sampling technique is definitely superior in this respect because the cause of death 

 of most larvae can be determined. Consequently, the bark-removal sampling method will 

 continue to be the choice for life-table sampling of mountain pine beetles where assess- 

 ment of mortality factors is of primary concern. 



Brood survival .-- Although estimates of brood by the bark-removal and radiographic 

 methods were comparable, the number of brood that completed development and emerged 

 was only 4.0 adults per 30.4 cm^ for the radiographed areas compared to 27.8 per 30.4 

 cm^ for nonradiographed areas. In addition, the sex ratio was more in favor of females 

 in radiographed areas than in nonradiographed areas--l : 2 .50 compared to 1:1.67 (males 

 to females) . A greater rate of drying in radiographed areas is suspected of causing 

 these differences (unpublished data on file at Intermountain Forest and Range Experiment 

 Station, Ogden, Utah). 



Bluestain fungi introduced by the parent beetles interfere with movement of water 

 in the infested tree (Nelson 1934) . However, after the initial sample was taken, bark 

 and wood in the area sampled by the bark-removal method appeared to contain more 

 moisture than that sampled by the radiographic method. The vertical slots made by the 

 saw to accommodate the film may have prevented moisture from moving horizontally from 

 inner sapwood to outer sapwood and bark being radiographed. In addition, evaporation 

 took place from the cut surfaces. 



The amount of radiation received by the developing brood should not have caused 

 the differences noted; no such abnormality was observed during laboratory investiga- 

 tions. In addition, the greatest total exposure to which any group of beetles was 

 subjected for the three sample dates was about 800 R. This exposure was far below that 

 generally found to sterilize insects or to interfere with insect behavior. For ex- 

 ample, Davich and Lindquist (1962) found that at an exposure of 2,500 R, behavior and 

 egg laying were about normal in the boll weevil (Anthonomus grandis Boheman) . 



Average size of mountain pine beetle brood adults did not appear to differ sig- 

 nificantly between radiographed and nonradiographed areas: males in radiographed areas 

 averaged 4.64 mm in length compared to 4.75 mm for males from nonradiographed areas; 

 females in radiographed areas averaged 5.07 mm in length compared to 5.11 mm for non- 

 radiographed areas (table 4) . 



Radiographic quality .-- A problem that affected radiographic quality was unequal 

 moisture content of the bark and wood within some of the radiographed areas. IVhere 

 moisture was uniform, high quality radiographs were obtained (fig. 3) ; however, where 

 moisture was uneven, radiographs were of unequal density and difficult to interpret 

 (fig. 4). Streaks of high moisture content occurred where bluestain fungi were absent; 

 absence was the result of beetle attacks that left some areas of bark lightly infested 

 and, consequently, uninfected by bluestain fungi. Uneven radiographic exposure oc- 

 curred when the probe for the electrical resistance meter was unknowingly placed in 

 a moisture streak. This error resulted in overexposure of the radiograph for the areas 

 surrounding the moisture streak. A less serious error occurred when the moisture probe 

 was placed in fungus- infected wood near a moisture streak, which resulted in under- 

 exposure of the moisture streak. Usually, this area contained few larvae from nearby 

 galleries; consequently, this error would not have affected the total count as much as 

 that of the first type. Where a moisture streak is indicated on the first radiograph, 

 possible solutions are either to take two radiographs of the area (one adjusted to the 

 moisture streak, the other to the area surrounding the streak) or to develop a filter to 

 reduce exposure of the area surrounding the streak. Poor radiographs necessitated a 

 return trip to the field to retake radiographs at a more optimum exposure. A portable 

 daylight developer unit would provide on-the-spot development; poor radiographs could be 

 retaken immediately. 



8 



