term "switch" to represent a shift in focus of attacks on 

 consecutive days, regardless of distance. It is likely, 

 however, that attacks are initiated on a single tree due 

 to a cumulative affect of all trees in the local vicinity 

 that are actively under attack. If our data are exam- 

 ined in this manner, it appears T6 may have been 

 attacked by redirected beetles from T2 and T3, yet 

 temporally further along than T6 in the mass-attack 

 sequence (fig. 2, 5). In our study, except in one case, 

 only one tree that was switched to on a particular day 

 was successfully mass attacked, although many 

 smaller trees were attacked at lower densities. Our 

 study was in an area with a sub-epidemic population, 

 however. Larger populations may behave differently. 



Large trees are believed to initiate and maintain 

 the switching process, while small trees are only killed 

 because they are near other attacked trees (Geiszler 

 and others 1980; Mitchell and Preisler 1991). Preisler 

 and Mitchell (1993) found the probability that large 

 trees are attacked to be greater than what a random 

 attack model alone would predict. In our study, average 

 d.b.h. of infested trees within the plot was 10.4 inches, 

 strip attacked trees 9.5 inches, and remaining green 

 trees 8.5 inches. These results indicate the mass at- 

 tack process was initiated on larger trees more often 

 than smaller ones. In fact, on day 5, attacks switched 

 from T4 to T5a (8.5 inches d.b.h.) and T5b (9.6 inches 

 d.b.h. ), while a 7.6 inch d.b.h. tree closer to T4 had only 

 a few attacks. This phenomenon could be due to either 



random landings on larger surfaces, or actual beetle 

 preference for larger trees (Cole and Amman 1969; 

 Gibson 1943; Hopping and Beall 1948). The largest 

 distance between consecutively attacked trees (non- 

 baited) in our study was 3.2 m, well within the maxi- 

 mum distance between attacked trees reported by 

 others. Raffa and Berryman (1983) stated that no 

 trees greater than 6 m apart were attacked, and 

 Geiszler and others (1980) suggested no switching 

 beyond 7.3 m. Preisler and Mitchell (1993) found that 

 in thinned stands, spacings of 4.3 to 6.9 m between 

 trees did not seem to affect switching of attacks to 

 surrounding trees. 



If only those trees that were successfully attacked 

 are considered, proportion of total attacks on the south 

 aspect (28.2 percent) were slightly greater than either 

 east (27.5 percent) or north (27.5 percent) aspects. 

 West aspect of all trees had the lowest proportion of 

 total attacks (16.7 percent). Typically beetles attack 

 trees on the cooler north to east aspects; fewest attacks 

 occur on hotter south aspects of the bole (Rasmussen 

 1974; Reid 1963; Safranyik and Vithayasai 1971; 

 Shepherd 1965). However, in this study area, there 

 was a nearby source of beetles to the south, and the 

 plot was on a west-facing slope. Therefore, the south 

 aspect may have been slightly cooler theui west aspect, 

 which may explain higher proportion of southern ex- 

 posure attacks. Also, beetles may have taken advan- 

 tage of upslope winds during the heat of the day. There 



300 



250 



-TO o~ 



-O- 



^ 200 



B 150 

 I 100 



V 



X 

 V 



X 



50 

 



6 7 8 

 Day of Attack 



10 11 12 13 



o T2 V T3 X T6 



Figure 5 — Cumulative number of attacks on three trees in plot that were success- 

 fully attacked. Day 1 is August 6, 1995 (JD 218). 



5 



