Chemistry Reference
In-Depth Information
10
Organophosphorus and
Carbamate Insecticides
10.1 Background
Organophosphorus insecticides (OPs) and carbamate insecticides are dealt with here
in a single chapter because they share a common mode of action: cholinesterase
(ChE) inhibition. Unlike DDT and most of the cyclodiene insecticides, they do not
have long biological half-lives or present problems of biomagnification along food
chains. When OCs such as DDT and dieldrin began to be phased out during the
1960s, they were often replaced by OPs or carbamates, which were seen to be more
readily biodegradable and less persistent, although not necessarily as effective for
controlling pests, parasites, or vectors of disease. They replaced OCs as the active
ingredients of crop sprays, sheep dips, seed dressings, sprays used for vector control,
and various other insecticidal preparations.
When OCs were phased out, the less persistent insecticides that replaced them
were thought to be more “environment friendly.” However, some of the insecticides
that were used as replacements also presented problems because of very high acute
toxicity. The insecticides to be discussed in this chapter illustrate well the ecotoxi-
cological problems that can be associated with compounds that have low persistence
but high neurotoxicity.
OPs were first developed during World War II, both as insecticides and chemical
warfare agents. During this time, several new insecticides were synthesized by G.
Schrader working in Germany, prominent among which was parathion, an insecti-
cide that came to be widely used in agriculture after the war. In the postwar years,
many new OPs were introduced and used for a wide range of applications. Early
insecticides had only “contact” action when applied to crops in the field, but later
ones, such as dimethoate, metasystox, disyston, and phorate, had systemic proper-
ties. Systemic compounds can enter the plant, to be circulated in the vascular system.
Sap-feeding insects, such as aphids and whitefly, are then poisoned by insecticides
(or their toxic metabolites) that circulate within the plant. Some OPs were developed
that were highly selective between mammals and insects, and showed low mam-
malian toxicity (e.g., malathion and pirimiphos-methyl), making them suitable for
certain veterinary uses, and protecting stored grain against insect pests.
The rapid growth in the use of OPs and the proliferation of new active ingredients
and formulations was not without its problems. Some OPs proved to be too hazard-
ous to operators because of very high acute toxicity. A few were found to cause
delayed neurotoxicity, a condition not caused by ChE inhibition (e.g., mipafox, lepto-
phos). There was also the problem of the development of resistance, for example, by
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