The little larvæ came safely yesterday and the specimens of bark this morning. Necessarily when the attack has been going on so long the burrows intersect each other so very much that they cease to show the typical patterning or tracks, but I do not see any reason at all to doubt that this is attack of the very great elm-pest, the Elm-bark beetle. With regard to its infestation of other trees besides elm, I have no knowledge of its ever attacking either oak or ash, but on careful search I find that one German writer records it as “sometimes” attacking the ash. I greatly doubt this having been observed in our country. Our ashes have, however, a bark beetle which tunnels much in the same manner between the bark and wood, and of which the presence may similarly be known by the shot-like holes in the bark. But you would distinguish the difference in pattern of gallery at a glance on raising the bark. As in the figure given, the mother-gallery is branched. This Ash-bark beetle, Hylesinus fraxini, does not do very much harm, for it chiefly attacks felled trunks, or sometimes sickly or damaged trunks and boughs. It is not to be compared in its ravages with the Scolytus, well-named destructor. I am not aware of this ever attacking oak.
Workings, showing forked “mother gallery,” with larval galleries from the sides.
FIG. 36.—TUNNELS OF THE ASH-BARK BEETLE, HYLESINUS FRAXINI, FAB.
April 12, 1899.
You have certainly two kinds of bark attack present in the specimens which you send me, but without the beetles I am not able to say at all what species may have been doing the mischief. I can say quite certainly that I do not see any signs of the presence of the Hylesinus fraxini (Ash-bark beetle), but I have never, so far as I remember, seen the very long, narrow borings, hardly wider than a thread of silk, which are a good deal represented on the inner surface of one of your pieces of bark.
There are two or three grubs in fairly good condition which I have gently inserted into a burrow in the little bit of bark and have put carefully aside in the little box, and if these develop, we shall then know what we have to deal with. Perhaps you may be able to secure some beetles in a month or two; it would be of interest to make out the attack with certainty.
November 7, 1899.
I have very carefully examined your beetle and find that it is Hylesinus crenatus, sometimes known as the “Large Ash-bark beetle” to distinguish it from Hylesinus fraxini, the “Ash-bark” or the “Small Ash-bark beetle.” The life history of each kind is stated to be the same, and I think, if I remember rightly, that some time ago, perhaps a year or so, in the course of our occasional correspondence, we have gone into the history of the fraxini, but if not I should have pleasure in either looking up the account in my Manual and sending the pages to you or condensing the points.
There appears to me to be this difference in method of larval proceedings: that whereas in the case of fraxini the parent galleries are formed somewhat in the shape of a T, with a short stem and long arms to the top, and the larval galleries placed at right angles to the others (fig. [36]), so far as I understand this form is not followed by crenatus fig. [38]).
The beetle obviously pierces the bark, for the orifice is visible; and in or under the bark there are the mother-galleries, but I do not find the larval galleries feathering as it were from these, and the figure before me gives the idea of the body of larvæ having by their united attack cleared a flat space from which they have continued their solitary tunnels. Perhaps in cutting up your trees you may come on some of these markings. It is said that there are two generations in the year, of which the flight time of one is in April and of the other in October. This species frequents oak as well as ash, which is an important consideration, and I find it noted as frequenting old trees. These are the main points which I see about the history. I should think that if you find the trees which you have felled much infested, it would be a good thing to strip the bark off and burn it.