Variation in complex semiochemical signals arising from insects and host plants

Chemical communication by many insect species involves complex signals of both insect and plant origin. Much attention has been focused on the behavioral activities of these components but less on their sources of variation, despite implications for evolutionary theory and pest management. We studied variation in chemical signaling at host, tree-within-host, and beetle-on-tree scales using tunneling male pine engravers [Ips pini (Say)] on jack, Pinus banksiana Lamb, red, P. resinosa Aiton, and white, P. strobus L. pines. Pine engravers are distributed transcontinentally, and stereoisomeric ratios of their principal pheromone component ipsdienol varies regionally. Linear mixed-effects models were used to examine variation in monoterpene and pheromone volatile profiles, determined by gas chromatography. Phloem from white pine had the greatest concentration of monoterpenes, although insects tunneling in white pine produced the smallest ratios of monoterpenes to pheromones (1:2) in their volatile plumes relative to jack and red pine (1:1). Beetle-to-beetle variation in plume composition was ≈29 times greater than the inter-tree variation within a tree species. The stereoisomeric ratio of ipsdienol was highly consistent within the pheromone component of the plume. The little variation present existed almost entirely at the level of the insects. Within the pheromone component of the plume in a given host species, there was up to 13 times more beetle-to-beetle than tree-to-tree variation. This magnitude was almost double the magnitudes of the ratios among components within the entire plumes. Implications to the behavioral ecology of bark beetle communication, such as potential strategies of cheating and predator avoidance, are discussed.