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.

Continuous trapping on the bark surface of Dendroctonus brevicomis LeConte infested trees in the central Sierra Nevada mountains was undertaken with the objective of determining the spatial and temporal arrival patterns of the natural enemies and other insect associates of the western pine beetle. Over 100 species of D. brevicomis associates were collected and patterns of arrival were described for many of these. The main bark beetle predators were trapped during D. brevicomis mass arrival and shortly thereafter. Enoclerus lecontei, Temnochila chlorodia, and Aulonium longum, all predaceous beetles on D. brevicomis adults and larvae, were among the first species to arrive, as was Medetera aldrichii (Diptera), a larval predator. The bark beetle parasites Roptrocerus xylophagorum and Dinotiscus (=Cecidostiba) burkei (Hymenoptera) were well synchronized with the beetles’ life cycle as they arrived late in the beetles’ larval stages when suitable hosts were available.Approximately twice as many associates were trapped in the first (spring) beetle generation as in the second (fall). Differences between species with regard to height distribution were common, and these often varied with seasonal beetle generation.Calculations of changes in species diversity through time, of the associate complex trapped at the bark surface, were made for both the first and second beetle generation. Linear correlation analysis indicated a highly significant increase in species diversity occurred from the time of the beetles’ mass arrival until brood emergence. This increase may correspond to an increase in diversity of the structure of the subcortical community, as more insect species arrived and progressively modified the habitat of the newly killed tree.

Estimating populations of both pest and natural enemy species is important in the planning and implementation of biological control. For example, synthetic pheromone lures are used to sample bark beetles, and sometimes their predators, in forest ecosystems. However, insect attraction to natural pheromone sources may differ from attraction to synthetic pheromone lures. Moreover, these differences may vary systematically between the target pest and some important natural enemies. Thus, the accuracy of both absolute and relative abundances of bark beetles and predators could vary with lure selection. We evaluated a series of synthetic lures to determine which lure gave the closest approximation to actual numbers of Ips pini (Say) and predators arriving at hosts infested with I. pini in Wisconsin. We deployed synthetic lures containing various ratios of the (ϩ) and (Ϫ) enantiomers of the principal I. pini pheromone component, ipsdienol, with or without an additional component, lanierone. I. pini showed strong preferences for speciÞc enantiomeric ratios of ipsdienol, and these responses were synergized by lanierone. Predators showed equally strong attraction to ipsdienol, but preferred different ratios of the stereoisomers. The addition of lanierone had no effect on predators. The most abundant predator, Thanasimus dubius (F.), showed greater preference for host material infested with I. pini than any synthetic lure. These disparities in responses, combined with strong disparities in seasonal ßight patterns, provided estimates of pest to predator ratios that varied by as little as 12% to as much as 12 times, from pest:predator ratios arriving at host material infested with I. pini. These results suggest that variation between herbivores and their natural enemies in their phenology, preferred pheromone blends, and infested host material should be considered when developing estimates of relative pest and predator densities for subsequent management options.

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