Try these options to save hemlocks from Asian insects
March 6, 2010 10:00 AM
Linda Spillers/Associated Press
Spraying and other techniques can save hemlocks from the destructive infestation of the woolly adelgids (white clumps, in center).
By Sandy Feather
Q. I live in Indiana Township and have hemlocks on my property. I understand that hemlock woolly adelgid has been found at Hartwood Acres. Should I have my trees treated?
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 and now cover about two-thirds of our state.
Since they were first identified in the early '50s, adelgids have devastated Canadian hemlocks throughout the Northeast and mid-Atlantic states. In areas where they are prevalent, Canadian and Carolina hemlocks (Tsuga canadensis and 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, Indiana, Marshall and Ligonier.
Hemlock woolly adelgids cause damage by sucking sap from host trees. 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.
Adelgid adults are aphid-like insects less than one-sixteenth-of-an-inch long. Slate gray in color, they are female and able to reproduce asexually. 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 0.3 millimeter. They sometimes leave their original host plant and move to another hemlock. They 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.
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 masses persist on the trees even after the insects are dead. 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. You should examine your trees to see if 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.
There are a number of options to control adelgids and keep hemlock trees healthy. Avoid placing bird feeders in or near hemlocks. 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.
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. 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. 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. 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 more than 8 inches in diameter at breast height (41/2 feet from the ground on the uphill side of a tree).
Trees less than 8 inches in diameter should wait until early spring (mid-March). It is important that the soil is not frozen or waterlogged prior to application. Imidacloprid is taken up with soil moisture. If the soil is not moist, 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 products such as Merit. If you are not sure of what you are seeing on hemlocks, hire a knowledgeable professional. Although woolly adelgid is a major pest of hemlocks, it is not the only problem they have, nor is it the only one that is white and fuzzy.
Send questions to Sandy Feather by e-mail at
or by mail c/o Penn State Cooperative Extension, 400 N. Lexington Ave., Pittsburgh 15208. First Published March 6, 2010 5:00 AM