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The group includes a large number of insecticides, herbicides and fungicides.
They all contain phosphorus and an organic structure made up of carbon atoms
arranged in a straight chain or ring. The group illustrates the great variety
of structures among pesticides, with some members fitting equally well into
other families such as the heterocyclics (compounds such as the pyridines
which have carbon ring structures that include nitrogen atoms).

Insecticides. Among the group are such well-known members as parathion,
which is no longer approved for use in the UK. It is a non specific insecticide
that is highly toxic to mammals, and destroys birds and non-target insects
such as bees.

Introducting chlorine atoms into this group reduced toxicity to mammals
without affecting the insecticidal power. A series of such formulations
appeared and was effective against insects which had become resistant to
DDT. Bromophos is a contact insecticide against mites as well as caterpillars.
It contains both chlorine and bromine and could also be included among the
halogenated hydrocarbons.

A series including sulphur atoms sprang from demeton-S-methyl and includes
phorate, a very toxic insecticide used against aphids and flies in crops
such as carrots and parsnips, and malathion. Malathion is an important treatment
for aphids, thrips and mites on a range of vegetables and fruits. A wide
range of insects are susceptible and it has a low toxicity to mammals. It
is a common ingredient in garden greenfly sprays.

Dichlorvos vaporises easily. It is used in household fly strips and
has been used for flea collars. Its relatives, chlorfenvinphos and tetrachlorvinphos,
fenchlorophos and mevinphos, are also nonspecific insecticides of varying
toxicity. All act on contact in the insect’s stomach. Dichlorvos and chlorfenvinphos
are much more toxic to mammals than tetrachlo-rvinphos.

Dimethoate is an insecticide and acaricide used against mites, thrips
and aphids on food crops. It persists longer on crops than does dichlorvos.
Livestock should be kept out of sprayed fields for a week. Most sprayed
crops cannot be harvested for seven days, but watercress must be left for
40 days.

Another group of organophosphates is based on heterocyclic structures
such as the triazines and pyrimidines. Azimphos-methyl and diazinon are
broad-spectrum insecticides. Chloropyrifos is a contact and stomach-acting
insecticide with a wide range of victims and a persistence of two to three
months. Although these three chemicals are widely used in the home and garden,
all are dangerous if swallowed and can irritate the skin.

Fungicides and herbicides. Wepsyn was an early fungicide based on the
triazole structure. Systemic fungicides were a great advantage against fungi
that penetrate and grow within the host plant’s tissues. They could permeate
all tissues of the plant and kill growing fungal threads. Pyrazophos, an
organophosphate based on a heterocyclic nitrogen structure, is an interesting
combination of systemic fungicide and insecticide.

A well-known and widely used herbicide is glyphosate, which is a broad-spectrum,
translocated herbicide active against couch grass as well as other perennial
weeds and annuals.

Environmental effects. Most organo-phosphates do not persist long in
the environment. Exceptions such as glyphosate and chlorfenvinphos are strongly
bound to soil particles, where they break down over a year or more. This
group is very toxic to fish, birds, earthworms and bees.

Carbamates were developed from organophosphate insecticides and also
work by disrupting the nervous system through blocking the action of the
enzyme cholinesterase. These compounds are now widely used as insecticides
or herbicides. One of these, aldicarb, is one of the pesticides most toxic
to people and other vertebrates.

This box is an edited extract from Robbins’ book, Poisoned Havest: A
Consumer’s Guide to Pesticide Use and Abuse, just published by Victor Gollancz,
London.