Herbivore-induced plant root volatiles phthalic acid mediate host selection for Holotrichia parallela larva

The soil-dwelling grubs of Holotrichia parallela inflict substantial damage to the root systems of key cereal crops, including maize and peanuts, thereby posing a serious threat to agricultural productivity. While the olfactory mechanisms through which above-ground insects utilize odorant receptors (ORs) and odorant co-receptors (Orco) to detect plant volatiles for host localization are well established, the role of OR-mediated chemoreception in the host-seeking behaviors of underground pests remains poorly understood. To address this gap, we systematically compared the volatile compounds released by rhizospheric and aerial plant tissues. Through antennal transcriptome analysis and sequence structure prediction, we identified an odorant receptor with biased olfactory expression-HparOR48. Tissue expression analysis revealed that HparOR48 is predominantly expressed in the larval antennae across developmental stages. The functional characterization of HparOR48 was performed using the Xenopus laevis oocyte expression system, which demonstrated its broad responsiveness to phthalic acid, a root-emitted acidic volatile. Electroantennogram (EAG) assays further confirmed that phthalic acid elicited significant electrophysiological responses in the larval antennae. Moreover, behavioral assays utilizing a four-arm olfactometer indicated that phthalic acid acts as an attractant for the larvae. Our findings provide clear evidence that H. parallela larvae rely on olfactory recognition of rhizosphere-specific acidic volatiles to guide host-seeking behavior, thereby extending our understanding of chemical interactions between soil-dwelling pests and their host plants.