ORGANOPHOSPHORUS ESTER INSECTICIDES
Our knowledge of the organophosphorus ester (OP) insecticides is substantially greater than for
any other class of pesticides except the carbamates -- and carbamates have the same mechanism
of insecticidal activity [inhibition of acetylcholinesterase (AChE)], so much of the toxicology
overlaps. We understand not only the mechanism by which OPs act, but the structural
considerations that affect the initial inhibition of AChE, regeneration of the enzyme, and aging of
the OP-enzyme complex. However, it is difficult to package this information tidily because
details differ between compounds and between species.
Historically, most toxicologic research focused on the OPs themselves, because during the 1950s
and 1960s, the emphasis was on developing new insecticides. Thus hundreds of structures were
examined for insecticidal activity in important target species, and then evaluated for nontarget
toxicity in a few "model" species. Inhibition of AChE was measured in house flies (Musca
domestica; rationale below); acute mammalian toxicity was usually measured in rats (Rattus
rattus) or in mice (Mus musculus). If unusual phenomena were seen, additional studies might be
carried out in other mammalian species. When evaluation of delayed effects (birth defects,
cancer) became a requirement for pesticide registration, rats or mice were the test species.
Environmental toxicity to aquatic species was measured either in commercially important fish
species such as trout (Salmo trutti, Salmo gairdneri) or model fish such as bluegills (Lepomis
macrochirus). Birds were represented by the few wild species that were easy to raise in
captivity, such as bobwhite quail (Colinus virginianus) or mallards (Anas platyrhinchos).1
Because so much of this early OP work was done under the aegis of the World Health
Organization (WHO or OMS), it was published freely. Additional data - provided by companies
to government agencies during the registration process for pesticides - are in government
archives; however, these data are confidential and unpublished.2 Thus, despite the wealth of data
available, comparable toxicity data - for the same OP in the same species, by the same route of
administration - were often not available. The reason is quite simple: there are too many OPs and
too many species to assay even a large fraction of all the permutations. For example, note the
use of both rats and mice, and of several routes of administration, in the comparison of di-Omethyl and di-O-ethyl OPs in the table from Gallo and Lawryk (Table 16.6, at end of handout).
Recently, a renewed interest in environmental toxicology has led to a resurgence of interest in
the toxicology of OP insecticides. The current emphasis is not on new chemicals, however, but
consists of re-evaluation of existing OPs in new species. This is to some extent providing the
missing comparisons. However, while it is undoubtedly important to identify interspecies
differences in response to environmental toxicants (and OPs are still the most widely used
insecticides in the U.S.), too many of these newer studies amount to body counts with the
addition of some in vitro assays of cholinesterase inhibition or some evaluation of environmental
variables that affect toxicity. It is an accumulation of details rather than of new insights.
Structure and nomenclature –
1
2
In addition, the World Health Organization (WHO or OMS) used chickens (Gallus gallus
domesticus) to identify the subset of OPs that cause delayed paralysis.
The companies' data have also not been subjected to peer review. In classifying chemicals as
carcinogens (probable, possible, or non-) the International Agency for Research on Cancer
(IARC) does not give the same weight to these data as to data that have been published in peerreviewed journals.
Given the general structure of the organophosphorus esters (Figure 1), where X, R1 and R2 may
or may not bond to P via an oxygen atom, the nomenclature is as follows:
phosphate: 4 (P-O); no P-C bond
phosphonate: 3 (P-O) + 1 P-C bond
phosphinate: 2 (P-O) + 2 P-C bond
phosphorothioate: 3 (P-O) + 1 (P-S); no P-C bond
phosphonothioate: 2 (P-O) + 1 (P-S) + 1 P-C bond
phosphinothioate 1 (P-O) + 1 (P-S) + 2 P-C bond
Figure 1: General structure of OP insecticides
Classes: P=O(S), R1, R2, X
A. X contains quaternary N . Such OPs are directly cholinergic, inhibit AChE (e.g., ecothiopate
iodide - anti-glaucoma DRG)
B. X = F: e.g., (sarin, DFP)3, mipafox, dimefox
C. X = CN, OCN, SCN, or a halogen other than F: e.g., Tabun3
D. X = other (includes the most OP insecticides)
1.
dimethoxy: e.g., azinphos-methyl, dichlorvos, fenitrothion, parathionmethyl, malathion, ronnel, trichlorfon
2.
diethoxy: e.g., parathion, chlorpyrifos, coumaphos4, diazinon, disulfoton,
TEPP, phorate
3.
other dialkoxy (dipropyl, di-isopropyl)
4.
diamino: e.g., schradan
5.
substituted dialkoxy (especially Cl): e.g., haloxon4
6.
trithioalkyl: e.g., DEF, merphos5
7.
triphenyl or substituted triphenyl: e.g., TOCP (tri-ortho-cresyl phosphate)6
8.
mixed substituents: e.g., cyanofenphos, leptophos, EPBP, EPN, DMPA
Most insecticides come from group D; a variety of structures are shown in the handout.
It is very important to realize that the insecticidal effects of all OPs result from inhibition of the
neurotransmitter acetylcholinesterase. This does not mean that they do not have other
mechanisms of action, which may case toxic effects in nontarget organisms.
3
Not all OPs are primarily insecticides. Sarin is a nerve gas; DFP is used in biochemistry. Tabun is also a
nerve gas.
4
Coumaphos and haloxon are veterinary drugs.
5
These are defoliants, not insecticides
6
TOCP is an industrial solvent and fuel additive.
MODE OF ACTION OF OP INSECTICIDES
Neuron: a nerve cell. Composed of cell body with nucleus, dendrites (bring impulses to cell
body); axon (carries impulses from cell body); may have insulating myelin sheath laid
down by Schwann cells. motor neurons carry impulses to muscles; sensory neurons carry
impulses from organs (e.g., muscles, pain receptors).
Nerve: a bundle of neurons, all oriented the same way.
CNS: central nervous system. brain and spinal cord.
PNS: peripheral nervous system. Divided into groups in several ways. a) autonomous nervous
system carries out involuntary functions (mucus secretion, peristalsis); voluntary nervous
system handles functions under conscious control. b) by neurotransmitter (below).
Transmission of nerve impulses:
Stimulation of neuron ---> electric impulse travels along axon. Between cells
(nerve/nerve, nerve/muscle, nerve/receptor), transmission is chemical. There are numerous
neurotransmitters in the central nervous system. In the peripheral nervous system, there are 3
major neurotransmitters: adrenaline, noradrenaline and acetylcholine (ACh). ACh acts at the
neuromuscular junction between nerves and voluntary muscles, and in many autonomic nerves.
Muscarinic effect: one which mimics treatment with acetylcholine7 in "muscarinic fibers" -smooth muscles, heart, exocrine glands. Causes bronchoconstriction, increased bronchial
secretion, salivation and lacrimation; nausea, vomiting, diarrhea, cramps, (due to increased
forward and reverse peristalsis) bradycardia (slowed heartbeat), miosis (pinpoint pupils),
urination and defecation.
Nicotinic effect: mimics treatment with nicotine, at the endings of motor nerves to skeletal
muscles and autonomic ganglia. Nicotine is an acetylcholine agonist (mimic) which, if it applied
to the end plate of muscle, causes muscle to contract8. Effect of OPs on voluntary muscles is
called nicotinic, to distinguish it from effects on autonomic nervous system. Symptoms include
muscular weakness and fatigue, twitching, cramps, convulsions, weakness of respiratory
muscles, tachycardia (fast heartbeat), pallor, elevated blood pressure.
CNS symptoms include slurred speech, confusion, restlessness, insomnia, emotional
instability/neurosis, coma.
Acetylcholine (ACh) is a neurotransmitter (of postganglionic parasympathetic nerve fibers,
preganglionic fibers of sympathetic and parasympathetic nerves, and of certain nerve fibers in
CNS) that consists of an acetylated choline. It contains a positively charged (quaternary) N.
Acetylcholinesterase (AChE) is an enzyme that decomposes ACh into acetate + choline. AChE
has an anionic site that forms an ionic bond with the N+; this bond is strengthened by
participation of 2 of the CH3 groups. This allows formation of a covalent bond between the
carbonyl C of the acetyl moiety; the ester linkage is broken, choline is eliminated; the acetylatedenzyme reacts with H2O to regenerate AChE + HAc.
7
8
This can result either from adding ACh, or from inhibiting ACh breakdown. Thus OPs have
muscarinic action.
Nicotine is not broken down by AChE, so it causes continuous contraction of the muscle - just like OPs, which
prevent AChE from breaking down ACh.
Figure 2: Interaction of ACh and AChE
When AChE binds to an OP, it is essentially irreversibly phosphorylated. In contrast to its action
on ACh, AChE do not metabolize OPs, and can dissociate from them only very slowly - in days
or weeks, not milliseconds. The degree to which such dissociation takes place affects the
lethality of individual OPs, and also the rapidity of recovery from poisoning. One type of
antidote to OPs increases the rate of dissociation between the OP and AChE; 2-PAM typifies this
type of antidote.
At some time after binding occurs between an OP and AChE, a hydrolytic reaction takes place,
resulting in hydrolysis of another portion of the OP molecule. Once this process of aging has
occurred, no further regeneration of enzyme can occur. It must be replaced by de novo synthesis.
Moreover, 2-PAM will no longer aid recovery.
`
Figure 3: Interaction between AChE and OPs
Antidotes:
Atropine blocks transmission of impulse from nerve to muscle - competes for ACh at receptor.
2-PAM (2-pyridine aldoxime; pralidoxime) reactivates AChE by dephosphorylating it.
Synthesis of Cholinesterases
Plasma cholinesterase (also known as pseudocholinesterase or aliesterase) is synthesized in the
mammalian liver.
Mammalian red blood cells (rbc) also synthesize AChE, but only while the cells are nucleated that is, AChE is synthesized in the immature rbc. Mature, circulating rbc do not contain nuclei.
If their complement of AChE is inhibited, replacement requires synthesis of new rbc.
Neurons in the brain9 synthesize AChE in the cell body; the enzyme then moves to nerve ending
by axonal transport. If neuronal AChE is inhibited, the cells can synthesize new enzyme.
AChE Levels
The time course of AChE inhibition by OPs depends on the interaction of inhibition, aging,
regeneration of old enzyme, and synthesis of new enzyme. Initially there is a rapid partial
recovery of inhibited AChE (and other ChEs) due to hydrolysis of the inhibited enzyme.
This reversal of inhibition depends not only on the structure of the OP, but also on the species,
and even on the tissue within a species, that is examined. For example, dichlorvos-inhibited
AChE undergoes spontaneous reactivation in both rat brain and rat plasma; in humans, AChE in
both plasma and rbc is renewed by de novo synthesis rather than by regeneration. Moreover,
differences occur for different enzymes in the same organism: OPs complexed with the human
plasma enzyme (ChE) age 5-10 times faster than human OPs complexed with human AChE.
Structure-activity considerations
Reactivation of the Op-enzyme complex depends on all parts of the OP molecule except the
leaving group (X) -- since, by definition, that part of the molecule is lost when the enzyme-OP
complex forms. For alkyl OPs and rat brain AChE, the reactivation of the OP-enzyme complex
by water is:
di-Methoxy > di-Ethoxy > di-n-Propyl > di-iso-Propyl > di-N-iso-Prop
For example, 25% of a di-Methoxy-OP/AChE complex is hydrolyzed in 1.3 hrs; of a di-EthoxyOP/AChE complex, in 20 hrs; of a di-n-propoxy-OP/AChE complex, in 40 hrs; and of a di-isopropoxy-OP/AChE complex, in > 1000 hrs.
However, remember that aging (the removal of a second O-alkyl/aryl group) of the enzyme-OP
complex occurs in competition with hydrolysis. Once aging has occurred, dissociation of the
OP-enzyme complex is no longer possible.
Structural requirements for aging of the OP-AChE complex differ from those for hydrolysis. In
human brain, the rapidity with which aging of alkyl OPs occurs is
di-Ethoxy < di-iso-Propyl  di-Methoxy < di-N-iso-Prop/di-Methoxy
9
Most studies have been done on brain, since it provides the largest quantity of neural tissue. It is
assumed that peripheral nerve cells do likewise.
In consequence of such competing reactions, the overall reactivation of cholinesterase in rat
plasma is roughly equal for the di-Methoxy-OPs and the di-Ethoxy-OPs, presumably because of
the very rapid aging of the di-Methoxy-OPs, which precludes hydrolysis.
Note, however, that there are large differences between species in the amount of reactivation that
occurs. The hen reactivates 25% of di-ethoxy-OPs in 0.9 hrs, while the human requires 310 hrs.
In rat diaphragm, 40% of ChE was reactivated 80 min after inhibition by paraoxon, while no
reactivation at all was apparent in lobster legs during the same period.
The one generalization that can be made form these observations is that the methoxy moiety
results is less toxic than the ethoxy (Table, from Gallo and Lawryk10, p 934).
Resynthesis
The competing reactions of aging and hydrolysis occur mostly during the first hours after
exposure to OPs. Later there is a slow, exponential recovery of AChE levels as new enzyme is
synthesized, either by existing cells (in brain), or by new cells (rbc); an average value for this
phase is the addition of 0.5% to 0.7% of AChE per day. It is generally assumed that normal
levels of AChE are present approximately one month after exposure ends.11
Measuring acute toxicity: AChE inhibition
Because we know the mechanism by which OPs act, we do not need to limit our studies to body
counts in assessing the toxicity of different compounds. Usually we begin with body counts, but
- especially if there are deviations from expected levels of toxicity - it is possible to examine the
action of the OP on its target, AChE, by making in vitro preparations of neural tissue (brain) and
actually measuring enzyme inhibition.
An early - and still the most common - assay used to measure AChE inhibition is the Ellman
assay12. Acetylthiocholine is used as a substrate for AChE. An aliquot of tissue is added to this
substrate, which AChE hydrolyses to thiocholine. Thiocholine is allowed to react with 5,5'dithiobis-2-nitrobenzene to form a colored product, 5-thio-2-nitrobenzoate. The rate of color
formation over a short period of time (during which color formation is shown to be linear) is
measured by observing changes in absorbance in a spectrophotometer at 412 µm.13
Toxicity of an OP to house flies can serve as a rapid test of insecticidal (but not vertebrate)
toxicity. In the fly LD50 assay, the test OP is applied topically to the fly's head (age and sex of
10
Gallo, MA and NJ Lawryk, 1991. Organic phosphors pesticides; pp 917-1123 in: Vol 2 of Handbook
of Pesticide Toxicology, 1st edition, WJ Hayes and ER Laws, eds, Academic Press, San Diego
11
In both birds and mammals, abnormally high levels of AChE are often seen several weeks after severe
inhibition. It is assumed that these reflect an over-synthesis of the enzyme during the recovery.
12
GL Ellman, KD Courtney, V Andres Jr, RM Featherstone, 1961. A new and rapid colorimetric
determination of acetylcholinesterase activity. Biochem. Pharmacol. 7:88-95.
13
Recent efforts to standardize results between laboratories suggest that the Ellman assay is extremely
sensitive to variations in protocol (e.g., changes in temperature); conclusions based on
comparisons of data from different labs (possibly even data obtained by different people in the
same laboratory) may not be comparable.
flies is standardized). Because transport of OPs across the exoskeleton is rapid and efficient, and
because fly brain contains high levels of AChE, this assay is very nearly a direct assay of AChE
inhibition. However, because vertebrate AChE differs in structure from insect AChE,
conclusions are not always applicable to vertebrates. From such comparisons, one knows that
the distance between the esteratic and anionic sites of ACh differs from insect to vertebrate.
On the other hand, if one tests vertebrates in vivo, structure-activity relationships are not nearly
so tidy as in insects. To a considerable extent this is due to differences in absorption, transport
and metabolism, both between compounds and between species. For example, different OPs are
more or less well absorbed through the skin: even those that are just more slowly absorbed will
be less toxic, especially if their structure promotes reactivation from the E-OP complex. An OP
like malathion will be highly toxic to insects when the P=S is converted to P=O; in mammals,
rapid metabolism of carboxylesterases (acting at arrows on structure) will prevent formation of
the P=O form, and the LD50 of malathion in rats is > 1000 mg/kg. The 1st graph from Francis et
al (1980) illustrates the differences in the correlation between sigma () and LD50 of a series of
phenylphosphonothionates in flies and in rodents. The correlation coefficient for the regression
line of LD50s in flies was 0.91; for rats, 0.71 - demonstrating a much closer relationship between
 and LD50 in the former than in the latter. The 2nd graph from Francis et al 1980 illustrates the
even looser linkage between  and non-cholinergic action of OPs.
ECOLOGICAL EFFECTS OF OP INSECTICIDES
Our view of the ecological toxicology of pesticides has been profoundly influenced by the
incredible persistence of the organochlorine insecticides (such as DDT, aldrin and dieldrin,
heptachlor and chlordane), whose half-lives are measured in years or decades. Because so many
of the early examples of ecological disaster involved persistent chemicals that bioaccumulated in
food chains, there remains a tendency to think that (relatively) rapid degradation is equivalent to
being environmentally benign. A careful toxicologist will add the proviso: provided the
pesticide is used as directed, since pesticide labels14 presumably include precautions that will
avoid serious damage to beneficial organisms.
It must be remembered, however, that toxicity testing is carried out on single species, in
controlled conditions that include optimum health of the test organism, free access to food and
water, and lack of concomitant illness or other toxicants. Species in the wild are rarely so well
protected. Moreover, assumptions about long term effects of pesticides on wild species are
usually extrapolated from their effects on laboratory species (for animals) or crops (for plants).
While toxicity testing for human health effects does include both carcinogenesis assays and
developmental toxicity assays, only the former can be extrapolated to other species with any
degree of reliability -- since we have the necessary theoretical basis for arguing that mechanisms
of carcinogenesis are the same between species and even between phyla. The extreme species
specificity of developmental effects, and the lack of a theoretical foundation for the mechanisms
by which they occur, means one cannot be certain pesticides do not cause developmental toxicity
in any species other than those tested. Reproductive toxicity - that is, impacts on the
reproductive capacity of potential parents - are somewhat more stable across species, but may
differ considerably between families, orders or phyla.
14
Note that the pesticide label is a legal document. Strictly speaking, it is a misdemeanor (both in
federal and Illinois law) to use a given formulation against a species, or under conditions, for
which it is not registered; or to use it at a level other than those listed. While noncommercial use
of pesticides by private individuals is rarely (if ever) prosecuted, the law does apply to all users.
In addition to differences between species, several factors must be considered in assessing the
ecological toxicity of a pesticide. These are:
 persistence or potential bioaccumulation of the parent compound, and of its metabolites and
impurities;
 secondary poisoning (which might be considered as a special case of (1);
 secondary effects arising from temporary changes in ecosystem composition;
 ecosystem simplification or other permanent changes in ecosystem composition, especially
in stressed ecosystems;
 reproductive toxicity, especially in non-mammalian species;
 evolutionary consequences of extreme pressure on a population.
Persistence and/or bioaccumulation This is not normally considered a problem with OP
insecticides. However, a few OPs are persistent enough to cause secondary poisoning.
Secondary Poisoning A chemical that is extremely toxic to a target species may kill predators
that eat the poisoned animals. For example, fenthion has an LD50 of approximately 5 mg/kg to
birds (but of 250 mg/kg to rats). It is marketed as an avicide by constructing perches soaked in
(or filled with?) fenthion, which are placed where the pest species roosts. Birds absorb the OP
through their feet; when lethal doses are absorbed, an additional amount of the pesticide is still
present on and in the skin of the feet. If a predator - such as a kestrel or other hawk - eats several
tainted carcasses, it could absorb enough fenthion to be poisoned in its turn. An episode of
secondary poisoning was claimed by owners of kestrels in central Illinois about 10 years ago.
(Less likely, in view of the low toxicity of fenthion to mammals, was the claim that some dogs
had been poisoned by eating poisoned birds.) More common are examples of birds eating OPpoisoned insects in lethal quantities.15 About 10 years ago, Michael Hooper established that
monocrotophos, an OP insecticide used against grasshoppers in Argentina, was responsible for
high mortality of Swainson’s hawk (Buteo swainsoni), which migrates between North and South
America16.
Changes in ecosystem composition usually involve temporary elimination of a food species
susceptible to the applied pesticide. For example, if fields are treated with insecticides in spring,
insect populations will plummet at exactly the time when birds are nesting or feeding young.
The resulting shortage of insects as food would be expected to decrease survival of offspring
(occasionally even of parents), especially if they have fed on poisoned insects and have absorbed
a nonlethal dose of toxicant. Algicides will remove food supply for many pond dwelling
herbivores, and may secondarily affect the food available to predators on such herbivores. If the
ecosystem is already stressed - e.g., by acid rain, size reduction, etc - populations of one or more
long-lived species in the system may be eliminated.17 In a healthy system, immigration from the
15
Fungicide-treated grain was a major cause of bird deaths in the 1950s, when mercury-containing
fungicides were used. The British banned mercury-containing fungicides because of the high
numbers of bird deaths, and the occurrence of several major episodes of human poisonings led to
their ban worldwide. The chemicals used as seed treatments today do not seem to pose acute
hazards to birds.
16
Golstein, MI; TE Lacher, ME Zaccagnini, ML Parker and MJ Hooper, 2004. Monitoring and assessment of
Swainson’s haws in Argentina following restrictions on monocrotophos use 1996-1997. Ecotoxicology
8:215-224, 1999.
17
This type of damage was made famous by Rachel Carson: the title of her book, "Silent Spring",
comes from the possibility that insecticides will silence bird song through contamination of both
insects and earthworms.
edges of the poisoned area will repopulate the ecosystem -- if the pesticide application is not
persistent, and if it is not repeated so often as to mimic persistence. In fragmented or stressed
ecosystems, damage may be irreversible.
Reproductive toxicity –
Because most of the assays for nontarget organisms are aimed at identifying either acute effects
or human health risks, and because different stages of the reproductive process are differentially
sensitive to toxicants (with considerable species-specificity), pesticides may affect the ability of
one or more species in a treated area to reproduce. Since there might be no immediately visible
consequences to such reproductive effects (in the absence of adult deaths, there are no bodies to
count) considerable harm can be done to a population before the cause is identified. The best
examples of population damage resulting from reproductive toxicity are avian declines resulting
from the presence of DDT and of PCBs in the environment. OP insecticides are suggested - but
have not been proven - to be part of the cause for declining bat populations in the U.S. It is
difficult to prove that pesticide residues in bats are high enough to cause either acute or
reproductive toxicity, however.
Evolutionary consequences of extreme pressure on a population have been postulated18. In their
simplest form these consist of genes for resistance to the pesticide (or class of pesticides)
spreading and becoming fixed in the population. Such a change affects only the results of future
spraying. If, however, a species became resistant to a highly toxic pesticide, resistant animals
could carry enough of the compound to poison predators. This degree of resistance has been
observed in mosquitofish (Gambusia affinis) exposed to the organochlorine insecticide endrin.
Endrin is extremely toxic to fish19 ; however, the resistant fish can tolerate 500 times the
normally lethal level of endrin. Moreover, a single resistant mosquitofish releases enough of this
organochlorine insecticide into a 10 gallon aquarium to kill all sensitive fish in the aquarium.
A very speculative example of the possibility of ecosystem change due to changes in gene
frequency is based on the following observation: the western corn rootworm, which from time
immemorial lived in Colorado on wild cucurbits, was increasing its range only very slowly until
the introduction of the insecticide dieldrin. Then it began to migrate eastward quite rapidly. RL
Metcalf suggests that the insecticide (or the selection pressure put on the population) somehow
altered its migration rate. Interestingly enough, it is also the western corn rootworm that is now
able to ravage corn when corn follows soybeans. Evidence has been accumulating over the past
10 years that there are 2 separate mechanisms for this pattern. In some areas, the female western
corn rootworm beetle has changed its pattern of egg-laying, and now lays eggs in fields not
planted to corn. In other areas, the eggs now remain in the soil for more than one winter before
hatching. In either case, the insect has responded to crop rotation by altering its reproductive
pattern to increase its survival.
Ecosystem simplification is an inevitable result of farming. We encourage the plants we want
(crops) and remove the plants and animals that interfere (pests). However, pesticide use may
cause additional simplification of the agroecosystem by differentially affecting species living at
the edges of planted areas. For example, grapes are very sensitive to the herbicide 2,4-D. Even
tiny amounts of spray drift, which would not give pause to any self-respecting weed, can kill
18
GA LeBlanc, 1994. Assessing deleterious ecosystem-level effects of environmental pollutants as a
means of avoiding evolutionary consequences. Environmental Health Perspectives 102:266-267.
19
And to rodents: it was used as a rodenticide until the late 1970s.
grape vines. In an area where wild grapes are found near cultivated fields, the vines are unlikely
to survive spraying of 2.4-D: sooner or later, spray-drift will kill them. Seed-eating birds can be
eliminated by toxic fungicides; disproportionate numbers of insect-eating birds may be killed by
eating insects dying from OP-intoxication (and therefore easy to catch). In most cases, it is the
desirable (i.e., rarer or more difficult to grow) species that are lost, and especially those that are
already under pressure (e.g., due to habitat destruction).
Summary
Among the ecological changes that have been identified are: a 50% mean global decrease in
amphibian populations; and a similar decrease in populations of songbirds that spend summers in
the U.S. and migrate to the Caribbean, Central and South America for the winters.
I do not know of any situations in which OPs have been clearly implicated in permanent
ecological changes. However, most ecotoxicologists assume that the relentless application of
pesticides to large fractions of global farmland, pasture, and forests contributes to changes in
ecosystems of the type described above. Undoubtedly the OPs - which are still among the most
heavily used insecticides in the U.S. and in the world - also play a role in whatever ecological
toxicity is occurring.
OP-INSECTICIDES: "CASE HISTORIES"
Parathion-ethyl
LD50: po, rats: 3.6 mg/kg, ; 13 mg/kg . Dermal, rats: 6.8 mg/kg, ; 21 mg/kg, .
po, birds: mallards, 1.4-3.0 mg/kg; pheasants, 12.4 mg/kg;
Long-term, chronic or delayed effects: in mammals, none proven; fairly strong possibility of
behavioral effects (anxiety, sleeplessness), shared by all insecticidal OPs. In birds,
teratogenic.
Persistence in water: The solubility of parathion is approximately 25 ppm, and its half-life, 15-16
days20.
Degradation at insecticidal levels: (see graph). EM in fish is 335. Polar materials accumulate to
about 45% of total extractable material by 33 days. At termination of microcosm, water
contained 0.30 ppb parathion. Levels for p-nitrophenol were 0.47 ppb (= 15% of polar
compounds) and for paraoxon 0.30 ppb (10% of polar cpds). These were considered
relatively minor components of the residues.21
Degradation at high levels: Biological degradation is slowed by high concentrations. If 31 lbs/A
are applied, residues remain for 16 yrs; if the level is 190,000 lbs/A, lethal residues
remain in the top inch of soil for 5 yrs.22 Both nonbiological and controlled microbial
degradation are feasible.23
20
in Metcalf microcosm
21
C-C Yu, JR Sanborn, 1975. The fate of parathion in a model ecosystem. Bulletin of Environmental
Toxicology and Chemistry 13:543-550.
22
DKR Stewart, D Chisholm, MTH Ragab, 1971. Nature 229:47. Long term persistence of parathion
in soil.
23
Sanborn, JR; BM Francis, RL Metcalf, 1977. The Degradation of Selected Pesticides in Soil: a
Review of the Published Literature. EPA 600/9-77-02.
Bioaccumulation: essentially nonexistent. Fish contain 335-fold levels of parathion found in
H2O; paraoxon was not detectable in fish.
Problems encountered in proper and legal uses of parathion: under dry, hot conditions, in the
presence of sunlight and dust, parathion is converted to paraoxon on foliage, and the
persistence of the paraoxon is significantly longer than under other conditions, resulting
in toxicity to workers even when legal re-entry times were observed. However, parathion
was eventually (1990s) banned in the U.S. because of its extreme acute toxicity.
Many of the deaths due to parathion were accidents outside of normal use (most often,
children who drank, spilled, or crawled over spilled parathion). In Dade County
(Florida), the death toll from pesticide poisoning in children decreased by 50% when
sales of parathion were restricted to licensed pesticide operators - meaning that parathion
was no longer sold in hardware stores. Many adults also died as the result of mistaking
parathion for less toxic chemicals, or because of careless response to spills. In addition,
parathion was a favorite method for committing suicide, especially in Japan, which
banned parathion in the 1960s primarily because of the high success rate of suicide
attempts.
Occupational poisoning due to parathion resulted both from employer carelessness in
enforcing lengthy waiting periods between spraying and re-entry of workers, from the
English-language illiteracy of many migrant workers, which made warnings on pesticide
labels irrelevant, and from the paradoxical persistence of paraoxon residues on foliage in
hot, dry conditions.
Diazinon
LD50: po, rats, 66-75 mg/kg (or 75-120 mg/kg?); dermal, 380 mg/kg
po, birds: 8.4 mg/kg, chicken; 10 mg/kg, bobwhite quail; 3.5 mg/kg, mallards. Redwinged blackbirds are remarkably resistant: po LD50 is 110 mg/kg.
LC50 0.079 mg/L water (bluegill); 0.635 mg/L water (rainbow trout);
Long-term, chronic or delayed effects: in mammals, none proven; fairly strong possibility of
behavioral effects (anxiety, sleeplessness), shared by all insecticidal OPs. In birds,
teratogenic.
Persistence in water: 1/2 life is longer in H2O than in soil: up to 6 months at pH 7.4, decreasing
rapidly with increasing pH.
Degradation at insecticidal levels: 2-4 week 1/2 life; degradation least rapid in cold, dry, alkaline
soils.
Degradation at high levels: not known?
Problems encountered in presumably proper and legal uses: runoff into water, especially after
use on golf courses. The high toxicity of diazinon to fish led to restrictions on its use for
commercial applications (notably sod farms and golf courses)24, and eventually to a ban
on diazinon-containing consumer products starting in 2005.
24
It became clear that as long as diazinon remained legal for use on private lawns, the ban on turf and
golf course use was circumvented. The extent of such illegal use was not well defined, but probably
contributed to the cancellation of consumer uses. This is a pity, because diazinon has no long-term human
health risks.
Chlorpyrifos
LD50: po, rats: 97-150 mg/kg (EPA list gives 163 mg/kg); dermal, 200. Rabbit, dermal, 2000
mg/kg.
po, birds: 25 mg/kg (hens); domestic duck, 75 mg/kg; quail, 16; wild bird (species
not given), 8 mg/kg
Long-term, chronic or delayed effects: in mammals, interacts with testosterone in ruminants only
to produce often lethal bloating syndrome. Almost certainly causes OPIDN. Fairly
strong probability of birth defects following early gestational exposure (humans). In
birds, teratogenic.
Persistence in water: microcosm studies show little bioaccumulation: Ecological magnification
(EM) is 72 in alga; 691 in snails, 45 in mosquito and 320 in fish.25 Chlorpyrifos-oxon
was not found in the organisms.
Degradation at insecticidal levels: Depending on location and soil type, 1/2 life is 5.7-17.2 days
if surface-applied; 33-56 days if soil-incorporated.
Degradation at high levels (under laboratory conditions): in Texas sandy loam, 1/2 life was 116
days; in Florida sandy soil, 1576 days (> 4 years). The (trichlorophenol) leaving group
has bactericidal activity that probably decreases degradation if present at high levels.
Problems encountered in proper and legal uses: at Cornell University, severe illness (including
death) in dairy bulls after bulls were treated with chlorpyrifos under supervision of
entomologists. A similar episode in another eastern dairy stud was mitigated by
information about the Cornell episode. Subsequent investigation showed that, in
ruminants, chlorpyrifos interacts with testosterone to cause accumulation of fluid in the
rumen. The mechanism is unknown.
In the late 1990s, Dow Chemical Co. was fined by U.S. EPA for not reporting over 150
incidents of untoward effects reported by users to Dow over a period of 10 years. It is
not known how many of the incidents (which include claims of permanent disability)
followed proper use, and how many resulted from inappropriate use (e.g., spraying food
preparation surfaces) or improper handling (e.g., a woman who cleaned up chlorpyrifos
granules in a summer cabin under the impression that they were grit/grime that had
accumulated over the winter).
A newborn infant brought back to a house treated with chlorpyrifos nearly died of
cholinergic crisis. Investigation by the Center for Disease Control (CDC) strongly
implicated contamination of clothes, directly or through contact with contaminated
counter-tops. This suggests improper application. The infant is reportedly not
developing normally. Four other infants, whose mothers were exposed during early
gestation, also reportedly suffer from developmental deficits. Such anecdotal reports,
while heart-wrenching, are not always reliable.
25
RL Metcalf, JR Sanborn, 1975. Pesticides and Water Quality in Illinois. Illinois Natural History Survey
Bulletin 31.