Hemlock Assassins

New growth on a healthy hemlock.

Off the corner of our property on the east side, and on the boundary of the west, are a couple of hemlock trees. I love these trees. Their finely-textured needles, which emerge all soft and fuzzyish, create an elegant evergreen refuge for songbirds. The smaller of these trees, the one on the west side of the lot, is doing very well this year, with at least three inches of new growth emerging from the tips of all its branches. It has draped itself over the slope, its lower branches creating a skirt of protection over the songbirds that spend more time on the ground than in the branches.

The larger trees in a cluster on the northeast corner of the property are not faring as well. I had looked at the trees last year during the summer but hadn't seen very many of the dreaded woolly adelgid (ah-DEL-jid). This year, I knew we had a problem. With the benefit of a younger, healthier tree to compare to, it was easy to see the less robust new growth, the "grayer" color, the thinning "hair" at the crown. A tree showing signs of male pattern baldness, if you will. On closer inspection, the woolly-coated insects were definitely in evidence.

Checking for eggs under the woolly protective layer.

Timing evidently played a role in my inspections. During the summer heat, just as in the depths of winter cold, both the hemlock and its parasite can enter a period of dormancy. If spring/fall has been a windy affair, the "woolly" coating--secreted by the adelgids--can be knocked off or minimized--making an infestation less evident. We always get wind.

I am not about to give up on these trees. They are the only true native evergreens in my immediate perimeter and are essential to the habitat of my little quarter acre. Not to mention that a dead tree that size would cost me a small fortune to have removed. (Leaving it would be an option only for a short time, since it would threaten TWO structures.) Fortunately, I knew who to turn to. In our neck of the woods,* you call the Hemlock Savers!

Hemlock Woolly Adelgids kill hemlocks primarily by the typical sucking of sap (think aphids on steroids) from the base of the needles. In mammal terms, the impact is like a flea infestation that bleeds an animal dry. Systemic insecticides are the only guaranteed method of elimination of the Hemlock Woolly Adelgid (HWA), and these insecticides will have to be reapplied as appropriate. Homeowners can treat small trees (under eight feet tall) themselves, but larger trees must be handled by a state licensed pesticide applicator. Foliar sprays (horticultural oils or insecticidal soap) can be used for some immediate relief for the tree, but will not provide lasting protection, which is why systemics are the preferred method of dealing with the invasive species.

To help protect hemlocks on your property, keep bird feeders away from them, if possible. Adelgids are carried by wind, birds and animals. Also, since drought increases the rate of decline of infected trees, you can mulch and water the trees. Do NOT fertilize an infested hemlock. New, succulent growth will only increase the impact of the adelgid. If a small hemlock on your property shows evidence of infestation, use Bayer's Advanced Tree and Shrub Insect Control liquid, which will give one year of systemic protection. A tree with no adelgids cannot spread them to other trees! There is no point, however, in "preventive" insecticide application. A waste of time and money.

Checking the health of the crown of the hemlock.

The recommended insecticides for professional application are Imidacloprid and/or Dinotefuran. Treatments are likely to start at around $45 to treat a 10" diameter tree by soil injection. I recommend that you locate a professional to assess your trees and advise you on the best treatment for your level of infestation/location. This is not a one-size-fits-all solution and it would be unwise to start throwing down poisons willy-nilly if your hemlocks are already past the point where they can be saved. The good news is that choosing a treatment of one or both of these pesticides is likely to protect your trees for a minimum of three years.

Some of you will no doubt be curious why I have not mentioned the imported predator beetles (

Sasajiscymnus tsugae

) as a method of control. Start with the $4.25 per beetle price tag. (The only place this option becomes economically sound is for large stands of trees when the cost of insecticide gets higher than the cost of beetles, which goes down with higher volume purchased.) My second reason is the nature of predators. Natural predators do not seek to eliminate their food source--they harvest only what they need, essentially planning for future meals. Which means that plenty of HWAs would remain to reproduce and infest other trees. One female = 300 eggs annually = 90,000 new adelgids a year. For the homeowner, systemics are the best option to protect the species until such a concentration of beetles has been reached in your area that the prey species will only rarely kill the host tree. Given funding levels for beetle releases, this concentration is likely to take awhile.

The Eastern Hemlock (

Tsuga canadnesis

) is the most shade tolerant of all eastern trees. Saplings have been discovered that measure between two and three inches in diameter

but are in excess of 200 years old

--just waiting for the moment when the giants above them fall to old age or disease. They are incredible trees--please check out the first link below for extensive information on the species, as opposed to its assassin.

*Our neck of the woods: Hemlock Savers will treat hemlocks in Buncombe, Henderson and Transylvania counties of Western North Carolina. My thanks to David for his patience in explaining the impacts of this insect and how to thwart it. Contact information is on the

Brevard, NC



Amherst: The Hemlock Woolly Adelgid


Fifth Symposium, HWA in the Eastern US


Recommendations for Hemlock Woolly Adelgid Control

, NC State: 

Hemlock Woolly Adelgid


A Threat to Hemlock in Tennesee

Cower, Aphids!

Cower, Aphids!

Made for Shade: Green and Gold

Made for Shade: Green and Gold