Wood resource and not fungi attract early-successional saproxylic species of Heteroptera - an experimental approach

1. The availability of dead wood and ability to colonise these ephemeral structures are crucial for sustaining vital populations of saproxylic insects. These insects locate suitable resources mostly visually and via olfactory cues emitted by dead wood and/or wood-decaying fungi. 2. For the conservation of saproxylic species of Heteroptera, a poorly studied group with a high proportion of threatened species, it is crucial to know which dead-wood structures are needed and how they are detected and colonised. 3. In a field experiment, we exposed different amounts of dead-wood logs and branches of the main tree species of montane beech forests (Abies alba, Fagus sylvatica) on sunny and shady forest plots. We sampled saproxylic heteropterans and sporocarps of wood-decaying fungi in two consecutive years to test the hypothesis that early-successional saproxylic heteropterans are more attracted to wood-decaying fungi than to wood itself. 4. The activity densities of saproxylic heteropterans measured with flight-interception traps increased with increasing surface of coarse woody debris and was higher under sunny conditions. Tree species, fine woody debris and abundance of sporocarps had no significant effect. 5. Our results suggest that during the early-successional forest stage, dead wood provides more important cues than fungi in the search of saproxylic heteropterans for suitable hosts despite assumed close associations of the insects and certain fungal species. To improve habitats for saproxylic heteropterans, we recommend increasing the supply of dead wood of large diameter (> 30 cm) in montane beech forests, particularly in sunny gaps. This can easily be realised during logging operations by gap felling.