Pirimiphos-methyl toxicity

The organophosphorons insecticides dimethoate and diazinon are mnch more toxic to insects (e.g., housefly) than they are to the rat or other mammals. A major factor responsible for this is rapid detoxication of the active oxon forms of these insecticides by A-esterases of mammals. Insects in general appear to have no A-esterase activity or, at best, low A-esterase activity (some earlier stndies confnsed A-esterase activity with B-esterase activity) (Walker 1994b). Diazinon also shows marked selectivity between birds and mammals, which has been explained on the gronnds of rapid detoxication by A-esterase in mammals, an activity that is absent from the blood of most species of birds (see Section 23.23). The related OP insecticides pirimiphos methyl and pirimiphos ethyl show similar selectivity between birds and mammals. Pyrethroid insecticides are highly selective between insects and mammals, and this has been attributed to faster metabolic detoxication by mammals and greater sensitivity of target (Na+ channel) in insects. 

Brealey, C.J., Walker, C.H., and Baldwin, B.C. (1980). A Esterase activities in relation to differential toxicity of pirimiphos-methyl. Pesticide Science 11, 546-554. 

Crane, M., Attwood, C., Sheahan, D. and Morris, S. (1999) Toxicity and bioavailability of the organophosphoms insecticide pirimiphos methyl to the freshwater amphipod Gammarus pulex L. in laboratory and mesocosm systems, Environmental Toxicology and Chemistry 18 (7), 1456-1461. 

Considerable species differences in A-esterase activity exist, ranging from very low or nonexistent in certain birds and fish, to very high in rabbits. Species differences in A-esterase activity could account, at least in part, for species differences in the relative sensitivity to certain phosphorothioate insecticides. For example, birds are much more susceptible to the toxicity of pirimiphos methyl than are mammals. 

In a study using a human neuroblastoma cell line, mixtures of three different organophosphates (azinphos-methyl, diazanon, and dimethoate), and mixtures of an organic phosphate (pirimiphos-methyl) and a benzim-idizole fungicide (benomyl) showed greater toxicity toward protein synthesis than the individual pesticides. I21 The authors of the study concluded that it is not feasible to predict the toxicities of pesticide mixtures on the basis of the toxicities of the single components. 

Hazardous Decomp. Prods. Heated to decomp., emits very toxic fumes of NOx, POx, and SOx Uses Broad-range insecticide Regulatory SARA reportable Pirimiphos-methyl CAS 29232-93-7... 

Pirimiphos-methyl, introduced by ICI (1970) and containing a pyrimidine ring, will control insects which affect stored crops, e.g. beetles and moths, and also those which are a health hazard, e.g. fleas, lice and cockroaches. It has very low mammalian toxicity and acts very quickly by both fumigant and contact action. 

Matrix bound coumaphos and trichlorfon and encapsulated toxicants containing ethyl and methyl pirimiphos, permethrin, and diazinon were found to be ineffective against the fire ant. However, the ineffectiveness was not due to the controlled release mechanism but rather the ability of the ant to filter particles in its infrabuccal pocket in its head. Particles as small as 0.88 microns can be filtered and never come in contact with internal parts. 

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