During the 1930s when the winds of war were brewing, there arose a concern that there might arise a shortage of silk if Japan were blockaded or if it became an adversary of the United States. To intercept this possibility scientists began to search for silk substitutes as they had been searching for rubber substitutes. Silk-making insects (mostly caterpillars and spiders which are actually arachnids) of many kinds were experimented with. A pair of gypsy moths escaped from a Massachusetts laboratory which began the out of control process that we are now experiencing with this species.
A voracious eater, a forest of oaks can be denuded in a matter of days. The trees can regenerate a second growth of leaves, but this places a severe metabolic stress on the plants. If they receive denudation for three consecutive years, even the mightiest oak will succumb. Many more will die even with only one or two seasons of denuding.
Different means of control have been employed with only limited and temporary success. Pesticides were widely used, but the insects became resistant to it and it also killed beneficial insects when applied. Sevin was the chemical of choice but is now seldom used. Eventually a bacteria strain was discovered which seemed to have qualities which killed the insects. What was more, this method seemed to only affect caterpillars and left others untouched. This strain, Bacillus thuringensis or BT is now the preferred method of control. It has serious drawbacks and shortcomings which render it difficult to apply as well as reducing its effectiveness.
It can only be applied within a very limited range of climatic conditions. It can only exist for enough hours for it to work properly if applied within a narrow band of temperatures. It also must only be applied under the correct humidity range. Furthermore, if there is a chance of rain within twenty fours hours before or after application, then the application would be washed away and be wasted. Finally, in order for it to work at its optimum capacity, it must be applied at least twice two weeks apart. This timing when combined with the narrow range of atmospheric circumstances necessary, makes it difficult to time it when the larvae are in the correct stage for the BT to have the desired affect. In short, this has not been a very successful method of control Gypsy moths.
Fortunately, population dynamics dictate that whenever a population explodes such as Gypsy moths do, other factors intervene to interrupt their life cycle. Eventually their population reaches a crescendo and there isn’t enough food for them to survive on. They become prone to starvation. This also weakens them which allows diseases to attack and kill them. This does cause them to be cyclic, only occurring in great numbers every so many years. But there is also an unexpected environmental factor which has appeared on the scene.
In 1989 the northeast was bracing for another year of high tree losses. Suddenly and out of nowhere, millions of Gypsy moth caterpillars began to whither and die. No reason was immediately discovered. It was later determined that an Asian fungus which acts as a natural predator against the insect, had arrived and was hard at work eradicating them. How it got here is still unknown. The fortunate thing is that with this added factor, it is hoped that Gypsy moths may never reach their earlier level of destructiveness.
When some refer to hemlock, they think of the poison hemlock (Conium maculatum), the deadly herb eaten by Socrates. This deadly weed can now also be found growing throughout much of North America (it looks much like Queen Anne's Lace which is a wild carrot). Here in the United States, hemlock is the common name for the conifer trees of the genus Tsuga. The species which is native to the northeast is Tsuga canadensis, or eastern hemlock. It used to be a prevalent tree throughout the east. The hemlock prefers cool, moist areas of the forest community. The dense, dark, and often extensive stands of eastern hemlock call to mind the coniferous forests of a more boreal climate.
The hemlock is one of the few tree species which are tolerant of shade. This offers the hemlock the ability to become successfully established even within a mature forest. Preferring moist soils, the hemlock has a shallow root system making it sensitive to droughts and prone to wind fall in exposed areas.
The average height of an eastern hemlock is over 60 feet although giants of 100 feet and more have been recorded. The oldest hemlock ever recorded was believed to be nearly 1000 years old!
In most of its range, the eastern hemlock is browsed by white-tailed deer and cottontail rabbits. The seeds and needles are eaten by ruffed grouse. Native Americans and woodsmen once used hemlock twigs and needles to make a tea which is a rich source of Vitamin C. For many thousands of years this beautiful native conifer prospered in the Northeast despite numerous types of assault. Some pests of hemlock are leafrollers, several species of spider mites, scale (including the elongate hemlock scale, Fiorinia externa) and needle miners (Coleotechnites sp.). Diseases which are known to infect the eastern hemlock include Cytospora and other canker diseases, blister and needle rusts, and leaf blight. Even the inconspicuous yellow-bellied sapsucker (Sphyrapicus varius varius) has inflicted serious injuries with is hole boring feeding activities. In the 19th Century, square mile after square mile of hemlock forest was destroyed to supply the northeast's tanning industry with hemlock bark tannin (as well as oaks).
The danger now faced by the eastern hemlock is the tiny, aphid-like insect, named the woolly adelgid (Adelges tsugae). This insect's native habitat is in eastern Asia where it is fairly innocuous to the Asian hemlock species, Tsuga chinensis, Tsuga diversilora, and Tsuga sieboldii. It was apparently introduced into the Pacific Northwest early in this century, having been first discovered in Vancouver, B.C. in 1922 and then in Oregon in 1924. Although present, adelgid damage to the resistant natives (western mountain hemlocks Tsuga mertensia) and western hemlock (Tsuga heterophylla) has been of little consequence to Northwestern forest hemlock populations. But in the Northeast, the situation is quite different.
First reported in the late 1960s in Pennsylvania, the hemlock woolly adelgid has slowly spread from the Smoky Mountains to Massachusetts. Hemlock woolly adelgid infestation is recognizable by the small, white cottony puffs of up to 3mm or more in diameter on hemlock the underside of the needles. These woolly little masses cover the adult female or her dead body and her eggs. The insect pierces the bark phloem tissues where the needles are attached and damages the tree by sucking sap. This late winter and early spring feeding activity causes the needles to discolor and to prematurely drop and the tree loses vigor. Severe infestations can kill the tree outright or weaken its defenses to fatal attacks from other insects or diseases.
It's generally agreed that the growth of adelgid populations, and the resulting damage to hemlocks, can be attributed to the fact that there are no naturally occurring parasites or predators of these insects in North America. While pesticide sprays have proven effective in adelgid control in small-scale settings, this management strategy would be impractical in forested areas. A natural enemy of Adelges tsugae may then be our best chance of control. As a result of research being conducted by Dr. Marc McClure of the Connecticut Agricultural Experiment Station (affiliated with Yale University), there may be hope of a biological means of control in the future.
Because the hemlock woolly adelgid is native to Asia, Dr. McClure explored mainland China and Japan for predators and parasites of these insects. He discovered five natural enemies that play an important role in keeping adelgid populations at low densities. Four insect predators, including a ladybird beetle (Coccinellidae), a green lacewing (Chrysopidae), a gall midge (Cecidomyiidae) and a flower fly (Syrphidae), were found to attack adelgids voraciously. It is, however, the fifth natural enemy that has really caught the researchers attention. An oribatid mite, Diapterobates humeralis was found at 17 forest sites and at 23 ornamental sites in Japan where it destroyed over 90% of all adelgid eggs. It was determined that these mites were not eating the adelgid, but were consuming the woolly material surrounding the eggs, causing them to fall to the ground beneath the hemlocks where they either dried up or were eaten by ants and spiders. Of particular interest were two of the ornamental sites where several Tsuga canadensis, our susceptible eastern hemlock, have grown vigorously in Japan for several decades in the presence of the hemlock woolly adelgid.
Healthy eastern hemlocks in the middle of "adelgid country" was enough to encourage further study and prompt the acquisition of state and federal permits to have Diapterobates humeralis and Pseudoscymnus (a ladybug beetle) imported from Japan to the federal quarantine facility in Ansonia, Connecticut. After tackling mountains of paperwork and fulfilling the rigorous requirements of federal environmental assessments, Dr. McClure is now rearing colonies of these natural enemies in his lab and has released Diapterobates humeralis in two experimental plots in Connecticut.
So far, the mites are reproducing successfully both in the lab and outdoors. They have also successfully overwintered at both release sites. Furthermore, the mites are consuming the woolly material and dislodging adelgid eggs just as they do in Japan.
As we have heard so many times before, the accidental release of pests and diseases on our continent has created many uncontrollable problems. Not the least of which has been the Africanized honeybee. In this case, it may literally be one which comes back upon its creator and bites him.
References

Smith, A. Home and Store News. Mahwah, New Jersey. 1993. Diseases and Pests Afflicting our Backyard Trees.
Tattar, T. Harcourt, Brace, Jovanovich. New York, New York. 1989. Diseases of Shade Trees.
Trotta, K. Museum Papers. Bear Mountain, New York. 1996. The Hemlock Wooly Adelgid.