Through the past few decades it has seemed that the forest stretching across northern Pennsylvania has been under fire by a steady stream of new invaders from across the oceans. Some are insects, some are fungi. It started long before, most notably with the chestnut blight which all but wiped out what had been called the greatest tree on Earth, the American chestnut. More recently, an ominous pest, the hemlock woolly adelgid, is threatening our state tree.
The magnificent American chestnut tree was so numerous that while in bloom the hills looked like they were covered by fresh fallen snow. The nuts they produced had long been a major food item for Native Americans, European settlers and many wildlife species. Its wood was the finest, beautiful and long lasting.
I was told by a man who lived before the chestnut blight that when we still had the American chestnut, squirrels weighed 4 pounds, and more, giants compared to the gray squirrels of today.
Just a few years after 1904, when the chestnut blight, a fungus, was found in the U.S., the American chestnut ceased to be a significant element of our forest. It made its way to North America on nursery stock which was imported from Asia.
Almost as devastating was Dutch elm disease. This fungus rode a load of elm trees, in 1931, from France to Cleveland, Ohio. It attacks and kills elms, particularly the American elm. By the 1980s, it was estimated that Dutch elm disease had wiped out 77-million American elms.
One of the more popular uses for the American elm was lining streets in towns and cities. That was the most noticeable place the tree was missed.
Even though this disease kills American elm trees in as short a time as one year, the American elm remains a significant component of bottom land forest areas. A resistant strain of the original American elm is now available.
In the news now is the hemlock woolly adelgid. It was first observed in the U.S. in the 1920s, but only on the West Coast. It may even be native to that area. The hemlock woolly adelgid is a small insect that feeds on nutrients stored in hemlock native to Japan, Taiwan, China and India. In its native range it is a harmless inhabitant of several hemlock species, as it is to native species on the West Coast from northern California to Alaska.
But once it made its way to the East Coast, in the 1950s, everything changed. To the eastern hemlock, a major component of eastern forests, it is fatal.
Hemlock woolly adelgid is not only a threat to eastern hemlock, it is a threat to wildlife that benefits from eastern hemlock. Most noteworthy is its link to brook trout, and thus to trout anglers. Eastern hemlock habitat includes wet soils such as might be found along streams, where it provides shade which is a significant factor in keeping these streams cool enough for brook trout.
Coincidentally, eastern hemlock is the official Pennsylvania state tree, and the brook trout is the official Pennsylvania state fish. High social and economic values to the Commonwealth of Pennsylvania are evident by these designations.
The first place it was identified in the eastern U.S. was Richmond, Virginia. Here in the East, hemlock woolly adelgid is a serious pest. It threatens the very existence of eastern hemlock. It also attacks Carolina hemlock, which is a rare ornamental species.
Hemlock woolly adelgid is barely big enough to be seen by the human eye. It does its damage during winter when it uses piercing and sucking mouth parts to extract sap from the twigs of hemlocks. It is believed that the insect injects a toxic substance into the tree which damages the vascular system, eventually causing death.
The hemlock woolly adelgid is unlike most insects because it is dormant through much of summer while it is in its nymph stage. Through fall and winter the nymphs develop into adults. Egg masses appear in March. Small, white egg masses along the bases of needles are the most noticeable evidence of the insect.
If a hemlock tree is already stressed, such as those trees which grow in less than perfect habitat, it might die from this disease in three years to five years. Healthy trees in a forest can last longer, perhaps 10 years.
Once the hemlock woolly adelgid enters an area, it is probably there forever, so hemlock trees should be closely watched. They should be checked several times each year.
The insect is naturally spread by wind and birds. It can also be spread by people through means including transporting fire wood, or trees used as ornamentals.
Hemlock woolly adelgid was first identified during the 1960s in Pennsylvania. Then in 2013 it was found in Cook Forest State Park and Clear Creek State Park. It also has made its way to various places in the Allegheny National Forest including Webbs Ferry, along the Allegheny River and in the Tionesta Research Natural and Scenic Area.
The best way now of dealing with the hemlock woolly adelgid is to closely watch hemlock trees, then deal with infestations on a tree to tree basis. This is quite expensive and work intensive. However, if left untreated infested trees will spread the insects to other hemlocks.