Should I stay or should I go? Modelling dispersal strategies in saproxylic insects based on pheromone capture and radio telemetry: a case study on the threatened hermit beetle Osmoderma eremita

To predict how organisms cope with habitat fragmentation we must understand
their dispersal biology, which can be notoriously difficult. We used a novel, multi-pronged
approach to study dispersal strategies in the endangered saproxylic hermit beetle Osmoderma eremita, exploiting its pheromone system to intercept high numbers of dispersing
individuals, which is not possible with other methods. Mark-release-recapture, using unbaited
pitfall traps inside oak hollows and pheromone-baited funnel traps suspended from
tree branches, was combined with radio telemetry (in females only) to record displacements.
Dispersal, modelled as a probability distribution of net displacement, did not differ significantly
between sexes (males versus females recaptured), observation methods (females
recaptured versus radio-tracked), or sites of first capture (pitfall trap in tree versus pheromone
trap – distance from original dispersal point unknown). A model including all
observed individuals yielded a mean displacement of 82 m with 1% dispersing1 km.
Differences in body length were small between individuals captured in pitfall versus
pheromone traps, indicating that dispersal is rarely a condition-dependent response in
O. eremita. Individuals captured in pheromone traps were consistently lighter, indicating
that most dispersal events occur relatively late in life, which agrees with trap catch data. In
addition, most (79%) females captured in pheromone traps were mated, showing that
females typically mate before leaving their natal tree. Our data show that integrating odour
attractants into insect conservation biology provides a means to target dispersing individuals
and could greatly improve our knowledge of dispersal biology in threatened species.