Toxicology and Biochemistry of Two Aliesterase Inhibitors as Synergists of Four Organophosphorus Insecticides in Boll Weevils (Coleoptera: Curculionidae)

Abstract

The toxicology and biochemical mechanisms of two aliesterase inhibitors, DEF (S,S,S-tributyl phosphorotrithioate) andS,S-diamyl phenylphosphonodithioate, as synergists of four organophosphorus insecticides were studied in boll weevils,Anthonomus grandisBoheman. The insecticides tested were parathion, methyl parathion, chlorpyrifos, and chlorpyrifos methyl. Both DEF andS,S-diamyl phenylphosphonodithioate effectively synergized the toxicities of these insecticides. The degree of synergism by DEF was chlorpyrifos > parathion > chlorpyrifos methyl > methyl parathion and that byS,S-diamyl phenylphosphonodithioate was chlorpyrifos > parathion > methyl parathion > chlorpyrifos methyl. The potency ofS,S-diamyl phenylphosphonodithioate was generally similar to that of DEF. Studies on the relationship between aliesterase inhibition, synergism, and acetylcholinesterase inhibition and recovery indicated that DEF synergized the toxicities of organophosphorus insecticides by selectively and persistently inhibiting aliesterase, thus preventing the protection of acetylcholinesterase by aliesterase. Aliesterase inhibitors appear to increase and prolong the toxic potential of the insecticides. The degree of synergism depends on both the insecticide and synergist. Insecticides with greater potency of inhibition toward aliesterase than toward acetylcholinesterase tend to be synergized by aliesterase inhibitor more effectively; the synergists which inhibit aliesterase more potently and persistently will provide higher and longer synergism to organophosphorus insecticides, and the degree of synergism is a function of the magnitude and persistency of the synergist-dependent aliesterase inhibition. Finally, to check whether mixed-function oxidase inhibition is also involved in this synergism, the synergism between these organophosphorus insecticides and a mixed-function oxidase inhibitor, piperonyl butoxide, was observed. A low degree of the synergism and different synergistic pattern of piperonyl butoxide from that of the AliE inhibitors suggested that the synergism of the organophosphorus insecticides by DEF andS,S-diamyl phenylphosphonodithioate was related primarily with the inhibition of aliesterases, and little or no mixed-function oxidase inhibition was involved.