The work of different kinds of insects, as represented by special injuries to forest products, is the first thing to attract attention, and the distinctive character of this work is easily observed, while the insect responsible for it is seldom seen, or it is so difficult to determine by the general observer from descriptions or illustrations that the species is rarely recognized. Fortunately, the character of the work is often sufficient in itself to identify the cause and suggest a remedy, and in this section primary consideration is given to this phase of the subject.
[Illustration: 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.]
[Illustration: 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.