Bark beetle populations may undergo dramatic fluctuations and are ofte
n important pests in coniferous forests. Their dynamics are thought to
be primarily driven by factors affecting the resistance of the host t
ree to attack, i.e., bottom-up forces, while natural enemies are usual
ly assigned a minor role in these systems. I present behavioral experi
ments that suggest that the clerid beetle Thanasimus dubius may be an
important source of mortality for the bark beetle Dendroctonus frontal
is during attack of the host tree, and determine the nature of the fun
ctional response of T. dubius under conditions close to natural. I als
o examine the numerical response of T. dubius to large-scale fluctuati
ons in D. frontalis density, and the relationship between bark beetle
population trends and predator density, and find that beetle populatio
ns tend to decline when predator densities are high. Combined with the
effects of clerid larvae on bark beetle broods, these results suggest
that top-down forces generated by natural enemies could also be an im
portant component of bark beetle dynamics. The implications of these r
esults for bark beetle dynamics are discussed in relation to the prolo
nged life-cycle of clerid beetles.