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
