Identification and behavioral bioassays of alarm pheromones from the onion aphid Neotoxoptera formosana.

Secreted from aphid cornicles, alarm pheromone is an important part of integrated pest management (IPM). As a destructive pest of economic crops, the onion aphid Neotoxoptera formosana threatens the production and quality of allium crops worldwide. However, the limited understanding of the alarm pheromone has hindered their control through IPM strategies. This work identified the alarm pheromone component from N. formosana and investigated their behavioral activities. The results indicated that 7 major components, α-thujene, sabinene, (+)-limonene, (E)-β-farnesene (EβF), cis-β-farnesene, cis-α-bergamotene, and α-farnesene were identified in the aphid. Among them, EβF and (+)-limonene were also detected in the cornicle of adult with a ratio of 67.8: 1. Quantitative results of the 2 terpenes through developmental stages indicated that amounts of the sesquiterpene EβF increased initially, then decreased from the third instar, and peaked in the adult stage, while the monoterpene (+)-limonene increased progressively with increasing stage. Behavioral bioassays revealed that both compounds exhibited repellent effects at all tested concentrations, with EβF generally displaying significantly higher repellency than (+)-limonene. Notably, the blend of EβF and (+)-limonene at the natural ratio (67.8: 1) showed the highest repellent activity. Pot experiments further confirmed the blend's optimal efficacy against aphids. This work indicated that EβF is the main effective component of the alarm pheromone from N. formosana, but the blend plays a critical role in the repellent behavior. This study enriches understanding of the chemical communication of N. formosana and lays the foundation for the development of green control methods against this aphid.