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wood worthless for finishing, staves, and woodenware, but apparently is 
not seriously detrimental to timber for other purposes. 
(2) The black check is caused in nearly every case by an injury to 
the cambium (growing layer) of the trunk by the hemlock bark maggot, 
Cheilosia alaskensis Hunter. Somewhat similar defects may be caused 
by barkbeetles, bark-borers, sapsuckers, or anything that makes a 
wound in the cambium, which afterwards heals. 
(3) The maggot enters the bark through an abandoned food burrow 
made by the hemlock barkbeetle (Hylesinus n. sp.). It gradually 
enlarges the burrow into a small wound, in which it lives for several 
years, feeding on the sap and cambium. In the spring, when full 
grown, it pupates in the resin mass which has formed on the outer bark 
around the entrance to the wound. The puparium soon changes to the 
adult, which emerges in April or May. The egg is probably laid on the 
resin exuding from the abandoned gallery of the barkbeetle. 
(4) A near relative (Cheilosia hoodianus Bigot) of the hemlock 
bark maggot causes, in a similar manner, a black check in the timber 
of lowland, grand, or white fir (Abies grandis). It enters the bark 
through the abandoned food burrows of several species of barkbeetles. 
(5) The alpine fir (Abies lasiocarpa), the Sitka spruce (Picea sitch- 
ensis), and probably many other trees are injured more or less by bark 
maggots that enter wounds and enlarge them or keep them open until 
the conditions are such that checks form in the timber, 
(6) For preventing losses in timber cut for purposes requiring clear 
stuff, select trees growing at altitudes above 1,800 feet and those at 
lower elevations which are free from the black indicating spots (old 
resin masses) on the bark. 
Approved : 
JAMES WILSON, 
Secretary of Agriculture. 
WASHINGTON, D. C., May 24, 1905. 
O 
