310 MISC. PUBLICATION 657, U. S. DEPT. OF AGRICULTURE 
Such killings have taken place in eastern and central New York, in 
Michigan, and elsewhere in the Northern States. Such outbreaks seem 
to occur during years in which precipitation is deficient during the 
summer months, and the increase in bark beetle numbers appears to be 
due to the direct beneficial effect on the insects of a deficiency of mois- 
ture and an abundance of sunshine, rather than indirectly because of 
lessened vigor in the trees. An excess of precipitation and a dearth 
of sunshine while the adults are feeding on the new growth and estab- 
lishing their burrows in the bark, and later when the larvae are young, 
results in a very high mortality and greatly reduces infestation in the 
following years. Control methods are discussed on pages 47-01. 
Further information on this species may be found in Swaine (407) ; 
Blackman (38, 39, 43) and Chamberlin.* 
The hackberry engraver (Scolytus muticus) is black, 3 to 4.5 
mm. long, and about half as wide. It is readily distinguished from 
other species by the rather long ashen hairs on the elytra and the sides 
of the pronotum. It breeds in the limbs of several species of hackberry 
(Celtis) and is known from New Jer sey to Florida and westward to 
Kansas and Texas. It is not a serious enemy, as it breeds in dying 
and dead limbs of hackberry and is not known to attack vigorous 
material. The egg galleries are quite similar to those of the hickory 
bark beetle. The larvae on hatching begin their mines at the con- 
tacting surfaces of the bark and sapwood, but as they become nearly 
full grown, the burrow is continued downward into the sapwood. 
Transformation to pupa and adult occurs beneath the surface of the 
wood. As the hackberry engraver seldom or never attacks living 
trees, control is not necessary, but could readily be accomplished by 
burning infested material. The reader is referred to Blackman 
(38, 43) for further information on the hackberry engraver. 
The shot-hole borer (Scolytus rugulosus), or fruit tree bark 
beetle, is reddish brown to black, averages about 2.5 mm. in length, 
and is distinctly more than twice as long as wide. The elytra 
are closely striate and well supphed with evenly distributed short 
hairs. The ventral abdominal outline is strongly ascending and 
slightly convex. This species, originally introduced from Europe, 
has been known in the United States since 1878 and at present is to be 
found in most of the States east of the Continental Divide, as well as in 
New Mexico, California, and Oregon. Trees serving as hosts for the 
shot-hole borers include our common cultivated fruit trees and also 
the wild plums and cherries. 
The habits of the shot-hole borer are similar to those of other species 
of Scolytus. They are not serious enemies of trees, because for breed- 
ing purposes they choose broken, cut, or dying material, and seldom 
or never attack healthy bark. Damage i is usut lly confined to orchards 
where sanitation has been neglected and the vigor of the fruit trees 
has been allowed to deteriorate, or in natural growths that have been 
injured by ground fires or in some other way. On at least one 
occasion, Specimens were taken from beneath the bark of sickly elm, 
Gossard (189) and Blackman (38, 43) have discussed this species. 
The smaller European elm bark beetle (Scolytus mu/tistriatus 
(fig. 61,4) is brownish to black in color, 2.5 to 3.5 mm. long, and about 
** See footnote 22, p. 302. 
