A few weeks ago, a lunch-time radio program featured a walk through Harvard Forest in Petersham and discussed the invasion of the non-native wooly adelgid insect into Massachusetts and its devastating impact on the hemlock trees in the eastern United States. The dire forecast had me wondering if and when the extensive hemlock stands in our town would soon meet the same fate as the chestnut or elm, since hemlock stands as near as Connecticut are experiencing devastating mortality.

Hemlock trees are conifers that tend to prefer cool moist woods, typically ravines and slopes where other trees find insufficient light to thrive. In the right conditions, they can form nearly pure stands. They are among the longest-lived trees in our forests and can reach up to 70 feet tall, although most in Harvard are significantly shorter. The most obvious stands of eastern hemlock in Harvard can be seen along the dirt portion of Warren Avenue, and on Shaker Road between Shaker Village and the golf course. You can find some beautiful stands on town-owned conservation land. The great stone outcroppings, ledge, and hemlocks form a fascinating landscape along the Holy Hill, Black Pond, Hermann, Kaufman, Maxant, Muller, Ohlin, and Prospect Hill trails.

The hemlock wooly adelgid (adelges tsugae) is believed to have been accidentally introduced into the United States as early as 1924 into the Pacific Northwest from Asia, and first detected in the eastern United States in Pennsylvania in 1967. Since its Massachusetts debut in Springfield in 1989, its range has expanded to include every county in the state (except for Nantucket, where there are no hemlocks), and has been detected as far north as southern Maine. The insect gets its name from the cottony egg sacs laid on the underside of the branches. The insect feeds on the fluid at the base of the needles, weakening the tree. After several years of this weakening, the tree loses needles and branches die back. The trees can survive in a weakened state for years, but eventually succumb. Signs of initial stress include dull grayish-green, rather than shiny dark green, needles. The tops of the trees usually show stress damage first.

The adelgid tends to be intolerant of extreme cold, and a large percentage of the nymphs won’t survive during very cold winters. But since the insect reproduces so rapidly, this natural control is only temporary, and is not a long-term deterrent. It is expected that the northernmost range of the hemlock may be effectively controlled by weather, unless the adelgid adapts, or climate warming reduces the frequency of cold weather.

What are the effects of the loss of this species? For one, the ecology of the area will change. Certain birds, like the black-throated green warbler, nest and forage in hemlocks. Concerns that rapid die-off would lead to erosion have not materialized. In Connecticut and New Jersey, where substantial die-offs have occurred, other hardwood species, such as gray birch, rapidly take hold. Some have speculated that eliminating the shade provided by these trees may raise water temperatures in streams, making them inhospitable to some fish, like trout. But the largest impact may be the loss of yet another species in our ecosystem, and a particularly visually dramatic part of the landscape altered.

It is doubtful that anything can be done to spare Harvard the large-scale mortality that is occurring in other areas. Some areas are experimenting with an Asian ladybug that feeds exclusively on the wooly adelgid. Initial results are promising, but uncertain. Also, the cost of application, even in small scale, seems prohibitive. At more than $150 per tree for chemical treatment, and $2 per ladybug, focus on saving important specimen or landscape trees is possible, but costly. Some efforts have been made in Purgatory Chasm in Sutton and Mount Tom in Holyoke to control the adelgid using dormant oil spraying. Results of spraying have been disappointing, possibly due to poor application coverage (how can you effectively spray a forest?) and the need for frequent application. The ladybugs have also been released in several locations, including Walden Pond, Blue Hills, and Hemlock Gorge in Newton. Over time, we’ll learn about their efficacy.

Given the progression of this infestation, and the possibility that our local hemlocks will eventually follow the same course as the chestnut and the elm, appreciate them now in their unique setting, before their inevitable decline.