Fig. 22. Work of Ambrosia Beetles in Tulip or Yellow Poplar Wood. a, work of Xyleborus affinis and Xyleborus inermis; b, Xyleborus obesus and work; c, bark; d, sapwood; e, heartwood.
Fig. 23. Work of Ambrosia Beetles in Oak. a, Monarthrum mali and work; b, Platypus compositus and work; c, bark; d, sapwood; e, heartwood; f, character of work in wood from injured log.
The characteristic work of this class of wood-boring beetles is shown in [Figs. 22] and [23]. The injury consists of pinhole and stained-wood defects in the sapwood and heartwood of recently felled or girdled trees, sawlogs, pulpwood, stave and shingle bolts, green or unseasoned lumber, and staves and heads of barrels containing alcoholic liquids. The holes and galleries are made by the adult parent beetles, to serve as entrances and temporary houses or nurseries for the development of their broods of young, which feed on a fungus growing on the walls of the galleries.
The growth of this ambrosia-like fungus is induced and controlled by the parent beetles, and the young are dependent upon it for food. The wood must be in exactly the proper condition for the growth of the fungus in order to attract the beetles and induce them to excavate their galleries; it must have a certain degree of moisture and other favorable qualities, which usually prevail during the period involved in the change from living, or normal, to dead or dry wood; such a condition is found in recently felled trees, sawlogs, or like crude products.
There are two general types or classes of these galleries: one in which the broods develop together in the main burrows (see [Fig. 22]), the other in which the individuals develop in short, separate side chambers, extending at right angles from the primary galleries (see [Fig. 23]). The galleries of the latter type are usually accompanied by a distinct staining of the wood, while those of the former are not.
The beetles responsible for this work are cylindrical in form, apparently with a head (the prothorax) half as long as the remainder of the body (see [Figs. 22], a, and [23], a).
North American species vary in size from less than one-tenth to slightly more than two-tenths of an inch, while some of the subtropical and tropical species attain a much larger size. The diameter of the holes made by each species corresponds closely to that of the body, and varies from about one-twentieth to one-sixteenth of an inch for the tropical species.