Maxillary palp gustatory receptors mediate aphid corpses avoidance in Harmonia axyridis

The ladybird Harmonia axyridis is an important predatory natural enemy with a broad dietary spectrum and few avoided food sources. Nevertheless, it shows avoidance behavior toward aphid corpses, likely to minimize the risk of pathogen infection in the wild. The molecular basis of this behavior remains unclear, and gustatory receptors (GRs) are hypothesized to be involved. Here, we examined the avoidance of H. axyridis toward pea aphid (Acyrthosiphon pisum) corpses and investigated the role of candidate GR genes in contact chemoreception via the maxillary palps. Using SEM and immunofluorescence, we characterized the microstructure of the maxillary palps and mapped neuronal distributions. Behavioral assays combined with RNAi were performed to evaluate three candidate GR genes associated with corpse recognition. Our results show that feeding behavior on A. pisum is largely determined by gustatory input through maxillary palp contact. The palp surface is densely covered with sensilla, and internally the tissue displays a complementary distribution of dopaminergic and serotonergic neurons. RNAi assays demonstrated that three GR genes (GR2-like, GR28b-like, and GR64e-like) mediate corpse avoidance. Silencing GR2-like alone significantly reduced avoidance, while simultaneous knockdown of GR28b-like and GR64e-like further weakened the response, suggesting functional interactions among gustatory receptors in H. axyridis. These findings reveal a gustatory mechanism underlying corpse avoidance in predatory insects and provide insights relevant to biological control and artificial diet development.