INSECT ENEMIES OF EASTERN FORESTS 299 
egg galleries in the pith. They occur only in twigs or very small limbs, 
and in the case of the last two genera, are found in the leaf-bearing 
portion of living pines. The larvae feed on the pith, the wood, or the 
inner bark. Forms making pith burrows are usually polygamous. 
(7) Woop Burrows.—These are excavated from the sapwood and in 
general chracteristics are similar to burrows occurring in the bark. 
The entrance gallery leads directly through the bark into the wood, 
where it may be continued as a simple egg gallery, may be branched, 
or may be expanded into a nuptial chamber from which several egg 
galleries arise. Various species of Zylocurus, Micracis, and Thysanoes 
and Lymantor decipiens Lec. construct wood burrows. The beetles 
producing these burrows may be either monogamous or polygamous. 
(8) Suwpite AmprosiaL Burrows (Pinuote Burrows).—The bur- 
rows of ambrosia beetles are always cut in the wood and are stained 
black by the action of the ambrosial fungi, which grow upon their walls 
and which serve as food for both larvae and adults. The simplest type 
of burrow, made by Yyleborus saxeseni Ratz. and X. pecanis Hopk., 
usually consists of an unbranched gallery extending into the sapwood, 
where it is shghtly widened to form a space for the eggs. The adults 
and larvae work together, enlarging this cavity, thus forming a wide 
flat room of varying area. This type of gallery is somewhat similar 
to Types 1 and 2 in the bark. 
(9) Brancuep Amprosta Burrows (PinnoLte Burrows) .—In this 
type the entrance gallery, after entering the sapwood, breaks up into 
several branches extending in various directions in the same general 
plane. The eggs are laid free in certain of the galleries and the larvae 
live there, feeding on the ambrosial fungi. The burrows of Yy/eborus 
celsus Eichh. have many branches, but those of Anzsandrus spp. have 
fewer. 
(10) Comrounp AmprostAL Burrows (PINHOLE Burrows).—The 
burrows of this type resemble those last described in that several egg 
galleries arise from the entrance gallery. They differ, however, im 
that the parent beetles make niches in the upper and lower sides of the 
galleries. In these niches the eggs are deposited and the young remain 
throughout their larval and pupal lives, the larvae being fed by am- 
brosia brought to them by the adults. As they grow, the larvae 
enlarge the niches more and more to form the characteristic larval 
cradles produced by species of 7rypodendron, Xyloterinus, Gnatho- 
trichus, and Monarthrum. 
ECONOMIC IMPORTANCE OF BARK BEETLES 
The family Scolytidae contains several of the most destructive forest 
insects found in the United States. Although more trees are killed by 
them than by any other group of insects, only a comparatively small 
number of species of bark beetles ordinarily attack and kill vigorous 
trees. Other forms, however, can breed successfully in twigs, cones, 
or other portions of living trees but in so doing destroy only the parts 
infested. By far the great majority of species, however, breed in 
recently cut trees, stumps, broken limbs, or in decadent, dying, or dead 
trees or portions of trees. Thus a number of species are economically 
neutral and some are even mildly beneficial. As a family, however, 
the injuries done by scolytids greatly outweigh the benefits they confer. 
Although losses due to bark beetles are by no means as severe in the 
