The paradox of attraction: Why push-and-pull doesn't always work for spruce protection against bark beetles

Outbreaks of the European spruce bark beetle (Ips typographus) increasingly threaten Picea abies forests under climate stress. Semiochemical “push–and–pull” systems, combining anti-attractants with pheromone-baited traps, are widely promoted, yet their performance under outbreak pressure and operational constraints remains understudied. In 2023, we evaluated push, push–and–pull, and control treatments in a multi-site field experiment across five Alpine localities during high-pressure outbreak. Push treatments used anti-attractant blend based on non-host volatiles and inhibitory host compounds (1-hexanol, 1-octen-3-ol, 3-octanol, 1,8-cineole, trans-conophthorin, trans-4-thujanol). Push–and–pull plots additionally included three MultiWit slot traps, baited with the aggregation pheromone Pheroprax© and placed along the forest edge ∼25 m from treated trees at low trap density. Contrary to prevailing expectations, pheromone trap addition increased tree infestation relative to both push and control treatments. Push and control did not differ significantly, although no infested trees occurred under push. Monitoring traps used to assess background beetle pressure showed similar catches among treatments. Spatial mapping revealed that beetle-killed trees were concentrated near push–and–pull plots, consistent with beetle attraction to trap lures combined with insufficient interception capacity of traps. Our results provide field evidence that semiochemical protection outcomes depend on deployment configuration, not chemical composition alone. Under operational conditions with limited trapping infrastructure, push treatments did not increase attack rates, whereas push–and–pull was associated with higher infestation. These findings demonstrate strong context dependence in semiochemical strategies, highlighting the importance of trap density and spatial arrangement when designing bark beetle protection programmes.