INSECT ENEMIES OF WESTERN FORESTS 183 
be used by inexperienced estimators with a fair degree of accuracy. 
In order to cover any large area, however, a great many strips must 
be run, which makes the method more laborious and consequently 
more time-consuming and expensive. 
In using this method, the observer travels through the forest along 
some routes of known position and length, such as a forest road or 
trail, but preferably along a section line or compass line, so as to 
obtain an impartial cross section of the area uninfluenced by the 
special types of trees which might be encountered along ridges or 
canyon bottoms. Distances are determined by pacing, or using the 
known distances between. fixed points, such as section corners or topo- 
eraphic features located on accurate maps. Without attempting to 
blaze or mark the trees, the numbers of fading, sorrel, or red-top 
trees are counted within a specified distance on either side of the 
line of travel. 
The width of the strip will depend on the density of the forest 
stand, and should be so chosen that the outer edges will correspond 
approximately with the average limit of vision within the stand. 
For open ponderosa pine stands a 10-chain strip (330 feet on each 
side of the center line) has been found generally satisfactory, but 
in the heavier stands this often needs to be reduced to an 8-, 5-, or 
even a 4-chain strip. In lodge pole and western white pine stands, 
red-top surveys usually are limited to 5-, 4-, 2-, and even 1-chain 
strips. However, in these types extensive surveys are usually con- 
ducted in the fall of the year to determine the number of new attacks, 
and the old loss represented by the red-top trees is ignored. Since 
the newly attacked trees are not discolored and can be found only 
by sighting the pitch tubes, very narrow strips are necessary, and 
a 1-chain strip (33 feet on each side of the compass line) has been 
adopted as standard for this work in the northern Rocky Mountain 
region. 
When a red-top strip count includes several ages of infested and 
recently abandoned trees, it 1s necessary to examine a representative 
series of trees, either on a sample strip or on a sample plot, to deter- 
mine the proportion of the different classes of insect attack and 
years of infestation which may be represented. A limited amount 
_ of intensive work is also necessary to determine the average diameters, 
heights, and volumes represented by the infested trees. The number 
of trees counted on the strips multiplied by the number of times 
the acreage of the strip would be contained in the acreage of the 
entire area will give the approximate number of trees for the entire 
unit. 
No fixed rule can be given as to the percentage of an infested 
area that should be covered during an extensive insect survey to 
obtain a reasonable degree of accuracy. The exactness of the survey 
will depend on the time and money available and the value of the 
timber stand under examination. Small units of valuable timber 
should be covered with a greater refinement of methods and a higher 
degree of accuracy than an extensive area of heavily infested lodge- 
pole pine. Ordinarily a 5-percent sample of an area should give a 
reasonably good estimate for control purposes, and on large areas 
a 1-percent sample is often sufficient. 
