INSECT ENEMIES OF WESTERN FORESTS 229 



along roads. Sample strips run at intervals back and forth across 

 infested areas permit a very comprehensive estimate of the 

 amount and distribution of infestation, and where time and money 

 are available these unquestionably furnish the most satisfactory 

 basis of estimates. The cruising- of sample plots has its place as a 

 supplement to topographic viewing, and with small units it is 

 often possible actually to survey a rather large portion of the area 

 in this way. Where large areas of diverse topography include a 

 number of different forest types, several methods may be used. 

 In fact, every source of information should be utilized in arriving 

 at the final estimate. 



The Topographic Method 



The topographic method, or red-top survey, is particularly well 

 adapted to estimating bark-beetle losses over large forested areas 

 of rough topography, where a large part of the forest can be 

 viewed from open valleys, ridges, or lookout points. It is the 

 cheapest and quickest method but is subject to a high degree of 

 error unless supplemented by intensive examinations of sample 

 plots or strips. 



In using this method, the estimator, equipped with binoculars 

 and a topographic map of the area, travels through the area 

 visiting all of the ridges, valleys, or lookout points that can be 

 found. At each selected point the opposite slopes and visible areas 

 are viewed, the red, sorrel, or fading trees counted, and an 

 estimate placed on the map as to the total number of dying or 

 dead trees per acre. Then strips or plots are actually cruised and 

 a ratio determined between the number viewed and the total 

 number actually found. Also, the proportion of newly infested and 

 abandoned trees, all of which have been counted in the general 

 survey, is determined. The total estimate is then corrected by 

 these ratios. 



The Sample-Strip Method 



The sample-strip method is adapted to estimating bark-beetle 

 losses on flat or gently rolling areas where viewing from a dis- 

 tance is impossible. It is also a more efficient and accurate method 

 and can 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 forest types 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 topographic features located on accurate maps. With- 

 out attempting to blaze or mark the trees, the numbers of fading, 



