Hemlock Woolly Adelgid

This insect pest is decimating Hemlocks


Q. I have quite a few hemlocks on my property and understand that hemlock woolly adelgid has been found in forested areas nearby. Should I have my Hemlocks treated for Hemlock woolly adelgid?

A. Hemlock woolly adelgid (Adelges tsugae) is an insect pest introduced from Asia that has been a problem in southeastern Pennsylvania since the mid-sixties.  They have spread westward in spite of the prevailing winds, and now cover about two-thirds of Pennsylvania. Hemlock woolly adelgids have devastated Canadian hemlocks throughout the northeast and mid-Atlantic States since they were first identified in the early 50's. In areas where they are prevalent, Canadian hemlocks (Tsuga canadensis) and Carolina hemlocks (Tsuga caroliniana) are no longer recommended for planting.

Geographic locations of adelgid damage

This destructive insect has now been identified in various parts of western Pennsylvania, including the City of Pittsburgh, Fox Chapel, Mt. Lebanon, Indiana Township, Marshall Township and Ligonier. It is quite likely that other infestations are present, too. 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, on a very tiny scale. 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. Fortunately, this pest is reasonably easy to control in the landscape. The real devastation of our beloved state tree occurs in the woods, where control is much more difficult.

Adelgid insect identification

Hemlock woolly adelgid adults are small aphid-like insects, less than 1/16th 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. Hemlock woolly 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 in color and extremely tiny, less than .3mm

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. 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 a 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.

Stately hemlocks growing in Cook Forest, Pennsylvania

Stately hemlocks growing in Cook Forest, Pennsylvania

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 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.

Perform a thorough inspection

You should examine your trees to see if hemlock woolly adelgids are present, or hire a certified arborist to examine them for you.  If they are present, have the trees treated. If not, follow the cultural guidelines below for keeping the trees healthy, but hold off applying insecticides until there is actually a problem. Scout your hemlocks on a regular basis so that you can start treating them promptly if the need arises.

Timing of Adelgid Controls

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 avoid encouraging more birds to land 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.

Quality Care

It is also helpful to promote the health of hemlocks by providing good growing conditions. If your lawn grows right up to the base of your hemlocks, consider removing the grass and applying a 1 to 2 inch layer of mulch to reduce competition for water and nutrients. Protect hemlocks from drought stress by watering them when we get into hot, dry weather to maintain their vigor. An inch of water weekly during such weather should be sufficient.

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 actually make fertilized hemlocks more susceptible to hemlock woolly 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.

Chemical Controls for Adelgids

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.

Sheared hemlock in a residential landscape

Sheared hemlock in a residential landscape

For larger trees that are hard to spray thoroughly, Bayer Advanced Tree & Shrub Insect Control containing Merit insecticide (imidacloprid) provides season-long systemic control of hemlock woolly adelgid. It is applied as a soil drench where the product is taken up by the root system and translocated throughout the tree. Merit moves rather slowly throughout the tree, 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 over 8-inches in diameter at breast height (DBH, measured at a height of 4½ feet from the ground on the uphill side of a tree). Trees less than 8-inches DBH should wait until early spring (mid-March). 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.

Other Adelgid Control Options

You also have the option of hiring a certified arborist to make soil drench applications or trunk injections of systemic products 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, nor is the only one that appears to be white and fuzzy.

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.  Research continues to find effective and practical means to control this destructive pest in Pennsylvania's woodlands.


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