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INSECT INJURY TO CHESTNUT AND PINE TREES IN VIRGINIA
AND NEIGHBORING STATES.
By F. H. CHITTENDEN. Recent years have witnessed extensive destruction of the pine and spruce forests in the United States, particularly in that portion of the South east of the Blue Ridge Mountain range, and to a lesser extent of chestnut trees in the same and other regions. This injury has been very generally attributed to insects, and there is evidence that certain wood and bark boring species have largely contributed to the work of demolition. The death of the chestnuts has been laid to the account of the buprestid, Agrilus bilineatus Weber, while the destruction of the coniferous trees has in like manner been credited to the scolytid bark-beetle, Dendroctonus frontalis Zimm.
The writer has always felt a certain degree of skepticism as to whether perfectly healthy forest growth would, save in exceptional cases like the present, succumb to insect attack, and were not this view shared by others it might sound like heresy to say that the insects mentioned are perhaps not the original cause of the injury. There is more than a suspicion that a predisposing agency has been at work in causing a weakened condition of the timber of the infested region. Whatever has brought this about, there is every appearance that the insects in question have multiplied in such numbers that they were forced to attack living, if not healthy, plants or perish, as they belong to groups that do not develop in timber that has been dead long enough for the bark to become separated from the wood.
Such an enfeebled condition as suggested might be caused by bacterial or fungous disease; or it might be due to a variety of other elements, among which might be numbered too close growth, defective drainage, insufficiency or a superabundance of subterranean moisture, too great dryness, or, again, it might be produced by a dry spell suddenly following an unusually wet one, or vice versa. In the present case it seems more than probable that the soil had undergone some change that has interfered with the growth of the trees, or that a combination of unfavorable atmospheric and soil conditions has accomplished the supposed predisposing debility. The withering and dying of leaves induced by a spell of hot, dry weather following cool, moist weather, and known as sun scald, might be provocative of the debilitated condition favorable to the attacks of these borers. Possibly "pine blight,” or what is known as winter desiccation, the latter occurring in mild, dry winters following autumn droughts, and particularly affecting conifers, might explain the original injury to this class of trees. A peculiar and interesting feature of what we may call the primary insect injury is that it is due very largely to native species not previ. ously known to be injurious; in fact, the principal pine destroying insect, Dendroctonus frontalis, is even at the present time comparatively rare in collections. Another singular fact is that the invading forces disappeared as suddenly as they came, and, as is usual in such cases, we are as ignorant of the reason for the insects' disappearance as we are of the inducing cause.
As usual in so many similar instances, these cases of forest-tree injuries were not brought to the attention of entomologists until too late for thorough investigation or experiment.
December 7, 1891, Mr. W. H. Farley wrote that chestnut trees in Botetourt County, Va., were suffering from the attacks of wood-boring insects, and although no specimens were obtained it is more than probable that Agrilus bilineatus was concerned.
The following year Hon. Gardiner G. Hubbard sent a piece of the bark of chestnut from his suburban residence in the district of Columbia from which were reared the ensuing spring specimens of A. bilineatus.
During June of 1893 Mr. A. J. Wedderburn, an extensive landowner in Virginia, in the neighborhood of Washington, D. C., reported seri. ous injuries from insects to trees, particularly chestnut and pine, on his property. An investigation as to the cause was requested, and the writer with Mr. W. H. Ashmead was detailed to visit the principal infested locality, which is situated in Fairfax County, about 17 miles from Washington, and now known as Wedderburn. The notes that follow are mainly the result of these and subsequent visits that were made in succeeding years to localities in the vicinity of Washington, where injuries to chestnut had been observed, but in spite of every effort no additional facts of value were learned and no opportunity offered for continuing the observations begun in 1893. It was in the hope that the full life economy of the principal depredator, which proved to be the buprestid beetle, Agrilus bilineatus, might be traced, as well as the causes that led up to the attack of this insect, that the publication of these notes has been deferred. In June, 1896, another visit was made to Wedderburn, but the Agrilus could not be found at this time, and as there is no immediate prospect that an opportunity for the further study and possible solution of the perplexing problem will be afforded it has been deemed desirable that such facts as are available be placed on record.
BORER INJURY TO CHESTNUT TREES.
The first visits to Wedderburn were made June 6 and 8, 1893. The principal damage was to chestnut (Castanea dentata) and scrub pine (Pinus inops), though very general injury to cedar and sassafras was also noticed.
The injury to chestnut was almost universal, the majority of the infested trees having been recently killed-within a year or two-few having been dead more than two or three years. The origin of the trouble may thus be traced back to about the year 1890 or 1891, or possibly a year or two earlier.
The estimated damage was placed by Mr. Wedderburn at about 75 per cent for this vicinity, and be further stated that a similar condition of affairs existed at this time in neighboring forests of the surrounding region. This was subsequently verified by the writer in conversation with other landholders of this and adjoining counties.
Two chestnut trees that had been attacked but were still living were felled, the bark stripped off, and the galleries of Agrilus bilineatus exposed. These galleries run mainly transversely just under the bark and are in length from 6 to 10 inches. Although none were found that completely encircled the tree, the result is practically that of girdling, through the combined work of many different individuals.
From the manner of the insect's work it can not be otherwise than exceedingly injurious, as it undoubtedly stops the flow of sap, cuts off the natural supply of plant food and moisture, and greatly weakens and eventually kills the tree. The galleries occur from the base of the trees well up toward the top.
The Agrilus was rather common on the foliage of both chestnut and oak, and a few larvæ and adults that had not yet issued were found under the bark of dead trees, but in the living trees the insects had evidently long before left the wood and had either issued or were still in the bark, as none were to be found after long search. Their characteristic galleries and exit holes, however, were noticed in all of the dead and dying chestnuts examined. From the observed fact that no other insect was present in any number in the infested trees that were still living there can be little doubt that the damage to the chestnuts is caused chiefly by Agrilus bilineatus, although, as stated in a previous paragraph, it is more than probable, judging from what is known of this and other similar bark-boring beetles, that the trees are first enfeebled from some other cause. A reason that may be adduced for this belief is that this buprestid is better known as an enemy of the oak, yet the trees of this genus did not show any evidence of attack, while the chestnuts were evidently all infested. Chestnut trees were also dying in the District, and it seems probable that this trouble is widespread. It is obvious that only the earlier stage of the larva is passed between the bark and the wood; indeed, it is likely that the entire life of the insect may be passed, in some instances at least, entirely within the bark.
The pupal chamber is evidently always constructed in the bark of living trees, but was also found in the wood just under the bark of small dead trees. In larger trees the last portion of the larval gallery is longitudinal and carried up to within a short distance of the outer
surface, when it gradually curves until this surface is nearly met. This last portion forms the pupal cell, and is wholly unlike the exit chamber of any other borer that the writer has seen on chestnut.
Later in the month, when the observations were made in the vicinity of Washington, the writer was in charge of the exhibit of the Division of Entomology at the Columbian Exposition. During his stay there Mr. J. S. Raymond brought in two large specimens of the work of what was undoubtedly the same insect in oak, with the report that damage of this nature was quite extensive in northern Illinois. The infested specimens were from Mr. H. E. Hamilton, Richmond, I., where they were attacking vaks growing thickly together on a lawn. Similarly affected trees were noticed at the same time by Mr. G. B. Sudworth, of this Department, in the neighborhood of Ann Arbor, Mich.; also by Mr. A. D. Hopkins, entomologist of the West Virginia Agricultural Experiment Station, who reported the occurrence in Insect Life (Vol. VII, p. 145), as follows:
Quite a serious trouble, affecting different species of oak in and around Madison [Wis.], was observed, which caused the death of isolated and groups of trees of different ages. Upon examination, I found the larva of Agrilus bilineatus quite common in the bark of most of those that were just commencing to die. I also observed what appeared to be this same trouble at different points through Ohio, Wisconsin and Indiana; and upon my return to West Virginia I noticed a number of trees dying in the same manner; one of which I examined, September 10, and found the larva of the same Agrilus mining through the inner living bark and outer sapwood. From what I have observed up to the present date with reference to the habits of this insect, it is very evident to me that it is capable of attacking and killing healthy oak and chestnut trees, and should the conditions at any time be especially favorable for its increase I anticipate that serious trouble will result from its attack.
Of this species the same writer remarks in the Canadian Entomologist for October, 1896 (Vol. XXVIII, p. 216), that until the fall of 1895 “ the dying of trees with which this insect was intimately associated was decidedly on the increase." In the vicinity of the District of Columbia, on the contrary, this condition has been constantly decreasing for at least the last three years.
THE SPECIES BRIEFLY DESCRIBED.
For the identification of the beetle and its larva the accompanying illustration (fig. 12) is presented. The beetle, shown at a, is elongate and subcylindrical, like others of its genus. It is black, with a more or less greenish tinge. The sides of thorax and elytra are clothed with a light golden-yellow pubescence, and the elytra are each marked with a stripe of the same color. An enlarged antenna is shown at the left (a), and below are the claws of the hind feet of both sexes. The larva, drawn to the same scale as the beetle, is illustrated at c. It is long, slender, and considerably flattened. In color it is milk white or slightly yellowish, except the mouth-parts and the peculiar anal fork, which are dark brown.
The distribution accorded this species by Dr. Horn comprises the United States from Maine to Texas and westward to the Rocky Mountains,
This borer undoubtedly has many insect enemies, as several genera of parasitic Hymenoptera are known to attack species of the same genus. Of the parasitic species observed in this vicinity, Spathius simillimus Ashm. was the most abundant, and is an undoubted parasite of Agrilus bilineatus, as the circumstances of its capture in the galleries of this buprestid indicate. It was exceedingly abundant at Wedderburn in 1893, and probably played an important role in reducing the numbers of its host. One other parasite
6 has been reared with this species, but it has not been identified at the present writing.
OTHER INSECTS ON CHESTNUT.
A considerable number of other insects were observed on the infested chestnuts,
a principally wood-boring Coleoptera. Many of these are well-known enemies of this tree, but the list includes also a number new to FIG. 42—Agrilus bilineatus: a, adult the chestnut. Space will not permit of the
beetle; b, antenna of same; 4, claws
of posterior targi of female; 9, mention of more than a few of the more same of male; c, larva-a, and c, en. important or interesting species at this time. larged; b, d, and f, more enlarged
(original). With scarce an exception they were only secondary in their attacks and it is hardly likely that more than a very small proportion are capable of initial injury to trees. The list follows:
Urographis fasciatus DeG.–This cerambycid was the most abundant borer in chestnut. It occurred in all stages under bark of recently dead trees, June 6 to 8. No imagos had issued at that time. Also reared from oak and maple.
Leptura zebra 01.—Recorded from oak. Found in different seasons and localities in abundance on the foliage of chestnut.
Chrysobothris 6-signata Say.-Found with Chrysobothris femorata on the trunks of recently felled trees. Reared by the writer from beech; also affects the birch.
Alaus oculatus Linn.—Larvæ of this species were noticed under bark at the base of trees.
Dendroides canadensis Latr.–Occurred in all stages and in some abundance under the bark.
Cryptorhynchus bisignatus Say.--Taken on trunk. Previously noticed by the writer on chestnut and beech; probably lives in bark.
Cryptorhynchus obtentus Ibst.—Isolated examples of the adults,