the wood. The grubs complete the destruction by boring through the
solid wood in all directions and packing their burrows with the
powdered wood. When they are full grown they transform to the adult,
and emerge from the injured material through holes in the surface.
Some of the species continue to work in the same wood until many
generations have developed and emerged or until every particle of wood
tissue has been destroyed and the available nutritive substance
extracted.

[Illustration: Fig. 28. Work of Powder Post Beetles, _Lyctus
striatus_, in Hickory Handles and Spokes. _a_, larva; _b_,
pupa; _c_, adult; _d_, exit holes; _e_, entrance of larvae
(vents for borings are exits of parasites); _f_, work of
larvae; _g_, wood, completely destroyed; _h_, sapwood; _i_,
heartwood.]

Conditions Favorable for Insect
Injury--Crude Products--Round Timber with Bark on

Newly felled trees, sawlogs, stave and heading bolts, telegraph poles,
posts, and the like material, cut in the fall and winter, and left on
the ground or in close piles during a few weeks or months in the
spring or summer, causing them to heat and sweat, are especially
liable to injury by ambrosia beetles (Figs. 22 and 23), round and
flat-headed borers (Fig. 24), and timber worms (Fig. 25), as are also
trees felled in the warm season, and left for a time before working up
into lumber.

The proper degree of moisture found in freshly cut living or dying
wood, and the period when the insects are flying, are the conditions
most favorable for attack. This period of danger varies with the time
of the year the timber is felled and with the different kinds of
trees. Those felled in late fall and winter will generally remain
attractive to ambrosia beetles, and to the adults of round- and
flat-headed borers during March, April, and May. Those felled in April
to September may be attacked in a few days after they are felled, and
the period of danger may not extend over more than a few weeks.
Certain kinds of trees felled during certain months and seasons are
never attacked, because the danger period prevails only when the
insects are flying; on the other hand, if the same kinds of trees are
felled at a different time, the conditions may be most attractive when
the insects are active, and they will be thickly infested and ruined.

The presence of bark is absolutely necessary for infestation by most
of the wood-bori