The invention provides a method for controlling Huanglongbing (HLB) disease of citrus plants through expressing genes encoding synthases for sesquiterpenes such as &bgr;-caryophyllene, and α-copaene, and combinations thereof, in citrus plants. Methods of controlling HLB comprising applying at least one purified sesquiterpene, which repels Diaphorina citri and/or Tryoza erytrae psyllid insects, so as to control the HLB disease of citrus plants, are also disclosed.