Differential responses among natural enemies and prey to bark beetle pheromones.

Predators that exploit prey pheromones may select for relatively subtle chemical changes that confer partial escape yet retain intraspecific functionality. Parallel experiments conducted with a transcontinentally distributed bark beetle, Ips pini, demonstrated that the most abundant predators in California and Wisconsin were more attracted to prey from distant sources than to those from local sources. Conversely, local I. pini populations were most attractive to local conspecifics. Any benefits to prey are probably only partial and temporary, however, because all predators showed some attraction to both I. pini populations. Responses by other insects exploiting I. pini communication systems varied with ecological guild. A parasitic wasp that attacks adult hosts was more attracted to local than to distant I. pini populations. Secondary subcortically feeding herbivores that cannot colonize living trees but rather breed in trees killed by I. pini were highly attracted to infested logs but did not consistently prefer either I. pini population source. Some possible underlying mechanisms, implications for natural enemy-prey coevolution, and suggested strategies for biological control are discussed.