INTRODUCED TREE DISEASES AND INSECTS

G. F. GRAVATT, D. E. PARKER.

Many people now are asking : Are there more insects and diseases than before? How does it happen that in a few years we have suffered scourges of gypsy moths and Japanese beetles and many others that were not here before?

The answer is that we are plagued by more insects and diseases and more destructive ones than our grandfathers were. The reason is easy to find.

Some of our present-day kinds of trees (as indicated by fossil remains, flourished in North America millions of years ago; trees and their parasites must have fluctuated in abundance long before the coming of the white man. Then, as now, periodic epidemics must have caused extensive losses but when the trees were attacked they usually could maintain themselves against borer and beetle.

This natural balance was upset by a new factor: The early settlers, who brought in new diseases and new insects along with their new plants. Some tree pests now considered native no doubt originated in foreign countries. Many serious diseases and insects are known to have come from abroad during the past 60 years, the entire span of any real study of the diseases and insects of tree species in North America. The end of such invasions is not in sight. All over the world disease-producing organisms and insects are lurking, ready to hitchhike to this country and pounce on our important forest and shade trees.

Before the enactment of our plant-quarantine laws, the gypsy moth, chestnut blight, and white pine blister rust were introduced. Since the enactment of the laws, the so-called Dutch elm disease has sneaked in. Other less well-known foreign diseases and insects also have been introduced and are attacking various kinds of trees.

In their native homes, many insects are kept under partial control by their parasites and other natural enemies, but when they are introduced into some other area they usually leave these enemies behind. For example, when the Japanese beetle and the gypsy moth reached this country, they multiplied rapidly, partly because of a lack of natural enemies. Insect, fungus, bacterial and virus parasites of these introduced insects now are being imported, but the parasites are valuable only when the environment favors their development. Unlike insects, the organisms causing our introduced diseases do not have any important parasites that directly affect them, although parasites may be used to reduce the populations of the insects that transmit certain of those diseases.

THE GYPSY MOTH illustrates the serious consequences of the introduction of a forest insect from Europe. In 1869 a number of egg clusters of the gypsy moth were brought from France to Medford, Mass., by a French mathematician and astronomer who hoped to develop a hardy silk-producing insect by crossing gypsy moths with silkworm moths. During his experiments some of the insects escaped. Some 20 years later the population of the gypsy moth had increased to a point where the damage was severe enough to attract general notice. At that time about 360 square miles was found to be infested. Within another 5 years, the infested area had increased to 2,200 square miles. Now the gypsy moth, which defoliates both deciduous and evergreen trees, is prevalent in New England, in an extensive area in eastern New York, and in an isolated area in Pennsylvania.

At least 65 million dollars have been spent by the Federal Government and various States, chiefly during the past 40 years, in fighting the gypsy moth. The main objective of the Federal control work, conducted in cooperation with the States, is to prevent the westward and southward spread of the insect. The discovery of the extreme toxicity of DDT to the gypsy moth and the development of airplane spraying of forested areas have furnished effective means of control to aid in the program. Timely applications of DDT by airplane will kill the gypsy moth, and prevent defoliation, subsequent growth retardation, and possible death of trees.

CHESTNUT BLIGHT has caused the complete destruction of our commercial chestnut from Canada to the Gulf States. This record is not approached by that of any other disease or insect. First reported in New York City in 1904, the disease spread rapidly.

For many years roots of killed trees continue to send up sprouts, but these sprouts are usually killed before they are more than a few inches in diameter. Unfortunately, search for 40 years has not resulted in the discovery of a single American chestnut tree with sufficient resistance to be of practical value. Blight has reduced millions of acres of forest land to a lower productive status for an indefinite period, because the native tree species replacing the chestnut are usually less valuable. It also has deprived us of cherished tasty nuts and has taken from wildlife a food.

Experimental plantings with blight-resistant Asiatic chestnuts and with hybrids of these and the American chestnut indicate that on suitable sites they will produce small telephone poles and abundant sweet nuts. Most of these resistant selections, however, are less straight-stemmed, less frost-resistant, and more particular in their soil requirements than the American chestnut. Some State forestry and game departments are beginning to grow resistant Chinese chestnuts for planting in farm wood lots.

Chestnut blight was found in commercial orchards and in ornamental chestnut plantings of the Pacific coast. Prompt eradication measures by State and Federal agencies almost completely eliminated the disease. The susceptible orchards of the West, however, are not safe, because of the danger of shipment of infected chestnut trees from the East.

Chestnut blight illustrates how an introduced pest can upset a phase of the national economy. The American chestnut has been the main source of our domestic tannin used in the manufacture of leather, and dead trees still are extensively used. Tannin, a strategic material especially vital in time of war, is extracted from the chipped-up chestnut wood. The chips are then used for paper or board pulp. This extensive industry, at present supplying most of our domestic tannin, faces its end when the supply of dead trees gives out.

The chestnut blight fungus is also seriously damaging the post oak, a widely distributed tree in the eastern half of the country with a forest stand of about 5 billion board feet. It kills some trees rather slowly but has not damaged others that have been exposed for long periods. So far no other kind of oak has been seriously damaged by the chestnut blight fungus.

THE SMALLER EUROPEAN ELM BARK BEETLE is an example of an introduced insect that was of little importance until it became associated with the introduced so-called Dutch elm disease fungus. That insect is known to have been established near Boston as early as 1904. It did little damage and was not considered a primary pest for about a quarter of a century. About 1930, when the Dutch elm disease fungus reached this country, the importance of the European elm bark beetle changed; it proved to be a carrier and transmitter of the fungus. The relationship worked to the advantage of the bark beetle. American elms inoculated by contaminated beetles develop disease symptoms, are partially or completely killed by the disease, and provide suitable breeding material on which increasing populations of beetles develop. A vicious circle thus has resulted from the relationship between the fungus and the insect.

The elm beetle unquestionably was introduced through different ports. It and the fungus were present in burl elm logs imported for veneer manufacture before quarantines prohibited the movement of elm wood into this country. Beetles and larvae have been found in elm wood used in certain types of crates received from Europe. A larger species of beetle, also a carrier of the Dutch elm disease fungus in Europe, has been introduced into this country in burl logs, but apparently it has not been successful in establishing itself here.

It is practicable to protect valuable trees from the Dutch elm disease where control measures are applied energetically, but losses are heavy in parts of New York, New Jersey, Pennsylvania, and Connecticut, and spot infections are known as far west as Denver. We may expect that these two pests and phloem necrosis, a virus disease, eventually will kill most of the elm forest growth in the northeastern quarter of the country.

The death of shade and ornamental elms is even more tragic. It is disheartening to all of us to see the large elm trees, so characteristic of New England and New York, decline and die. Sometime in the not too distant future, when the total value of the elms killed and the annual costs of removing dead trees and of spray and other control measures for those still alive are totaled, a loss figure of hundreds of millions of dollars is not unlikely.

A EUROPEAN-ASIATIC FUNGUS that causes white pine blister rust entered the country some 50 years ago on imported white pine seedlings. Although this fungus cannot spread from pine to pine but must first attack an alternate host in this case currants and gooseberries it found plenty of the hosts here. Thus it was able to complete its life cycle and spread widely. Its dependence on currants and gooseberries, however, proved its partial undoing, because spread of the disease can be stopped by removal of the plants within 900 feet of white pine.

Whitebark pine, a picturesque member of the white pine group that grows at high altitudes in the West, usually does not have sufficient commercial and esthetic value to justify the cost of removing the numerous wild currants and gooseberries near them. Thus, most of the trees of this species will be killed by the rust and many park and wilderness areas will become less interesting. Several other high-altitude species of white pine may be largely killed in the future.

OTHER INTRODUCED INSECTS damage our forest and shade trees. Among them are the brown-tail moth, satin moth, European pine shoot moth, elm leaf beetle, European pine sawfly, and the European spruce sawfly.

Various other diseases also have been introduced or are suspected of having been introduced. Not all introduced diseases become established. The European larch canker, for example, was introduced into Massachusetts, but it spread slowly and was successfully eradicated. We do not know how it would act in the main larch stands of this country.

A canker disease from Asia and a scab from Europe are causing serious damage to some kinds of willows, especially in New England. Twig and leaf diseases do not excite so much interest, but their action is a perpetual drain on the productivity of the affected trees.