Detection of stag beetle oviposition sites by combining telemetry and emergence traps

Abstract

The European stag beetle, Lucanus cervus, is a flagship species for biodiversity conservation of old-growth forests and is protected under the Habitats Directive. Although it has been the focus of active research in the last two decades, many aspects of its ecology and habitat requirements for the larvae remain poorly known, particularly to what extent certain factors limit larval development. The objectives of this preliminary work were: (1) to explore the feasibility of a non-invasive method for detecting oviposition sites; (2) to attempt the characterisation of above-ground ecological factors recorded in the oviposition sites and (3) to quantify the number of traps and operators needed for obtaining a number of beetles suitable for statistical analysis. In 2014, twelve females were followed by means of radio-telemetry to detect potential oviposition sites in a relict broadleaf forest of northern Italy. In 2015, emergence traps were set in nine sites selected from the 21 sites where females were recorded digging deeply in the soil near to dead wood during the previous year. Traps were checked during the 2015 and 2016 flight seasons. Overall, 15 stag beetles were detected (8 males and 7 females) from five emergence trap sites which were therefore regarded as real oviposition sites. All oviposition sites were characterised in terms of typology of dead wood, tree species, canopy openness, trunk diameter, dead wood volume, decomposition stage (five classes) and wood hardness (four classes). All the detected emergence sites belonged to the genus Quercus, two being from the allochthonous Q. rubra, but no preferences for a dead wood species nor for a typology were shown and a broad variation was apparent for all the considered variables. The mean values of canopy openness, diameter, dead wood volume, decay status and wood hardness were 2.54%, 51cm, 4.92m3, 3 and 3.4 respectively. These data suggested an important heterogeneity in the oviposition sites selection. Although this method (telemetry + emergence traps) provided substantial aid to finding newly emerged beetles, it required a large amount of fieldwork effort, both in terms of time and man-hours. The advantage of the method is its low degree of invasion while its drawback is the amount of effort needed. Calculations were made to assess the minimum number of operators and traps needed to gather a number of data suitable for statistical analysis. It was found that two full time operators should be able to detect about 50 potential oviposition sites in one flight season, 28 of which were expected to be real oviposition sites.