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THE TREE. 15 
TOLERANCE. 
Western yellow pine is intolerant of shade, except in the seedling 
stage on very favorable situations. Investigations conducted at the 
Coconino Forest Experiment Station indicate that most seedlings 
get their start under protection of the seed trees, while in small 
openings the bordering stand shields the seedlings from the full 
effect of the weather. Saplings may exist under the shade of veter- 
ans, and a 74-year-old pole was found growing in full shade, though 
it had but a few more years to live. If once suppressed during the 
sapling stage, western yellow pine can not recover and develop into 
saw timber. Dense mature stands often decline in vigor and become 
stag-headed through lack of side light. 
CAUSES OF INJURY... 
INSECTS. 
The Dendroctonus beetles, of which six species, including the 
Black Hills beetle, have been observed in Arizona and New Mexico, 
are perhaps the most destructive enemies of western yellow pine.t 
Records of depredations by the Black Hills beetle indicate that these 
have been far more continuous and extensive in comparatively hu- 
mid sections, like the Black Hills of South Dakota and certain sec- 
tions of Colorado, than under the more arid conditions of Arizona 
and New Mexico. The fact, however, that practically all of the 
Dendroctonus beetles are known to be primary enemies of western 
yellow pine in that they attack and lill perfectly healthy trees 
makes it important that Forest officers should watch for any evidence 
of the abnormal dying of pine timber, and should take prompt 
steps, if it is found to be due to the work of these beetles, to check 
further depredations, in accordance with the methods advised by the 
Bureau of Entomology. 
FUNGI. 
The bluing and red rot of western yellow pine are due to the 
attacks of fungi, and have been described fully in Bureau of Plant 
Industry Bulletin 36, “ The Bluing and Red Rot of Western Yellow 
Pine, with Special Reference to the Black Hills Forest Reserve.” 
The blue fungus attacks the contents of the wood cells and not the 
cells themselves; consequently the blued wood is not rotten. The 
spores of the fungus which causes red rot lodge in bark grooves of 
dying trees, and after germinating grow through the cambium and 
sapwood, and attack and destroy the cell walls of the heartwood. 

1For complete information concerning the work of these insects and their control, see 
Bureau of Entomology Bulletins 83, Part I, and 58, Part V, and Circulars 125, 126, 127, 
and 129, 
