Interaction of some organophosphorus compounds in susceptible and resistant houseflies (Musca domestica L.)

The effect of tri-ortho-cresylphosphate (TOCP) on poisoning by parathion and paraoxon in susceptible and resistant houseflies (Musca domestica L.) was investigated.

TOCP influenced the penetration of parathion and paraoxon, and this could explain the contradictory results of both an antagonistic and synergistic effect of TOCP on parathion and paraoxon poisoning, described in the literature.

Studies were also carried out on inhibition of cholinesterase (ChE) and aliesterase (AliE) from susceptible and resistant houseflies by TOCP, parathion or paraoxon in vivo and in vitro.

TOCP proved to be a poor ChE inhibitor, but 50 % of AliE in susceptible and 15 % in resistant houseflies were inhibited by this compound.

It was concluded that at least 2 aliesterases were present in houseflies, one sensitive to inhibition by organophosphorus compounds, and the other one almost insensitive. In vitro TOCP protected ChE against inhibition by parathion or paraoxon.

The importance of these findings for the explanation of resistance to organophosphorus compounds in insects was extensively discussed. It was concluded that penetration was an important factor in resistance of houseflies to parathion and paraoxon.