was no observable trend in a side of a tree initially 

 attacked and direction of nearest attacked tree. 



Discussion 



We use the term "switch" to represent a shift in 

 attacks on consecutive days between a focus tree and 

 surrounding trees. In our study, switch in attacks 

 from a focus tree to nearby trees typically occurred the 

 second day of attack, and at time of peak daily attack 

 density on the focus tree (fig. 3, 4). Similarly, Raffa and 

 Berryman (1983) observed highest rate of attack on 

 day 2. Maximum attractiveness of the host coincides 

 with the period of high ^rans-verbenol production; 

 majority of production in female hindguts occurs after 

 24 hours of feeding (Pitman and Vite 1969). Female 

 mountain pine beetle that had fed in lodgepole pine 

 bolts for 24 hours also contained significant amounts 

 of verbenone (Hunt and Borden 1989). Similarly, large 

 amounts of volatiles of both verbenone and the aggre- 

 gation pheromone (cis-verbenol) of Ips typographus 

 were measured from individual entrance holes on the 

 second day of attack (Birgersson and Bergstrom 1988). 

 Large concentrations of both aggregation and inhibi- 

 tory pheromones in the first few days of attack on a 

 tree may cause mountain pine beetle to be attracted 

 to the focus tree, and both space out on the bole and 

 participate in the tree-switching mechanism (Bedard 

 and others 1980; Birgersson and Bergstrom 1988). 

 The function of inhibitory pheromones such as 

 verbenone in this tree-svntching mechanism is un- 

 clear, however. 



Based on results of our study, shift in attacks to a 

 new host occurred before the focus tree was fully 

 colonized, although each tree continued to be attacked 

 at a decreasing daily rate for 5 to 6 days. If beetles were 

 responding to verbenone before landing on a tree, as 

 suggested by Bertram and Paine (1994), range of 

 perception in our study plot was smaller than the 

 distance between the two closest trees attacked within 

 a day (less than 1 m). If verbenone was providing an 

 inhibitory plume around a tree, most attacks on that 

 focus tree would cease, signaling a fully utilized re- 

 source, although additional attacks may occur due to 

 variability in the response threshold of individual 

 beetles. However, because attacks shifted to a new 

 tree while less than 50 percent of total attacks had 

 occurred on the focus tree, verbenone (or other inhibi- 

 tory pheromones) could be functioning at a more local 

 scale, perhaps around a particular entrance hole, as 

 was suggested previously (Raffa and Berryman 1983; 

 Renwick and Vite 1970). While large amounts of ag- 

 gregating pheromones are still being produced, 

 verbenone may provide a means to reduce competition 

 within a tree, while some other component of the 

 pheromone system is the primary cause of attack 



switch to a new tree. The "threshold" hypothesis of 

 Gara and Coster ( 1968) is supported by this reasoning. 



Our results indicate that a shift in attacks to a new 

 tree occurred on the day of greatest attack rate on a 

 nearby tree. If we assume that time of greatest attack 

 rate on a particular tree coincides with peak trans- 

 verbenol emission from that tree, switch in attacks to 

 a new tree could be explained by a spill-over effect. 

 This would occur when the concentration of aggrega- 

 tion pheromone was leu-ge enough to envelope sur- 

 rounding trees, resulting in random attacks on trees 

 enveloped by the plume, and establishment of a new 

 focus tree. The new focus tree then has an increase in 

 number of attacks, while attacks on the old focus tree 

 decline due to a reduction in attacks and concomitant 

 trans-verhenol production (Pitman and Vite 1969). 

 These results were seen in our data: number of attacks 

 on recipient trees the first day of switching was always 

 less than number of attacks on the focus tree that same 

 day. The following day, attacks on recipient trees were 

 greater than on the previous focus tree (fig. 3, 4). Each 

 beetle is more attractive when joined by other beetles 

 than when alone, thereby resulting in the nonlinear 

 increase in attacks with time observed. Throughout 

 the 18 days we monitored the plot, many trees were 

 under attack at the same time, resulting in overlap- 

 ping plumes of aggregating pheromones. Although we 

 have evaluated our data on a daily basis by looking at 

 consecutively attacked trees, overlapping plumes of 

 aggregation pheromones could become increasingly 

 complex, greatly influencing beetles dispersing in the 

 area. Although we do not know the size of aggregation 

 pheromone plumes, our data suggests that the inhibi- 

 tory, or antiaggregation, pl\ime being released is 

 smaller than the distance between two consecutively 

 attacked trees spaced less than 1 m apart. 



Conclusions 



We have observed response of a nonepidemic popu- 

 lation of mountain pine beetle to naturally occurring 

 semiochemicals. Results from our study indicate that 

 beetle attacks may switch to a new tree before the 

 original focus tree is fully colonized. What role aggre- 

 gation and inhibitory compounds play in the process of 

 switching attacks to new trees, and the geographic 

 extent of this phenomenon, are unclear. Because 

 verbenone is used by so many species of aggressive 

 bark beetles, and a large portion is produced by micro- 

 organisms in decaying wood, it may be a signal to 

 beetles that the immediate substrate is no longer 

 suitable for colonization (Borden and others 1987; 

 Leufven and Birgersson 1987). The question remains — 

 are beetles responding to verbenone prior to or after 

 landing on the tree? Our results suggest that verbenone 

 may be acting within a tree rather than between trees. 



6 



