This Hemlock insect pest is spreading

Hemlock woolly adelgid

By: Sandy Feather 2006
Penn State Extension

Q. My neighbors are having their trees treated for an insect called hemlock woolly adelgid. I have quite a few hemlocks on my property and would like to know more about it.

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. They have devastated Canadian hemlocks throughout the Northeast and mid-Atlantic states since they were first identified in the early 1950s.

In areas where they are prevalent, Canadian hemlocks (Tsuga canadensis) and Carolina hemlocks (T. caroliniana) are no longer recommended for planting. 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.


Cause of Hemlock Damage

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. In turn, this leads to reduced growth and dieback of major limbs. Severe infestations can kill a mature tree in about four years.

Tightly sheared Canadian Hemlock
Tightly sheared Hemlock anchors the
corner of this foundation planting

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.

Adelgid Description

Hemlock woolly adelgid adults are small aphid-like insects, less than 1/16-inch long. They are 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.

Overwintering Pests

They 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 0.3 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, where they insert their piercing-sucking mouthparts and begin feeding. They will 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 enter an unusual summer dormancy, and their development is delayed until the onset of cooler weather in October. They mature and overwinter to begin the cycle anew the following spring.

Adelgid Identification

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 actually a waxy covering produced by the settled life stages to protect themselves and their eggs from predators and from drying out.

The cottony-looking masses persist on the trees even after the insects are dead, for up to a year. New infestations will have a very white color 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.

Canadian Hemlock
Tsuga canadensis
By trimming them early in the season
they can be maintained without
having a formal sheared look.

Adelgid Control Options

There are a number of options to control them and keep hemlock trees healthy. Avoid placing bird feeders in or near hemlocks so as not to encourage birds to spread them to 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.

It is also helpful to promote the health of hemlocks by providing good growing conditions. Protect them from drought stress by watering in hot, dry weather. An inch of water weekly during such weather should be sufficient.

Be Careful with 'Helpful' Fertilizer Applications

Avoid the temptation to "help" infested trees by fertilizing until the infestation is under control. While fertilization can be helpful to maintaining the vigor of trees, high-nitrogen fertilizers make fertilized hemlocks more susceptible. 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.

VIDEO: Trimming a Hemlock Naturally

Chemical Controls

There are a number of chemical control options to fight hemlock woolly adelgid. 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 where it is taken up by the root system and 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 greater than 8 inches in diameter at breast height. It should be applied in early spring (mid-March) for trees less than 8 inches in diameter.

It is important that the soil is not frozen or waterlogged prior to application. Since imidacloprid is taken up with soil moisture, it is also important that adequate soil moisture is present. If that is not the case, be sure to irrigate the tree thoroughly prior to application.

You can hire a certified arborist to make soil drench applications or trunk injections of systemic product such as Merit. If you are not sure of what you are seeing on your hemlocks, 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.

Biological Controls for Adelgid

Forest entomologists have been researching biological control agents in order to preserve hemlocks in our forests. Two insects in particular, a mite and a ladybird beetle imported from the hemlock woolly adelgid's indigenous range, show promise. In the meantime, cold winter weather is very helpful in reducing the numbers of this pest, so get out there and do your best snow dance!
UPDATE: Researchers are also testing different species of hemlocks for natural resistance to hemlock woolly adelgid. So far the Chinese hemlock, Tsuga chinensis shows the most promise although it is not yet widely available.


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