Geographic variation in prey preference in bark beetle predators

1. Bark beetles and their predators are useful systems for addressing questions concerning diet breadth and prey preference in arthropod natural enemies. These predators use bark beetle pheromones to locate their prey, and the response to different pheromones is a measure of prey preference. 2. Trapping experiments were conducted to examine geographic variation in the response to prey pheromones by two bark beetle predators, Thanasimus dubius and Temnochila virescens. The experiments used pheromones for several Dendroctonus and Ips prey species (frontalin, ipsdienol, and ipsenol) and manipulated visual cues involved in prey location (black vs. white traps). The study sites included regions where the frontalin-emitter Dendroctonus frontalis was in outbreak vs. endemic or absent. 3. There was significant geographic variation in pheromone preference for T. dubius. This predator strongly preferred a pheromone (frontalin) associated with D. frontalis at outbreak sites, while preference was more even at endemic and absent sites. No geographic variation was found in the response by T. virescens. White traps caught fewer insects than black traps for both predators, suggesting that visual cues are also important in prey location. 4. The overall pattern for T. dubius is consistent with switching or optimal foraging theory, assuming D. frontalis is a higher quality prey than Ips. The two predator species partition the prey pheromones in areas where D. frontalis is abundant, possibly to minimise competition and intraguild predation.