Significant uncertainty in bark beetle trap catches due to varying pheromone release from dispensers and local trap position

Tree-killing bark beetles are important pests severely affecting forests worldwide. An understanding of their spatio-temporal swarming intensity, typically assessed with pheromone traps, is crucial to guide management actions. While multiple factors have been shown to affect trap catches, we lack knowledge of the effects of inherent dispenser-dependent variations in pheromone release and of local trap position. In a laboratory experiment, we assessed the influence of filling level and temperature on the release rate (Rr) of three commonly used pheromone dispenser products for bark beetles (Pheroprax®, Chalcoprax®, Curviwit®). By conducting a complementary field study at two sites in Germany, we quantified the effect of varying Rr of Pheroprax® and trap position on the number of Ips typographus trapped. Rr of all three products correlated with temperature and strongly declined during the application period in Pheroprax® and Chalcoprax®. In the field, both the temporal variability in filling level and the ambient temperature similarly affected Rr, which in combination led to a fivefold change in trap catches. Additionally, catches varied by a similar magnitude due to local trap position, partly explained by the distance from the forest edge. The large uncertainties found in pheromone trap catches, which may also apply to other pest species, highlight the need for careful interpretation (or correction) of trap data. As a potential improvement of monitoring, we propose swarming models to facilitate more accurate predictions of infestation risk by (i) incorporating uncertainties arising from trap-related factors and (ii) providing continuous information on the spatio-temporal abundance of pest species.