Soil substrates affect responses of root feeding larvae to their hosts at multiple levels: Orientation, locomotion and feeding

The role of soil characteristics for cue-directed behaviour of rhizophagous insects remains largely unexplored to date. Here, we studied behavioural responses of polyphagous cockchafer larvae (Melolontha melolontha; Scarabaeidae) to roots of one of its preferred host plants, dandelion (Taraxacum sectio ruderalia; Asteraceae). Plants were grown in substrates with different adsorptive capacities, i.e. in vermiculite or sand. A behavioural assay was developed which allowed both monitoring the release of attractive CO2 from roots and observation of larval behaviour at the same time. In sand, larvae got closer to the roots than in vermiculite where they eventually stopped orienting towards the roots: However, their total locomotory activity and the number of turning events/stationary points were higher in vermiculite. Larval behaviour was not correlated with CO2 concentrations in both vermiculite and sand. On average, larvae kept a greater distance to vermiculite-grown roots. However, a feeding bioassay revealed that the larvae consumed more of vermiculite-grown roots than of sand-grown ones. This result showed that the weaker orientation of larvae towards vermiculite-grown roots was not due to the lower palatability of these roots. Vermiculite might adsorb foraging-relevant cues of dandelion roots, alter composition of root exudates, and/or physically impair oriented movements of soil-dwelling insects. Our study shows that the type of substrate affects responses of rhizophagous larvae to host roots at multiple levels independently of root-derived CO2 gradients.