Development of multifunctional metabolic synergists to suppress the evolution of resistance against pyrethroids in insects that blood feed on humans.

BACKGROUND: Pyrethroids are the insecticides of choice when exposure to humans is likely, such as occurs in vector and public-health-related control programs. Unfortunately, the pyrethroids share a common resistance mechanism with dichlorodiphenyltrichloroethane (DDT), knockdown resistance (kdr), and prior extensive use of DDT has predisposed the pyrethroids to cross-resistance via kdr. Given the widespread occurrence of kdr, the use of synergists with pyrethroids is considered to be prudent to guard against the selection of multiply resistant insects.
RESULTS: 3-Phenoxybenzyl hexanoate (PBH) was synthesized as a multifunctional pyrethroid synergist that, besides being a surrogate substrate for sequestration/hydrolytic carboxylesterases, now also functions as a substrate for oxidative xenobiotic metabolism. The addition of PBH to permethrin-treated females of the ISOP450 strain of Culex pipiens quinquefasciatus resulted in a threefold increase in synergism, as judged by the synergistic ratio. Similarly, PBH synergized the action of deltamethrin sixfold on females of the common bed bug, Cimex lectularius, and was 2.8-fold more synergistic than piperonyl butoxide (PBO).
CONCLUSIONS: PBH synergized the action of both type I and type II pyrethroids in a mosquito vector (Cx. p. quinquefasciatus) and in a public-health pest, C. lectularius, respectively, indicating a broad spectrum of action on blood-feeding insects. PBH appears to have residual properties similar to permethrin and is itself non-toxic, unlike PBO, and therefore should be compatible with existing pyrethroid formulations used for insecticide-treated nets and home/residential sprays.