How does a rhizophagous insect locate host plant roots? A case study with Melolontha melolontha larvae

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Rhizophagous insect larvae can use CO2 released from respirating plant roots as chemical signal to find their food. Even though this is an ubiquitous gas in soil, few studies addressed the question: Which chemical cues other than CO2 guide rhizophagous insects to roots? We intend to study this question using larvae of the polyphagous common cockchafer, Melolontha melolontha, and Taraxacum sect. ruderalia (dandelion) as host plant. Our previous laboratory studies using a vermiculite soil arena revealed that (a) these larvae oriented to CO2, however, (b) the attractiveness of CO2 vanished when an aqueous rhizosphere extract of dandelion was added. We hypothesized that – at the conditions (vermiculite) used - dandelion roots may hide the attractiveness of CO2 by producing masking exudates. Dandelion roots are often heavily infested in the field by cockchafer larvae. We therefore ask whether plants at natural soil conditions do no longer have the ability to mask the attractiveness of CO2 and, if so, which are the factors responsible for this? How does the type of soil or the natural soil microfauna affect the attractiveness of plant roots? Which signals render plant roots attractive? In particular, how do mycorrhizal fungi usually associated with Taraxacum roots affect root attractiveness? We aim to study these questions in the lab by investigating cockchafer larval orientation to CO2 and Taraxacum roots in different soil types with and without soil microbiota (i.e. natural soil biota, mycorrhizal symbionts). We will compare the chemical pattern of attractive and nonattractive rhizospheres. Furthermore, we want to elucidate by electrophysiological tools which soil compounds can be perceived by cockchafer larval chemoreceptors. Our investigations aim to provide new insights into the complex mechanisms of belowground plant - herbivore interactions. (characters: 1597)