With insecticide-resistant mosquitoes becoming an ever-growing concern, new vector control technologies are needed, including the development of novel synergists that improve performance of already utilized compounds. Two plant extracts, black pepper and Sichuan pepper, significantly synergize the effects of natural pyrethrins when applied topically to Aedes aegypti females. Synergism was assessed by synergism ratio (SR); LD50 compound alone / LD50 of compound + synergist. Black pepper extract (SR = 17) and Sichuan pepper (SR = 15.3) synergized the effects of natural pyrethrins on Ae. aegypti when applied topically in combination with piperonyl butoxide (PBO), a commercially available synergist. This synergism of natural pyrethrins was also observed directly on the mosquito central nervous system, indicating that this synergism is mediated (at least in part) via action directly on the nervous system, which is unique from PBO. Piperine, one of the active constituents of black pepper extract, and alpha-hydroxysanshool, the primary active constituent of Sichuan pepper extract, were purchased and screened individually as synergists of natural pyrethrins. Both piperine and alpha-hydroxysanshool were shown to synergize natural pyrethrins both topically (SR = 13.9 and 8.5, respectively) and in vitro on nerve preparations. Interestingly, only piperine was capable of synergizing natural pyrethrins on the larval central nervous system of a pyrethroid-resistant strain of mosquitoes, indicating that both molecules possess a slightly different mechanism of action. To probe this question further, we evaluated the effects of both compounds on American cockroach giant axons. Both piperine and alpha-hydroxysanshool applied at 10 µM produce continually diminishing action potential amplitudes after continual stimulation when axons are stimulated at 10 Hz. However, alpha-hydroxysanshool produces marked depolarization in axon resting membrane potential, whereas piperine does not. This activity-dependent block of action potentials in both compounds, but the differences in their ability to depolarize nerve membranes, may help us better understand how these two pepper constituents synergize pyrethroids on both a pyrethroid-susceptible and pyrethroid-resistant strain. It is our hope that the identification and characterization of the effects of target-site synergists may lead to the development of more selective and potent pest control chemistries in the future.

Pepper extracts significantly enhance the toxicity and neurophysiological effects of natural pyrethrins in Aedes aegypti

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