The leaf pests are far more serious. They include the true and false caterpillars, moths, gall insects and plant lice.
Of the bark pests, the bark beetles are the most destructive. These are also called Engraver Beetles from the smoothly cut figures which are their burrows under the bark, Figs. 89, 90, 91.
Fig. 89. Work of the Spruce Destroying Beetle: a. Primary gallery; b. Borings packed in side; c. Entrance and central burrow thru the packed borings; d. Larval mines. Note how the eggs are grouped on the sides. [Agric. Year Book, 1902, Fig. 24, p. 268].
Fig. 90. Complete brood Galleries of the Hickory Bark Beetle in Surface of Wood. [Agric. Year Book, 1903, Fig. 28, p. 316.
Fig. 91. Brood Galleries of the Oak Bark Beetle, showing Character of Primary Gallery at b; Larval or Brood Mines at a. [Agric. Year Book, 1903, Fig. 30, page 318.]
Many pairs of beetles make a simultaneous attack on the lower half of the main trunk of medium-sized to large trees. They bore thru the outer bark to the inner living portion, and thru the inner layers of the latter; they excavate long, irregular, longitudinal galleries, and along the sides of these at irregular intervals, numerous eggs are closely placed. The eggs soon hatch and the larvae at once commence to feed on the inner bark, and as they increase in size, extend and enlarge their food burrows in a general transverse but irregular course, away from the mother galleries (see illustration). When these young and larval forms are full grown, each excavates a cavity or cell at the end of its burrow and next to the outer corky bark. (Hopkins, Agric. Yr. Bk., 1902.)
Some of the species attack living trees, causing their rapid death, and are among the most destructive enemies of American forests.