42 BULLETIN 1496, U. S. DEPARTMENT OF AGRICULTURE 
not less than 1,200 trees to the acre. Even closer planting would be 
preferable. This close planting protects the trees from injury by 
the white-pine weevil, as the insect does not attack so readily trees 
growing in dense stands, and the effects of the attacks which do 
occur are rapidly outgrown by the trees. The extra cost in planting 
a larger number of trees per acre is justified in the long run, partly 
by the reduction of the possible loss through the pine weevil and 
partly by the higher quality of the timber. 
SPRUCE BUD WORM 
Evidence is strong that the serious outbreaks of the spruce bud 
worm are due to the presence of many balsam firs in the black-spruce 
forests. The bud worm attacks first of all balsam fir and, when it 
finds favorable food, breeds to such an extent that it begins to attack 
black spruce. Apparently, the only way to prevent epidemic out- 
breaks of the spruce bud worm is to reduce the proportion of balsam 
fir in the spruce stands; and this can be done, if at all, only by 
proper handling of the spruce forests. 
JACK-PINE SAWFLY AND TIP MOTH 
The appearance of the jack-pine sawfly and the jack-pine tip moth 
is due to the great increase in quantity of jack pine in this region 
as the result of fires. The only preventive measure is to limit 
the amount of jack pine by replacing it with Norway and white pine. 
If the proportion of jack pine is reduced, the danger of jack-pine 
sawfly outbreaks will be greatly lessened. 
LARCH SAWFLY 
Very little is yet known concerning the protection of forests 
against the larch sawfly. It seems that tamarack in swamps is not 
safe against attacks by the sawfly. For this reason tamarack is not 
the most promising species in the swamps. There is a possibility, 
however, that tamarack grown on the better-drained soils is more 
resistant to sawfly attack and on such lands may once more become 
one of the best trees. 
GENERAL MEASURES 
The most important general principle to bear in mind in the protec- 
tion of forests against all insect outbreaks is that a forest made up of 
conifers and hardwoods is always safer from insects than is a pure 
coniferous forest or a pure hardwood forest. This is particularly 
true in regions where the natural forests are of a mixed type. Pro- 
tection against insect attacks, therefore, lies in the improvement of 
the present forests and coincides closely with the measures advocated 
for intensive forest practice. 
SPECIFIC RECOMMENDATIONS FOR HANDLING THE 
FORESTS IN THE LAKE-STATES REGION 
Owners of forest land, aside from a general knowledge of the 
conditions essential for keeping their lands productive, must also 
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