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 wdiich 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- 

 graphic 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 Avestern 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 Kocky Mountain 

 region. 



When a red-top strip count includes several ages of infested and 

 recently abandoned trees, it is 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. 



