246
J. R. Plimmer
is dependent upon carboxyesterase activity and its action may therefore be potentiated by esterase inhibitors. Some impurities in the technical material such
as O,S,S-trimethyl phosphorodithioate may increase its mammalian toxicity. This
impurity was detected in one study (Pellegrini and Santi, 1972) at the 1 % level,
and two related homologs [(MeOhP=O(SMe) and (MeOhP=O(OMe)] were
detected at the 0.1 % and 0.2% levels. Their mammalian toxicities are high (LDso
450, 47, and 96 mg/kg orally in rats) and they have a considerable effect on the
toxicity of malathion.
Malathion of 92.2% purity had an LDso of 1580 mg/kg, but 98% malathion
had an LDso of 8000 mg/kg; this marked difference indicates the magnitude of
the effect of impurities.
A homolog of malathion has been identified as a residue on crops, and this
compound, the ethyl butyl analog of malathion, is believed to be a contaminant
of the commercial product (Gardner et ai., 1969). More recently, the presence
of a toxic contaminant, isomalathion, has been implicated in the deaths of workers
(Baker et ai., 1978).
Malathion is generally decomposed in the environment by hydrolysis and
is also degraded by ultraviolet radiation. It does not appear to accumulate in the
environment or to build up in food chains. Its biological transformations have
been investigated in many species; since it is susceptible to degradation by
phosphatases and carboxyesterases, the products are species-dependent. For example, Krueger and O'Brien (1959) found seven metabolites in mice that were
fed malathion. The dose was 70-80% detoxified in 0.5 hr, and the major metabolites were the monoester (68%) and phosphatase hydrolysis products
(20.5%).
It has been stated that malathion is one of the most benign pesticides
available today (von Rumker et ai., 1974). However, the onset of insect resistance
associated with continued intensive use has been reported for a number of species.
A recent F AO report of a global survey of pesticide susceptibility of stored-grain
pests (Champ and Dyte, 1976) summarizes the position throughout the world.
Resistance was found in 78 of 86 countries surveyed. It was present in all species
tested but was most prevalent in Tribolium spp. and in Rhyzopertha dominica.
The implications of the report are profound and extend beyond the future of
malathion: there is evidence of resistance to fumigants as well as to insecticides.
In its conclusion, the report stresses the need to maintain high standards of
hygiene, because the ease with which control could be obtained in the past with
insecticides, even with poor standards of hygiene, led to complacency and inadequate concern with the basic principles of food storage. It was recommended
that chemical methods be used to give as complete a kill as possible to delay
onset of reinfestation. Fumigants should be used for initial control of established
infestations and insecticides should be applied when needed to prevent reinfestations.
J. R. Plimmer
is dependent upon carboxyesterase activity and its action may therefore be potentiated by esterase inhibitors. Some impurities in the technical material such
as O,S,S-trimethyl phosphorodithioate may increase its mammalian toxicity. This
impurity was detected in one study (Pellegrini and Santi, 1972) at the 1 % level,
and two related homologs [(MeOhP=O(SMe) and (MeOhP=O(OMe)] were
detected at the 0.1 % and 0.2% levels. Their mammalian toxicities are high (LDso
450, 47, and 96 mg/kg orally in rats) and they have a considerable effect on the
toxicity of malathion.
Malathion of 92.2% purity had an LDso of 1580 mg/kg, but 98% malathion
had an LDso of 8000 mg/kg; this marked difference indicates the magnitude of
the effect of impurities.
A homolog of malathion has been identified as a residue on crops, and this
compound, the ethyl butyl analog of malathion, is believed to be a contaminant
of the commercial product (Gardner et ai., 1969). More recently, the presence
of a toxic contaminant, isomalathion, has been implicated in the deaths of workers
(Baker et ai., 1978).
Malathion is generally decomposed in the environment by hydrolysis and
is also degraded by ultraviolet radiation. It does not appear to accumulate in the
environment or to build up in food chains. Its biological transformations have
been investigated in many species; since it is susceptible to degradation by
phosphatases and carboxyesterases, the products are species-dependent. For example, Krueger and O'Brien (1959) found seven metabolites in mice that were
fed malathion. The dose was 70-80% detoxified in 0.5 hr, and the major metabolites were the monoester (68%) and phosphatase hydrolysis products
(20.5%).
It has been stated that malathion is one of the most benign pesticides
available today (von Rumker et ai., 1974). However, the onset of insect resistance
associated with continued intensive use has been reported for a number of species.
A recent F AO report of a global survey of pesticide susceptibility of stored-grain
pests (Champ and Dyte, 1976) summarizes the position throughout the world.
Resistance was found in 78 of 86 countries surveyed. It was present in all species
tested but was most prevalent in Tribolium spp. and in Rhyzopertha dominica.
The implications of the report are profound and extend beyond the future of
malathion: there is evidence of resistance to fumigants as well as to insecticides.
In its conclusion, the report stresses the need to maintain high standards of
hygiene, because the ease with which control could be obtained in the past with
insecticides, even with poor standards of hygiene, led to complacency and inadequate concern with the basic principles of food storage. It was recommended
that chemical methods be used to give as complete a kill as possible to delay
onset of reinfestation. Fumigants should be used for initial control of established
infestations and insecticides should be applied when needed to prevent reinfestations.