Studies on the mode of action of organophosphorus insecticides on Sancassania berlesei (Michael) (Acarnia: Tyroglyphidae) and Tetranychus urticae (Koch) (Acarina: Tetranychidae)

Abstract

The literature concerned with organophosphorus insecticide mechanisms in the Acarina is reviewed. Two different mechanisms have been found to be responsible for organophosphorus insecticide resistance in the Acarina. In one of these mechanisms, organophosphorus insecticide resistant strains contain a modified cholinesterase insensitive to these inhibitors, while the second mechanism involves a greater detoxification ability by resistant strains.

The toxicity of a number of organophosphorus insecticides to Sancassania berlesei Michael and Tetranychus urticae Koch was investigated using a slide dip toxicity test. After correcting for differences in weight and differences in the amount of insecticide picked up by the mites during dosing, S. berlesei was found to be more resistant to organophosphorus insecticides than T. urticae.

The cholinesterase from crude mite homogenates was investigated with respect to a number of choline substrates and organophosphate inhibitors using a colorimetric technique. S. berlesei was found to have a cholinesterase more insensitive to organophosphate inhibitors than T. urticae; the cholinesterase sensitivity differences between the two mites correlating with their toxicity differences to the corresponding organophosphorus insecticides. The cholinesterase of S. berlesei hydrolysed choline esters at a slower rate than T. urticae. The esteratic site of S. berlesei cholinesterase was found to be more hindered sterically than that of T. urticae cholinesterase. Separate acetylcholine and acetyl β-methylcholine hydrolysing enzymes were demonstrated in both mite homogenates. The significantly higher sensitivity of acetyl β-methylcholinesterase to organophosphate inhibitors precluded any involvement of the acetyl β-methylcholine substrate in the organophosphorus insecticide resistance process.

The response of the non-specific esterases, separable by polyacrylamide gel electrophoresis from mite extracts, to a number of substrates and select inhibitors was characterised. S. berlesei was found to contain a greater number of nonspecific esterases which were considerably more insensitive to organophosphate inhibition than those present in T. urticae. The possible role of these non-specific esterases in the resistance mechanism of S. berlesei to organophosphorus insecticides is discussed. The inability to classify these non-specific esterases according to common esterase classification schemes from their substrate and inhibition characteristics was demonstrated.

The development of a specific staining technique to identify the acetylcholinesterase isozymes separable by polyacrylamide gel electrophoresis and the subsequent difficulties encountered in acetylcholinesterase solubilisation and acetylcholinesterase gel penetration is described. Major differences in acetylcholinesterase isozyme structure and membrane binding properties of these two mites are indicated by their different abilities to penetrate into gels, and their different electrophoretic mobilities and solubilisation characteristics. The sensitivity of the acetylcholinesterase isozymes to inhibitors was investigated by visually assessing their response to inhibition and in some cases using a gel scanning technique to derive their bimolecular rate constants. The sensitivity of the acetylcholinesterase isozymes to organophosphate inhibitors further confirmed the findings obtained using total soluble cholinesterase and non-specific esterases, that S. berlesei contain enzyme systems which are considerably more insensitive to organophosphate inhibitors than T. urticae. The possible involvement of an insensitive acetylcholinesterase isozyme component in the organophosphorus insecticide resistance of S. berlesei is discussed.

The in vitro degradation of mevinphos was investigated using a bioassay technique in which S. berlesei acetylcholinesterase was used as the reference enzyme. As the technique was found to be relatively insensitive further investigations using ¹⁴C -parathion and ¹⁴C -paraoxon were developed. Attempts to measure the activation of ¹⁴C -parathion to ¹⁴C -paraoxon by mite homogenates were unsuccessful. T. urticae homogenates degraded mevinphos and ¹⁴C -parathion at slightly faster rates than S. berlesei homogenates whereas the reverse was found for ¹⁴C -paraoxon. These inconsistent degradation results are discussed with respect to toxicity and organophosphorus resistance mechanisms.

The rates of penetration of ¹⁴C-parathion into the bodies of the two mite species were compared using a topical application technique and the difference in the losses of radioactivity from the two mite cuticles is discussed. A smaller ¹⁴C-parathion penetration rate was found for S. berlesei than for T. urticae and this possibly contributed to the greater resistance of S. berlesei to parathion. The differences in cuticle structure of T. urticae and S. berlesei are detailed and the effect on penecration is discussed.

The results of the biochemical investigations are considered with special emphasis on their involvement in the mechanism whereby S. berlesei is more tolerant to organophosphorus insecticides than T. urticae. It is concluded that the organophosphate insensitive nature of S. berlesei cholinesterase is a major contributor to the resistance of this mite species to organophosphorus insecticide, although other factors are also involved. Relationships originally found to hold for organophosphorus insecticide resistant and susceptible strains of acarine species were also found to hold in this interspecies comparison when S. berlesei assumed the role of the 'resistant' species and T. urticae that of the 'susceptible' species.