α-Copaene is a potent repellent against the Asian Citrus PsyllidDiaphorina citri

The Asian Citrus Psyllid (ACP), Diaphorina citri , severely threatens citrus production worldwide by transmitting the greening(= Huanglongbing)-causing bacterium Candidatus Liberibacter asiaticus. There is growing evidence that the push-pull strategy is suitable to partially mitigate HLB by repelling ACP with transgenic plants engineered to produce repellents and attracting the vector to plants with a minimal disease transmission rate. Species that pull ACP away from commercial citrus plants have been identified, and transgenic plants that repel ACP have been developed. The concept of a repellent-producing plant was first demonstrated with an Arabidopsis line engineered to overexpress a gene controlling the synthesis of β-caryophyllene and other sesquiterpenes. We have analyzed the volatile organic compounds released by this Arabidopsis line and identified α-humulene, α-copaene, and trace amounts of β-elemene, in addition to β-caryophyllene. Behavioral measurements demonstrated that α-copaene repels ACP at doses ca. 100x lower than those needed for β-caryophyllene repellence. In contrast, α-humulene is innocuous at the level emitted by the transgenic plant. We confirmed that a mixture of the three sesquiterpenes in the ratio 100:10:1 repels ACP. Likewise, a commercial sample of refined copaiba oil containing the three sesquiterpenes, in a proportion similar to that in the transgenic plant, repelled ACP.