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 <i>Osmoderma eremita</i>

Abstract:
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.