Page:Popular Science Monthly Volume 68.djvu/217

Rh 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