Pheromone monitoring of rare and threatened insects: exploiting a pheromone-kairomone system to estimate prey and predator abundance.

Pheromone-based monitoring is a promising new method for assessing the conservation status of many threatened insect species. We examined the versatility and usefulness of pheromone-based monitoring by integrating a pheromone-kairomone trapping system and pitfall trapping system in the monitoring of two saproxylic beetles, the hermit beetle Osmoderma eremita (Coleoptera: Scarabaeidae) and its predator Elater ferrugineus (Coleoptera: Elateridae), which live inside hollow trees. We performed mark-recapture studies of both species with unbaited pitfall traps in oak hollows combined with pheromone-baited funnel traps suspended from oak branches to intercept dispersing individuals. For O. eremita, the integrated trapping system showed that the population in the study sites may be considerably higher than estimates based on extrapolation from pitfall trapping alone (approximately 3400 vs. 1100 or 1800 individuals, respectively). Recaptures between odor-baited funnel traps showed that males and females had similar dispersal rates, but estimating the number of dispersing individuals was problematic due to declining recapture probability between subsequent capture events. Our conservative estimate, assuming a linear decrease in capture probability, suggested that around 1900 individuals, or at least half of the O. eremita population, may perform flights from their natal host trees, representing higher dispersal rates than previous estimates. E. ferrugineus was rarely caught in pitfall traps. One hundred thirty-nine individuals, likely almost exclusively females, were caught in odor-baited funnel traps with approximately 4% recapture probability. If recapture probability over consecutive capture events follows that of O. eremita, this would correspond to a total population size of 2500-3000 individuals of the predator; similar to its supposed prey O. eremita. Our results demonstrate that pheromone-based monitoring is a valuable tool in the study of species or life-history stages that would otherwise be inaccessible.