Semi‐field experiments investigating facilitation: arrival order decides the interrelationship between two saproxylic beetle species

1. Acanthocinus aedilis (Linnaeus) and Rhagium inquisitor (Linnaeus) both colonise the cambial layer in newly dead Scots pine Pinus sylvestris L. and thus are potential competitors. 2. Species interactions and in particular priority effects were investigated in an experiment with a replacement series design. Four pairs of adult beetles were released on each of 91 caged pine bolts (0.35 m long, 13 replicates). To test for priority effects, a 2‐week interval was used to separate the species. 3. The interrelationship in simultaneous coexistence was positive for A. aedilis and negative for R. inquisitor (+,?). Acanthocinus aedilis produced 161% more offspring per female in coexistence than alone. Rhagium inquisitor had lower offspring quality in coexistence where its larvae weighed 39% less than in one‐species bolts. 4. The interrelationship depended on arrival order. When A. aedilis had priority the interaction was again advantageous to A. aedilis (+,?) but when R. inquisitor had priority no species interaction occurred (0,0). Both species therefore fared better when having priority. 5. Facilitation in cerambycids is novel and the facilitative effect on A. aedilis could be oviposition incitement or resource enhancement by R. inquisitor.