Localized Spatial and Temporal 

 Attack Dynamics of the Mountain 

 Pine Beetle in Lodgepole Pine 



Barbara J. Bentz 

 James A. Powell 

 Jesse A. Logan 



Introduction 



Many species of bark beetles in the genus Dendroc- 

 tonus (Coleoptera: Scolytidae) use kairomones and 

 pheromones in the processes of host selection, attack, 

 and colonization (Borden 1982; Wood 1972). Female 

 mountain pine beetle {D. ponderosae Hopkins), which 

 in this species are usually responsible for initiating a 

 new attack, bore into the bark and through a chemi- 

 cally mediated synergistic reaction with host chemi- 

 cals, release ^rans-verbenol. Trans-verbenol is an aggre- 

 gative pheromone attracting both sexes (Hughes 1973; 

 Pitman 1971; Pitman and others 1968). At higher 

 concentrations of ^rans-verbenol, higher proportions 

 of males are attracted (Renwick and Vite 1970). Males 

 produce exo-brevicomin, which at low concentrations 

 primarily attracts females (Conn and others 1983), 

 although it may inhibit beetles at higher concentra- 

 tions (Borden and others 1987; Rudinsky and others 

 1974). This system of chemical communication en- 

 ables a massive aggregation of individuals on a single 

 "focus" tree that for species such as mountain pine 

 beetle, is selectively advantageous in overcoming host 

 defensive mechanisms. As with most herbivores, how- 

 ever, there is an optimal density range of individuals 

 on an exhaustible food resource (Berryman and others 

 1985). If beetle densities on a single tree are too high, 

 mortality can result from within-tree competition for 

 limited breeding and feeding space. 



Several hypotheses attempt to explain the termi- 

 nation of colonization on a single tree at optimal beetle 

 densities. The first assumes that anti-aggregative or 

 inhibitory pheromones such as verbenone and exo- 

 brevicomin deter incoming beetles, thereby terminat- 

 ing additiongd attacks and ensuring that beetle attack 

 density does not exceed the threshold for optimum 

 brood survival (Borden and others 1987). Verbenone is 

 a semiochemical produced by mountain pine beetles 

 (Rudinsky and others 1974), through autoxidation of 

 host terpenes, and by enzymatic conversion by yeasts 

 (Hunt and Borden 1989). Based on this hypothesis, as 

 verbenone is released, the majority of beetles are 

 dispersed at some distance, switching attacks from 



the focus tree to adjacent recipient trees (Geiszler and 

 others 1980). 



A second hypothesis emphasizes the role of host 

 resistance, citing cessation of resin exudation as the 

 primary cause for termination of colonization on a par- 

 ticular tree (Raffa and Berryman 1983; Renwick and 

 Vite 1970). As with the first hypothesis, attacks switch 

 to adjacent trees once the focus tree becomes fully 

 utiUzed — the difference lies in the roles of inhibitory 

 pheromones and host-tree resins. In the second hy- 

 pothesis, inhibitory pheromones are one component 

 of a suite of territorial behaviors that in conjunction 

 with a decrease in resin exudation, enable individual 

 colonizers to maximize reproductive potential (Raffa 

 and Berryman 1982, 1983). From this perspective, 

 inhibitory pheromones are perceived and function at 

 the local scale; attacks switch to neighboring trees 

 because the focus tree has become fully colonized and 

 resin exudation ceases. This role for inhibitory com- 

 pounds is comparable to studies performed with D. 

 brevicomis, wherein inhibitory compounds such as 

 verbenone were postulated to function as short-range 

 regulators of attack density rather than long-range 

 inhibitors that indicate a fully colonized host (Beyers 

 and others 1984). 



A third hypothesis, labeled the "threshold model," 

 assumes that as a tree is mass attacked, the liigh 

 concentration of frans-verbenol being emitted in the 

 local area causes incoming beetles to attack neigh- 

 boring trees that are enveloped in the pheromone 

 cloud (Coster and Gara 1968; Gara and Coster 1968; 

 Geiszler and others 1980). A threshold level of trans- 

 verbenol is necessary to cause landing and attack on 

 adjacent trees. 



The presence of inhibitory pheromones in the genus 

 Dendroctonus is known (Hunt and others 1989; Libby 

 and others 1985; Pitman and others 1969; Rudinsky 

 and others 1974; Ryker and Yandell 1983). However, 

 our lack of knowledge about the explicit function of these 

 pheromones in mountain pine beetle community ecol- 

 ogy is exhibited by confounding results in past re- 

 search endeavors (see Amman and Lindgren 1995 for 

 a review). It is unclear whether inhibitory pheromones 



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