es felled the previous winter.
They are not attracted to any other kind of timber, because they can
live only in the bark or wood of pine, and only in that which is in
the proper condition to favor the hatching of their eggs and the
normal development of their young. As they fly only in April, they
cannot injure the logs of trees felled during the remainder of the
year.

There are also oak insects, which attack nothing but oak; hickory,
cypress, and spruce insects, etc., which have different habits and
different periods of flight, and require special conditions of the
bark and wood for depositing their eggs or for subsequent development
of their broods. Some of these insects have but one generation in a
year, others have two or more, while some require more than one year
for the complete development and transformation. Some species deposit
their eggs in the bark or wood of trees soon after they are felled or
before any perceptible change from the normal living tissue has taken
place; other species are attracted only to dead bark and dead wood of
trees which have been felled or girdled for several months; others are
attracted to dry and seasoned wood; while another class will attack
nothing but very old, dry bark or wood of special kinds and under
special conditions. Thus it will be seen how important it is for the
practical man to have knowledge of such of the foregoing facts as
apply to his immediate interest in the manufacture or utilization of a
given forest product, in order that he may with the least trouble and
expense adjust his business methods to meet the requirements for
preventing losses.

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.

Ambrosia or Timber Beetles

[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