Forests, Trees, and Pests

Insects and Spread of Forest-Tree Diseases

Curtis May, Whiteford L. Baker.

Insects spread several important forest- and shade-tree diseases. Chief among them are the blue stain fungi, the Dutch elm disease fungus, and the virus of phloem necrosis of elms. The relation of insects to the spread of the blue stain fungi was the subject of early extensive investigations, and this relationship has been known for several years. Recently the large losses caused by the two elm diseases have stimulated intensive research, and much new information has been obtained on the insects that spread them and on their control. Additional investigations undoubtedly will disclose that insects are responsible for the spread of many other diseases of forest and shade trees.

The wind spreads the fungi that cause some forest-tree diseases; for example, the spores of the fungus that causes white pine blister rust. The same is true of the one that causes chestnut blight; insects have only a minor part in its distribution. At best, insects act only as accidental, secondary carriers when they happen to come in contact with spores of the fungus on a diseased chestnut tree and then move to a healthy one where the spores are dislodged from their bodies. No doubt the spores of many other parasitic fungi are thus carried from tree to tree and place to place by accidental insect carriers. The fortuitous relationship between insect and fungus in such instances offers practically no opportunity for the development of control measures through sprays or other treatments for the insects. But there are fungi and viruses that are spread primarily or entirely by insects. Then control of the disease may depend upon finding a satisfactory control for the insect carriers. The relation of insects to the spread of Dutch elm disease and phloem necrosis and their control is illustrative.

THE DUTCH ELM DISEASE is caused by the fungus Ceratostomella ulmi. Bark beetles carry the fungus from diseased trees and from dead elms or elm wood or logs in which it is growing to healthy trees or to other dead elms or elm wood. Were it not for these bark beetles, the disease would be relatively unimportant. By the same token, were it not for the destructiveness of the fungus, the bark beetles themselves would be relatively unimportant before about 1930 they were so considered; only after the introduction of the Dutch elm disease into the United States in the late 1920's did they assume their present importance.

Two kinds of elm bark beetles are important in the spread of the disease. One, the smaller European elm bark beetle, is European in origin. The other, the native elm bark beetle, has been here right along. The European species was introduced 25 or 30 years before the Dutch elm disease fungus came in and, because it bred only in dying or recently dead elm or cut elm wood during the interval, it was relatively unimportant. When the fungus also was introduced, however, the combination of beetle and fungus produced a disastrous situation. This sequence of events emphasizes the importance of the need for constant vigilance to exclude dangerous or potentially dangerous parasites.

The story of how the elm bark beetles spread the Dutch elm disease fungus was brought to light through the research of many plant pathologists and entomologists in Europe and in this country. Workers in State agricultural experiment stations and divisions of agriculture, several colleges, and the Department of Agriculture have made contributions that have helped to fill out the picture.

The fungus that causes the Dutch elm disease lives in the water-conducting vessels and adjacent cells of the elm, where it grows and produces spores in the vessels. The spores are so small they can be transported rapidly in the sap stream in the trunk, branches, leaves, and roots. When the fungus has become established in the tree or a part of a tree, it causes wilting, yellowing, dying, and dropping of leaves. The symptoms may develop suddenly or slowly and may involve the whole tree or any part of it. The affected tree may be so severely diseased that it dies in a few weeks, or it may decline gradually for several years before it finally succumbs. Occasionally an infected tree may recover. The fungus can live for several years in standing trees; it can also live for a year or two in dead standing elms or in elm logs, wood, and branches broken off or partly broken off during storms. Without aid from elm bark beetles, however, the fungus has no effective way to reach other elms except through natural grafts of roots of diseased elms with nearby healthy elms. Unfortunately for those who wish to save their elms, the dying trees, broken branches, and recently cut logs and wood are soon invaded by the elm bark beetles, which may later carry the disease to the healthy trees.

The beetles bore into the bark where the females make egg galleries and lay eggs. The young larvae, when they emerge, eat the inner bark and make characteristic channels in it. After a period of feeding, they pupate and come out as adult beetles. The fungus may be carried into the original egg gallery made by the female beetles in the bark, or it may be present in the underlying wood if it was previously infected. The fungus can grow luxuriantly in the beetle galleries and produce spores in great abundance. When the adult beetle emerges from the bark it may be well seeded externally and internally with the fungus spores. It begins to feed soon after it emerges, either in living parts of the tree from which it has emerged, or after it has flown to another elm, which may be nearby or several miles distant.

The adult beetle feeds in the crotches of twigs or at the point of junction of a leaf and twig. While it does so, spores of the fungus may be rubbed off its antennae, mouth parts, feet, or body and become lodged in the feeding wound. Some of the spores, or indeed one of them, may start to grow in the feeding injury and infect the tree. That can happen only if the injury in which the spore is released reaches the wood, however: otherwise, no infection takes place. This limitation on the ability of the fungus to cause infection clinches the need for the assistance of the beetle in spreading the parasite from diseased to healthy trees. On its part, the fungus makes the tree suitable for invasion by egg-laying beetles.

Beetles may lay their eggs in dying elm of elm wood that has not previously been invaded by the Dutch elm disease fungus. If the invading beetles have emerged from diseased trees, however, they may carry the fungus with them into such wood, and it in turn becomes a source of danger for nearby healthy trees when the new brood of beetles emerges the following spring or later during the same season. The smaller European elm bark beetle therefore does not require the presence of the Dutch elm disease fungus to survive, although the fungus helps to provide it with the type of wood it needs for reproduction. Storms, drought, certain construction activities of man, and old-age decline of elm trees provide elm wood suitable for reproduction of the beetles. These alone would insure perpetuation of the species as it was perpetuated for years in Europe. With the additional assistance of the Dutch elm disease fungus in providing suitable wood there is little reason to hope that the beetle can ever be eradicated.

We have reason to hope, however, that it can be controlled. We know, for instance, that the beetle is highly susceptible to DDT sprays and that its numbers can be kept at a relatively low level by the systematic removal of all elm wood that might serve as breeding material. For weeks or months the beetles will not feed on individual trees that have been thoroughly sprayed with the correct formulation of DDT. If all breeding material is removed from a locality, the beetle population will be limited to migrants flying in from the outside, and will thus be held to a low level. If recommended spraying and sanitary measures are combined, it is likely that little loss from disease will result. Naturally, the larger the area in which sanitary measures are undertaken, the better the results will be.

THE NATIVE ELM BARK BEETLE can also spread the Dutch elm disease fungus. Its life history is different from that of the European species and it has been a less effective carrier in the United States than the latter. The native beetle hibernates mostly as an adult in the bark of healthy elms, although occasionally it overwinters in the larval stage. An adult may carry the Dutch elm disease fungus into its hibernation chamber where the fungus will survive until the following spring. Before it emerges in early spring, the beetle feeds for a short time on the inner bark, it may then come in contact with the wood underlying its hibernation chamber. Elms are invaded by the fungus by way of these contact points, but they are often made so early in the spring that growth of new wood in the trees has not started and no new water-conducting vessels have been formed. When new growth does begin, therefore, it entombs the injury along with any spores that may be in it. Because the fungus has only weak power to penetrate cell walls, few infections can occur.

This beetle has not been an important carrier of the disease in the United States. Nevertheless, its habit of over-wintering in healthy elms makes it impossible to eliminate as a source of disease transmission from an area, without at the same time eliminating all the elms of more than I -inch trunk diameter. The beetle can be controlled, however, by the same spray program recommended for the European species.

In Canada the native elm bark beetle appears to be an important carrier of the Dutch elm disease fungus. We have no satisfactory explanation for the difference in its importance as a carrier in the two countries. We suspect that the seasonal history of the beetle may be more closely synchronized in that country with the development of the water-conducting vessels of elm in the spring. In our more northern and colder sections, therefore, it may prove to be a more effective carrier than it has been farther south. Just how effective the smaller European elm bark beetle will be in the more northerly regions we do not know, because it has only recently invaded these territories.