Attractiveness of CO2 released by root respiration fades on the background of root exudates

Plants are endangered at their roots by soil-dwelling rhizophagous insects. These below-ground living herbivores may orient to the source of carbon dioxide (CO₂), an ubiquitous volatile released by respiring plant roots. Here, we studied the interaction of CO₂ and other plant root-derived chemical stimuli with regard to the chemical orientation of the polyphagous larvae of Melolontha melolontha L. (Scarabaeidae). A soil arena was developed that enabled both determination of the actual soil CO₂ concentration and the behavioural response of an insect to (a) CO₂ gradients per se, (b) chemical stimuli released from respiring, undamaged roots of plants potted into vermiculite in this arena and (c) combinations of CO₂ gradients and root-derived stimuli. In a root-free arena, larvae of M. melolontha oriented to the source of synthetic CO₂. However, similar CO₂ gradients generated by host plant roots did not attract the larvae. Neither did a synthetic CO₂ gradient combined with aqueous extracts from rhizospheres with undamaged plant roots elicit an attractive effect. Our data suggest that orientation of cockchafer larvae within CO₂ gradients generated by respiring roots is ‘masked’ by an aqueous extract from a rhizosphere with undamaged roots. The results emphasise that effects of behaviour modifying plant-derived compounds need to be investigated against the background of naturally co-occurring chemicals. The significance of our results for orientation of soil living insects is discussed with respect to abiotic conditions in natural soil and the role of soil microorganisms for the attractiveness of plant roots.