continues.
Send questions to Sandy Feather by e-mail at
or by regular mail c/o Penn State Cooperative Extension, 400 N. Lexington Ave., Pittsburgh 15208.