Can chemical communication be cryptic? Adaptations by herbivores to natural enemies exploiting prey semiochemistry

Abstract

Predators and parasites commonly use chemical cues associated with herbivore feeding and reproduction to locate prey. However, we currently know little about mechanisms by which herbivores may avoid such natural enemies. Pheromones are crucial to many aspects of herbivore life history, so radical alterations of these compounds could be disadvantageous despite their exploitation by predators. Instead, minor modifications in pheromone chemistry may facilitate partial escape while maintaining intraspecific functionality. We tested this hypothesis using Ips pini, an endophytic beetle that develops in the phloem tissue of pine trees. Its predominant predators in the Great Lakes region of North America are Thanasimus dubius and Platysoma cylindrica, both of which are highly attracted to I. pini’s pheromones. However, there are significant disparities between prey and predator behaviors that relate to nuances of pheromone chemistry. Thanasimus dubius is most attracted to the (+) stereoisomer of ipsdienol, and P. cylindrica is most attracted to the (−) form; Ips pini prefers racemic mixtures intermediate between each predator’s preferences. Further, a component that is inactive by itself, lanierone, greatly synergizes the attraction of I. pini to ipsdienol, but has a weak or no effect on its predators. A temporal component adds to this behavioral disparity: lanierone is most important in the communication of I. pini during periods when its predators are most abundant. The difficulties involved in tracking prey are further compounded by spatial and temporal variation in prey signaling on a local scale. For example, the preferences of I. pini vary significantly among sites only 50 km apart. This chemical crypsis is analogous to morphological forms of camouflage, such as color and mimicry, that are widely recognized as evasive adaptations against visually searching predators. Presumably these relationships are dynamic, with predators and prey shifting responses in microevolutionary time. However, several factors may delay predator counter adaptations. The most important appears to be the availability of alternate prey, specifically I. grandicollis, whose pheromone ipsenol is highly attractive to the above predators but not cross-attractive with I. pini. Consistent with this view, the specialist parasitoid, Tomicobia tibialis, has behavioral preferences for pheromone components that closely correspond with those of I. pini. These results are discussed in terms of population dynamics and coevolutionary theory.