Investigating the potential of selected natural compounds to increase the potency of pyrethrum against houseflies Musca domestica (Diptera: Muscidae).

Background: A study was undertaken to determine the efficacy of seven natural compounds compared with piperonyl butoxide (PBO) in synergising pyrethrum, with the intention of formulating an effective natural synergist with pyrethrum for use in the organic crop market.

Results: Discriminating dose bioassays showed PBO to be significantly more effective at synergising pyrethrum in houseflies than the seven natural compounds tested, causing 100% mortality in insecticide-susceptible WHO and resistant 381zb strains of housefly. The most effective natural synergists against WHO houseflies were dillapiole oil, grapefruit oil and parsley seed oil, with 59, 50 and 41% mortality respectively, compared with 18% mortality with unsynergised pyrethrum. Against 381zb houseflies, the most effective natural synergists were parsley seed oil and dillapiole oil. Esterase inhibition by the natural compounds and PBO in vitro showed no correlation with pyrethrum synergism in vivo, whereas the inhibition of oxidases in vitro more closely correlated with pyrethrum synergism in vivo.

Conclusion: Dillapiole oil and parsley seed oil showed the greatest potential as pyrethrum synergists. PBO remained the most effective synergist, possibly owing to its surfactant properties, enhancing penetration of pyrethrins. The results suggest the involvement of oxidases in pyrethroid resistance in houseflies, with the efficacy of synergists showing a high correlation with inhibition of oxidases.