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Popular Science Monthly/Volume 68/March 1906/The Black Locust Tree and its Despoliation



WITHIN the past few years an increasing interest has been manifested in the black locust tree, Robinia pseudacacia. Many persons have begun to propagate it, not only as a wayside tree, but as a forest product; and issues of the public press lately have contained many articles and paragraphs pointing out the excellent qualities of the wood and recommending its general cultivation for economic uses. Several of the articles referred to have mentioned the fact that one of the great railroad companies has, within the past two years, planted on its Pennsylvania lands nearly a million and a half of trees of this species with the intention of using the product for railroad ties and fence posts, and for other purposes requiring exceptionally durable wood. It has been publicly announced that large additions to that company's planting of this tree are to be made, and it is also known that many smaller, but still extensive, enterprises of this kind, under both corporate and individual management, are in progress in different parts of our country. The aggregate of these enterprises requires the expenditure of so much labor and money before any profitable returns could be expected, that one who is acquainted with the past history of the tree can not but wonder at the apparent lack of business precaution, or of sound advice, which they imply. The fateful destiny of this tree has been long known and until recently it has been generally neglected; but by most persons the facts concerning it apparently are now forgotten or disregarded. From personal observations, extending through many years and over a large part of the United States and adjoining parts of Canada and Mexico, I am convinced that all attempts to cultivate this tree in any part of North America, with the possible exception presently to be mentioned, will result in failure so far as suitable returns in practical value of the product is concerned. The subject therefore has, with comparative suddenness, become of public importance, and my chief object in writing this article is the utterance of a public caution concerning it, especially directed to industrial interests.

The excellent quality of the wood of this tree is all that has been claimed for it, and doubtless it is this quality, together with the knowledge of the vigor of its early growth, that has encouraged the extensive preparations that lately have been made for its artificial propagation. Our soils and climate are almost everywhere well adapted to its growth, its abundant seeds germinate readily, and it responds promptly to the forester's care. The danger of failure in growing this tree for economic uses lies, not in the character of the tree nor in that of our soils or climate, but in the persistent attacks of destructive insects which are natives of the same region with the tree, which follow it in its geographical distribution, and which presently will be further referred to.

There are two North American trees which bear the popular name of locust, the one already mentioned and the honey locust, Gleditschia triacanthus; but it is only the black locust that is referred to in these remarks. This tree originally was known only in that region which lies east of the Allegheny Mountains and between New York and Louisiana. By natural dispersion and artificial propagation, however, it has grown for many years more or less commonly, but for the most part unthriftily, in nearly all the eastern half of the United States, as well as in other parts of North America. Early after its first discovery its seeds were carried to other countries, where the tree was successfully progagated from them. In Europe, especially, where its American insect despoilers never have been introduced, where the indigenous insects never molest it, and where it readily adapts itself to the local climatic and terreous conditions, it has always grown thriftily and symmetrically, reaching a maximum size comparable with that of the oaks. Being there esteemed as an ornamental tree, it is often grown in public parks, and it is also much cultivated in preserved forests for its valuable wood.

This European experience with the black locust tree well illustrates its extraordinary vigor and its ability to reach full maturity of growth under a wide diversity of conditions of soil and climate. Its completely successful growth to trees of medium size in formerly isolated North American districts west of its native regional habitat, and its persistent struggle for existence against its insect despoilers wherever it has been established in our country show that our soil and climate are entirely favorable to its growth and that it is only accessory, but dominant, conditions that are unfavorable. These accessory conditions are now known to be the result of ravages upon the living parts of the tree by the insects referred to. Indeed this tree presents the remarkable case of a strong arboreal species doomed on its native ground and in contiguous regions, to a constant state of suppression of its natural development, and even to local extermination, by insect despoilers which are natives of the same region with the tree and wholly dependent upon it for their own existence. There is no other North American tree, perhaps, excepting the common mezquite of our southwestern states and Mexico, which is so disastrously damaged in its growing condition by indigenous insects as is the black locust, and both of these trees would be of very great industrial value if it were not for the ravages of those insects. Other insects, the imported gypsy moth for example, commit their terrible ravages upon the foliage of different kinds of trees indiscriminately, but the insect pests of the black locust and the mezquite are indigenous, and each species attacks only its own destinate tree. The chief injury to each of these trees is done by the larva? which burrow in its living wood.

There are at least three species of insects which injure the black locust tree. The small larvæ of one of them tunnel the parenchyma of the leaflets, and another species produces a gall-like enlargement of the tips of tender twigs as a result of depositing and hatching its eggs there. But worst of all, the large, vigorous and abundant larvae of one of the longicorn beetles, Cyllene robiniæ, burrow throughout the wood of the entire trunk and larger branches, rendering it unfit for economic uses. All these insect species are known to be dependent for their own existence upon the black locust tree because all three of them deposit their eggs nowhere else than in its tender tissues; all three of them pass their entire larval stage, the only stage in which the insect really increases in growth, in its living substance, and all three of them derive their only incremental nourishment from that tree. If, therefore, the black locust tree were exterminated, all those insects would necessarily perish; and if all those insects were first exterminated we should have restored to us one of the most valuable of our forest trees. But none of those contingencies is likely to occur.

Great damage is sometimes done to the black locust tree by the two insect species first mentioned, but usually their depredations are so much less disastrous than are those of the tree borer that, for only the present occasion the two former species may be regarded as negligible and only the latter need be specially noticed. Because this article is written with reference to a matter of public interest it is thought desirable to give a brief popular account of the characteristics and habits of that destructive insect. The beetle, which dies naturally soon after the function of reproduction is completed, is nearly an inch long, somewhat slender, and has a pair of slender curved antennae as long as the body; and the larva is a vigorous grub nearly or quite as long as is the beetle. The metamorphosis from the larva to the pupa stage and from that of the pupa to the imago or beetle stage occurs as the insect is about to emerge from its burrow in the tree; the final change and emergence beginning to occur in late summer and continuing through autumn. The beetles soon mate and hover about the Solidagoes and other late flowers, feeding scantily and harmlessly upon the pollen. The females immediately seek the black locust trees by flight, pierce the bark and deposit their eggs in the soft cambium layer beneath it. The resulting larvæ burrow at once into the tree, traversing the wood of the trunk and larger branches in all directions. The insect there completes the annual cycle of metamorphoses and emerges as a beetle of that generation about a year after the egg is hatched. The burrows are made by the strong horny jaws of the larvæ, which shred every particle of the wood in the course of the burrows, all of it passing through the intestinal canal of the larvae. Only the scanty protoplasmic contents of the wood cells, however, are digested for nourishment, and the dry refuse, resembling fine saw-dust, is packed behind the larva as it progresses in its burrow. The burrows are comparatively large and when numerous, as usually they are, they cut across the wood fiber so frequently that the trunk and larger branches are often completely riddled by them.

Such is the condition to which the wood of the black locust tree is habitually reduced by those insects and to which it is the special object of this article to call public attention. It is almost needless to add that such burrows render the wood useless for timber, of little value as fuel, and more subject to decay than is the uninjured wood. Many and various kinds of insects burrow in the dead wood of different kinds of lumber and fuel and thereby do much injury, but comparatively few species bore exclusively in living wood, and these are extremely injurious. The destructive borer of the locust tree and the smaller but hardly less destructive borer of the mezquite tree, already mentioned, are two of the best-known examples of the latter kind. Perhaps the best-known example of the former kind is the hickory wood borer, which householders often find in their fuel; especially that which has been felled in late winter or early spring. These borings in hickory wood are closely like those which are made in the living locust trees, and the locust and hickory borers are so nearly alike in appearance in all three of their metamorphoses that it is difficult for the ordinary observer to distinguish them apart. The hickory borer, however, burrows only in recently felled dead hickory wood, its incubation therein beginning in the spring; while the locust borer burrows only in the living wood, its incubation beginning in late summer and continuing until frosts prevail. This last-mentioned fact is important with reference to any remedies against the ravages of the locust borer that may be proposed. Hickory wood which is felled in autumn or early winter is likely to escape its borers by becoming too dry to serve their needs when they reach the beetle stage in the spring; but for the locust tree, after its sapling stage, there is no immunity from its borers so long as it lives.

Apparently there are several reasons why the ravages of the locust borer have largely escaped popular attention, such as the destruction of fruit and foliage by insects receives. The bark of the tree usually remains intact long after the wood beneath it is greatly injured. The small pits and punctures which are made in it by the female beetle for depositing her eggs are not ordinarily noticeable, and even the holes by which the beetles escape from the burrows are not conspicuous. While the bark remains unbroken the tree lives and usually continues annually to produce its seed. The latter fact not only belies its hidden trouble, but it is characteristic of the strong vitality of the tree. This vitality is also exhibited by the roots which send up vigorous suckers, especially after the borers have attacked the trunk. Again, the reciprocal relation of the tree and the insects which prey upon it, although it is never wholly interrupted, is of more or less unstable equilibrium, sometimes the tree, and sometimes the insects being ascendant. That is, it has often happened in a given district that the tree became reduced in numbers to a few scattered and injured specimens, and its insect enemies were correspondingly reduced in numbers because of the reduction of their only means of subsistence. The native vigor of the tree then gives it such advantage that it so thrives again that one naturally hopes for its permanent immunity. But that improved condition of the tree itself invites, and sooner or later receives, renewed attacks of the insects, which lurk there or which come in from contiguous districts. Such an oscillation of relative conditions occurs with the borer especially,thus deceiving local observers as to the great average damaged condition of the tree. The two insect species which have been mentioned as preying upon the leaves and tender twigs respectively have their needs supplied by even the youngest, as well as the older, growth of tree, but the borer requires a body of living wood of some inches in diameter in which to produce burrows of sufficient extent for its needs. Therefore this greatest of the insect enemies of the black locust tree is held at bay until the tree has reached sufficient size for boring, during which time the planter must await the issue. Meantime the young of this tree generally grows as thriftily as the average of other trees, and often it produces seed before it is large enough for the borers. It is not strange that this early thrift of the tree should encourage disbelief of impending evil for it, but the facts here mentioned are too well established to admit of serious question.

Exceptionally large and healthy specimens of the black locust tree are sometimes found growing as a part of the native arboreal flora, their, at least partial, immunity from insect injury doubtless being due to local causes, some of which are obvious and some obscure. For example, some of the best American specimens of the tree are found to have grown in, or around, cattle-pens, barnyards or other farmstead inclosures where domestic animals are gathered, the conditions of which places are known to be favorable to the tree, and they are apparently unfavorable to the insects. Again, the isolation of the tree by planting its seeds in districts remote from those in which both the tree and its insect enemies prevail, has resulted in the healthy growth of the tree for many years; but in most of such cases the trees have been overtaken by the borers and destroyed or rendered valueless long before they could have reached the full size of which the species is capable. My present information indicates that there are yet some districts in the western part of the United States, notably portions of California, in which the black locust tree, which was originally grown there from eastern seed, has not yet been injured by the borers. But the borers are surely lurking in all other parts of our country in which that tree has grown for any considerable number of years. Furthermore, judging from the facts already stated and the remarkable case of destruction of that tree by the borers which occurred in the great valley of the Upper Mississippi between forty and fifty years ago, one may reasonably fear that those farther western groves will yet suffer like disaster.[1]

The case referred to is peculiar, and fraught with important suggestions to those who are now contemplating the* artificial propagation of the black locust tree. In the early half of the past century there occurred a strong migratory movement of families overland from eastern states to Illinois and Iowa, who carried with them in their wagons the seeds of various kinds of trees, among which were those of the black locust. Those seeds were planted in the fertile soil of the new homesteads, where they germinated promptly and the seedlings grew vigorously and healthfully. The settlers thus 'stole a march' of many hundred miles on the borers, for neither they nor the tree upon which they exist had then occupied a large part of the country which the emigrants traversed. The seedling trees of the earlier settlers soon reached reproductive maturity and furnished abundant seed for further planting. The streets of the towns and villages were bordered with the trees and the farmers who possessed prairie land planted groves of them with the expectation of using the product for much-needed fence posts, and for other purposes. But while the trees were rapidly increasing in size by healthy growth, ominous reports began to reach the settlers that the borer was moving westward and, finally, that it was approaching the great region in which they had made their homes. In due time the borers arrived there, for the intervening country eastward had become so dotted with artificial groves of the black locust tree that the insects in their beetle stage, easily spread from grove to grove by natural flight. None of those trees in the Mississippi valley had then reached the maximum size of the species, but they had fruited for many years, and some of them had reached a foot or more in diameter at the base. They had grown so thriftily that their continued growth was naturally expected by those who had planted them; but when the borers came the trees were everywhere injured or destroyed as by a pestilence. Within a very few years after the first appearance of those insects in that western region nothing remained of the almost numberless groves and rows of those trees except their blasted remnants and the young shoots that the vigorous and unconquered roots were striving to bring forth. That condition remains there to this day, essentially unchanged except that even the stumps and roots of the blasted trees have, from time to time, been removed to reclaim for other uses the soil which they formerly occupied. Because of that wide destruction one may now go many miles in that great region without seeing more than a few neglected outcasts of that once popular tree; just enough to afford breeding places for a few of the hardy and prolific borers, which are always ready to commit their ravages. I was an eye-witness of that great destruction from its beginning to its consummation, and afterward had unusual opportunity to observe its effects when investigating the subject of foresting the prairie soils of Iowa. Similar destruction also occurred as the borers traversed the country between their native region and the Mississippi valley, the effects of which still remain there.

The question now arises whether there is any known remedy for the attacks of those insects. Unfortunately no effective remedy of general applicability has yet been discovered. The attacks of the insects are not upon any of the parts concerned in reproduction, such as might interfere with the propagation of the tree, but upon its growing substance, which is constantly exposed at all seasons of insect activity. Therefore its inflorescence and fruitage need no protection, and proposed remedies must be applied to the surfaces of the tree and directed against some important function in the life of the insect, mainly that of reproduction. The killing of the insects in any considerable numbers seems to be quite impracticable. The few remedies which have been proposed are fluid applications which are harmless to the trees and so repugnant to the insects that they will not puncture any surface to deposit their eggs which has been so covered. But many difficulties attend the application of such remedies. The leaves and terminal portion of the twigs are doubtless too delicate for such treatment, .but the bark is not easily injured by it. The application with a brush to the bark of the trunk and branches of lime whitewash mixed with a solution of whale-oil soap has seemed to prevent the female of the borer in the beetle stage from puncturing the bark to deposit her eggs. But to be effective all such applications must carefully be made to the entire surface of the trunk and to that of the branches which have reached a couple of inches or more in diameter. They must be applied not later than midsummer, that is, before the newly matured beetles have begun the function of reproduction, and they must be continued intact until frosts have killed the last of the beetles for the year. They also must be renewed annually as long as the tree stands. One might feel justified in employing such a remedy to save a few favorite trees, but it involves too much labor to be of practical value for general application.

The foregoing statements, which are assumed to be indisputable, show that the prevailing condition of the black locust tree in the land of its origin warrants the conclusion that it is fated, not to extinction, but to destruction as a profitable natural product; and that this fate is beyond the effective reach of any known general remedy. The biological aspect of this subject is one of very great interest, but this article has purposely been confined to a presentation of its economic bearings, the significance of which is too obvious to need explanation.

  1. A personal communication from Mr. A. E. Schwarz, of the U. S. Department of Agriculture, who has long made special studies of the habits of these insects, confirms my own observations as to the extent and imminence of their ravages. He thinks, however, that the Locust borer possibly may not invade our Pacific Coast region because the insect fauna there being so different from that of which the borer is an original member is likely to prevent its geographical range over that region. He thinks it probable also that a similar faunal influence has prevented the introduction of the borer into Europe, where the tree has been so fully acclimated.