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OCR for page 358
Environmental Effects of DDT
Dichlorodiphenyltrichloroethane, better known as DDT, was a potent
insecticide when first used in the late 1930s. It had no obvious side effects
and was active against many insect pests. As a result, it was extremely
widely used, both in the United States and elsewhere. But, as has since
happened with so many other chemicals, DDT had unforeseen effects.
Those effects resulted from its persistence-one of its initially attractive
features. Because of this persistence, DDT was transported from its initial
site of application by both biotic and abiotic factors until almost no part
of the earth's surface was free of it. Animals in the oceans, lakes, and
rivers, on land, and in the air had detectable amounts of DDT in their
tissues. The resulting concern led to detailed monitoring, to increased
understanding of how chemicals are transported through the environment
and how quickly resistance to pesticides can evolve, and to a new aware-
ness of how even the most careful scoping of a problem does not guarantee
freedom from unpleasant surprises.
358
OCR for page 359
Case Study
JOHN BUCKLEY, Whitney Point, New York
INTRODUCTION
DDT (dichlorodiphenyltrichloroethane) was first synthesized in 1874,
but its properties as an insecticide were not discovered until 1939. By the
early 1940s, the United States was producing large amounts of DDT for
control of vectorborne diseases, and its use is credited with saving millions
of lives from diseases with insect vectors, such as typhus and malaria. Its
use in the United States for agricultural purposes expanded rapidly after
World War II, reached a peak of 80 million pounds in 1959, and decreased
thereafter until it was terminated by cancellation of its registration in 1972.
Approximately 1,350 million pounds of DDT were used in the United
States during those 30 years. In addition to domestic use, hundreds of
millions of pounds were exported.
Public concern over use of DDT became widespread in 1962, when
Rachel Carson's Silent Spring was published. As a result of this concern,
the pros and cons of DDT use were considered by the President's Science
Advisory Committee, whose report, Use of Pesticides, issued in May
1963, recommended an orderly phasing out of DDT over a short period.
As early as 1957, the federal government began restricting its own use of
DDT, and, beginning in 1967, many registered uses of DDT were can-
celed. Finally, on June 14, 1972, after a hearing that generated 9,312
pages of testimony (from 125 experts) and more than 350 documents, the
administrator of the Environmental Protection Agency (EPA) announced
the cancellation of all remaining uses of DDT on crops. Uses for public
health and quarantine purposes were not affected, and export was per-
mitted. After appeals by both sides, the Court of Appeals for the District
of Columbia ruled on December 13, 1973, that there was "substantive
evidence" in the record to support the ban of DDT.
The cancellation of DDT was so strongly opposed by some agricultural
interests that the 1974 report of the Committee on Appropriations of the
House of Representatives (Congressional Record, November 1973, H.
9619) stated:
The LEnvironmental Protection] Agency was also directed to initiate a complete and
thorough review, based on scientific evidence, of the decision banning the use of
DDT. This review of DDT must take into consideration all of the costs and benefits
and the importance of protecting the Nation's supply of food and fiber.
359
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SELECTED CASE STUDIES
The resulting review, DDT: A Review of Scientific and Economic As-
pects of the Decision to Ban Its Use as a Pesticide, was published by
EPA in July 1975. All data reviewed supported the 1972 findings.
ENVIRONMENTAL PROBLEMS
The cancellation of DDT was based on its persistence, transport, bio-
magnification, and toxic effects and on the absence of benefits of DDT
that were not available through less environmentally harmful substances.
In the course of research on DDT and its metabolites, many insights into
the movement, fate, and effects of pollutants in the environment were
developed. DDT was the first pesticide to which rapid and extensive
resistance evolved.
The earliest field studies were of DDT applied at 5 lb/acre for spruce
budworm control. These studies concentrated on acute effects, such as
deaths of birds, fish, and insects and other invertebrates. The acute LDso
(the dose lethal to 50% of the test organisms) in laboratory mammals
varied from 60 mg/kg in dogs to 800 mg/kg in rats. Bioassays of fish,
however, revealed LCso values (concentrations in water lethal to 50% of
the test organisms in a specified number of hours) of a few parts per
billion (ppb). Captive birds had acute LCsoS (concentrations in food lethal
to 50% of the test organisms) of 400-1,200 ppm (parts per million) when
given in the diet for 5 days (Hill et al., 1975~. Because of the extreme
toxicity of DDT to fish, attempts were made to avoid bodies of water in
the course of aerial spraying. Avoiding such areas completely was not
possible, because more than 50% of the DDT applied typically drifts
outside the target area, often onto water.
DDT had been in widespread use only a short time before resistance
developed in some pest species. The first recognized evidence of resistance
to DDT was the failure of house fly control with DDT in southern Italy
in 1947 after exceptional success in 1946. Shortly after discovery of
resistance to DDT, resistance to other insecticides was observed in the
field, but development of resistance was not universally recognized as late
as 1956 (Brown and Pal, 19711. Standardized laboratory methods for
detection of resistance were worked out under the sponsorship of the World
Health Organization. By the time of cancellation of DDT in 1972, more
than 200 species of invertebrates were resistant to DDT, cyclodiene, or
organophosphorus insecticides; some species were resistant to all three.
Resistance in vertebrates is less common, but has been reported to occur
in some frogs in Mississippi (Boyd et al., 19631.
Resistance has several ramifications in relation to environmental effects.
Indirectly, inability to control resistant insects has led to an increase in
OCR for page 361
ENVIRONMENTAL EFFECTS OF DDT
361
entomological research to find alternative methods of control. Generally
referred to as IPM (integrated pest management), these methods depend
on greater understanding of the life history and ecology of the pest, and
they result in much more selective control. Pesticides are used in most
applications of IPM, but only sparingly and in combination with other
methods, so that there is less contamination of the environment.
In the mid-19SOs, massive spraying programs were undertaken to save
the American elm by killing the elm bark beetle, which spread the imported
Dutch elm disease fungus from tree to tree. Robin mortality was wide-
spread, and robins dying with tremors characteristic of DDT poisoning
were commonly seen in treated areas. Dying robins appeared not only at
the time of spraying, but also in the following spring before additional
spraying. Chemical analyses showed that soils in these areas contained
DDT at up to 18 ppm, and earthworms that fed on the leaves in the soils
contained DDT at 53-204 ppm. Dead robins that had fed on the worms
had up to 3 mg of DDT in their tissues (Barker 19581. Tests for residues
in forest soils revealed that DDT disappeared slowly (Woodwell, 19611.
Here was unequivocal evidence of the persistence of DDT and of the
movement and concentration of DDT through a terrestrial food chain.
Also during the mid-19SOs, dichlorodiphenyldichloroethane* (000, also
called tetrachlorodiphenylethane, or TDE) was applied to Clear Lake,
California, to control gnats. Initial concentrations of 0.02 ppm in water
yielded DDD residues of 10 ppm in plankton, 903 ppm in the fat of
plankton-eating fish, 2,690 ppm in the fat of predatory fish, and 2,134
ppm in the fat of grebes that fed on fish (Hunt and Bischoff, 19601.
In the late l950s, the first indication of worldwide spread of DDT
residue was observed. It was discovered (Buckley, 1979)
that shark liver contained detectable amounts of DDT and its metabolites. This dis-
covery was so surprising that it was not believed. The initial presumption was that
the sample analyzed had been inadvertently contaminated; but a second sample, taken
with precautions to avoid contamination, also contained DDT residues. Because DDT
was not applied to the oceans, it appeared that the residue in shark liver had to be a
naturally occurring substance, but residue analysis of pre-DDT shark liver oil detected
no DDT. Finally, it was assumed that the DDT must have been accidentally dumped
into the ocean and that the shark liver contamination was local. Residue analysis
showed at least small amounts of DDT widely distributed in sharks and other marine
fish, thus requiring us to acknowledge long-range movement of DDT and biological
magnification of these residues. Laboratory studies confirmed the bioaccumulation.
Later analyses of tissues from penguins, skuas, and seals from the
*000 was used directly as an insecticide, but also formed by metabolism of DDT.
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SELECTED CASE STUDIES
Antarctic and fur seals from the Pribiloff Islands in the Bering Sea con-
firmed the occurrence of DDT and its metabolites far from any area where
it had been applied. DDT was truly ubiquitous (Anas and Wilson, 1970;
Sladen et al., 19661.
The degree of bioconcentration of DDT from the environment into living
tissues and of residues from prey to predator was so striking as to be
almost unbelievable (U.S. EPA, 19751:
Experimental data showed that DDT can be biologically concentrated by a variety of
aquatic organisms at all trophic levels. Phytoplankton, the dominant oceanic vegetation
and primary food source for marine animals, concentrates DDT from seawater into
its cell membranes. Water-fleas (Daphnia), a food source for many freshwater fish
species, accumulated 9.0 ppm in tissues after three days exposure to 80 pptr (parts
per trillions. This represents a bioconcentration factor of 112,500 times the exposure
level. Rainbow trout exposed to 1.0 ppm DDT (wet weight) in food and 10 pptr in
water for 84 days contained 2.3 ppm as whole body residues. Exposure to food alone
resulted in residues of 1.8 ppm (a concentration factor of 1.8 X) and exposure to
water alone yielded residues of 0.72 ppm (a concentration factor of 72,000 X). In
fish fed 1 mg/kg DDT/day, 73% of the DDT residues were present 90 days after the
fish were transferred to clean food.
DDT is not equally toxic to all species of animals. Control programs
for arthropods have sometimes resulted in increased pest damage, because
predators of a pest were eliminated by DDT. For example, DDT appli-
cations in apple orchards eliminated populations of predaceous ladybird
beetles, so that red-mite populations formerly controlled by the ladybird
beetles reached outbreak proportions. This particular mite is not suscep-
tible to DDT and was hardly influenced directly by the DDT that killed
the beetles (Helle, 19651. In other cases, less susceptible populations have
flourished after control programs, presumably because of reduction in
competition as a result of elimination or reduction of more susceptible
species. Cope (1961) reported that, after treatment of 72,000 acres of the
Yellowstone River watershed, the total numbers of invertebrates had re-
covered within a year, but the species composition was still altered. Ple-
copterans and ephemopterans were reduced, but trichopterans and dipterans
occurred at higher numbers at the end of the year. In some cases, selective
toxicity has resulted in reductions of species that are normally used as
food by valued predators. Ide (1967), in studies of the effects of DDT
applied at 0.5 lb/acre in the forested watershed of the Mirimachi River in
New Brunswick, observed that fewer insect species emerged in streams
affected by DDT, and that those most severely reduced were the large
ones, such as caddie flies, on which salmon mainly feed. Recovery of
stream fauna required up to 4 years.
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ENVIRONMENTAL EFFECTS OF DDT
363
Studies carried out at the Patuxent Wildlife Research Center demon-
strated that stored residues of DDT could be mobilized during weight loss,
and this resulted in mortality after exposure to DDT had stopped (Van
Velzen et al., 1972~. These studies provide a basis for understanding the
cause of some of the delayed mortality observed in the field.
During the 1960s, more subtle effects of DDT and its metabolites were
discovered. The striking eggshell thinning in some predatory birds such
as bald eagles, ospreys, peregrine falcons, and pelicans was clearly the
cause of decreases in their populations. Eggshells of the affected species
collected before 1945 were found to be noticeably thicker than those
collected in the late 1940s and later (Anderson and Hickey, 1972~. It was
hypothesized that this eggshell thinning resulted in decreased reproductive
success, and field studies of the brown pelican supported the hypothesis
(Risebrough et al., 19711. Laboratory studies confirmed that thinning of
shells occurred in mallards (Davison and Sell, 1974), American kestrels
(Peakall et al., 1973), and ring doves (Haegele and Hudson, 1973) when
dichlorodiphenyldichloroethylene (DDE) was incorporated in the diet of
these birds. Since the cancellation of DDT in 1972, there has been a
marked decrease of DDT residues in brown pelicans and improved re-
production in those pelicans on the Pacific, Gulf, and Atlantic coasts and
in ospreys in the eastern United States (U.S. EPA, 19751.
DDT alters the behavior of some fish, and some of the changes are
clearly detrimental to the welfare of the species. Atlantic salmon Parr, for
example, select water temperatures below those to which they are accli-
mated when exposed to DDT at 10 ppb or less and temperatures higher
than acclimation temperature when exposed to DDT at 10-100 ppb. Fur-
thermore, exposed salmon are less active than control fish. Rainbow trout
select higher than acclimation temperatures and exhibit high-temperature
shock followed by death (Javaid, 1972a).
KEY ISSUES
By 1970, it was evident that DDT residues were everywhere and that
reducing the residues would require huge reductions in the use of DDT.
Contamination of fish used for human food beyond the contamination
limit approved by the Food and Drug Administration (FDA) occurred
widely, especially in freshwater fish. Residues also appeared in many
meats and dairy products. Surveys of human fat revealed virtually universal
contamination by DDT, and residues in human milk gave rise to concern
over the desirability of breastfeeding. That DDT residues appeared far
from where DDT had been applied and lasted for years or even decades
suggested that most uses would have to stop; control of application sites
OCR for page 364
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SELECTED CASE STUDIES
and care in application would not be enough to reduce environmental
residues substantially.
The low cost of DDT, its relatively low toxicity to mammals (including
humans), and its persistence made its continued use highly desirable for
public health vector control programs. Although concern for its toxicity
is greater now than when DDT was first used in the United States (FDA
workers are said to have attested to its harmlessness by putting a spoonful
in their coffee), there is still no clinical or epidemiological evidence of
damage to man from approved uses of DDT, despite its demonstrated
tumorigenicity in mice.
ECOLOGICAL APPROACHES TO THE ENVIRONMENTAL
PROBLEMS ASSOCIATED WITH DDT
Ecologists first became concerned about ecological effects of DDT when
it began to be used for control of forest insects, especially when applied
from aircraft over hundreds of thousands of acres. The earliest studies
consisted of field observations of treated areas. Mortality at the time of
treatment or shortly thereafter was the criterion of the effect of treatment.
These first field observations revealed some mortality of birds and fish in
streams in treated areas and massive mortality of stream insects. For
example, it was noted in one study that cutthroat and brook trout stomachs
contained 99% crayfish after spraying, compared with none before, pre-
sumably because of the almost complete elimination of their insect food
(Adams et al., 19491. An experimental area was treated annually for 4
years with DDT, and breeding bird populations were studied. Three species
decreased substantially, but no changes were detected in 23 others (Rob-
bins et al., 1951~.
Studies during the early 1950s, principally in the laboratory, continued
to concentrate on acute effects of DDT. In the late 1950s, the Dutch elm
disease control program resulted in insights into food-chain transfer and
accumulation of toxic materials in terrestrial environments. Studies of
DDT (and other chlorinated organic pesticides) thereafter regularly looked
for phenomena associated with food-chain accumulations. The general
belief among toxicologists had been that metabolism of toxic substances
invariably decreased their toxicity, but it was discovered that some me-
tabolites of DDT, especially DDE, were more persistent and in some cases
more active biologically than the original compound. This discovery laid
the groundwork for research that eventually explained the decline of a
number of bird species through eggshell thinning.
Pesticide-wildlife studies also led to insights into repopulation by song-
birds of areas from which birds were removed. A study by Robbins and
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ENVIRONMENTAL EFFECTS OF DDT
365
colleagues demonstrated that, if half or more of the songbirds were re-
moved from a 40-acre tract of bottomland forest in Maryland during the
early part of the nesting season, they were replaced within a few days,
as judged by counts of singing males before and after the removal (U.S.
FWS, 19631.
By the early 1960s, it had become evident that many freshwater sport
and commercial fish were becoming so contaminated with DDT and its
metabolites (and with other chlorinated hydrocarbons) that they were not
suitable for human food. Concentrations were sometimes high enough to
impair fish reproduction. For example, lake trout from Lake George, New
York, produced eggs containing DDT and metabolites at 2.9 ppm or more;
these eggs hatched, but the fry died (Burdick et al., 19641.
USES OF ECOLOGICAL KNOWLEDGE AND UNDERSTANDING
Valued Ecosystem Components
No formal effort was made to identify valued ecosystem components,
although it was the effects on valued ecosystem components that eventually
resulted in eliminating the use of DDT in the United States. Among these
effects were the decrease in songbird populations caused by massive con-
trol programs, such as those for spruce budworms in forests and Dutch
elm disease in suburban areas; the declines in sport fish, such as lake trout
in Lake George, caused by blackfly control programs; the contamination
of fish and game desired as human food to a point greater than that
permitted in domestic animals used for food; and the decline of avian
predators at the top of food chains, particularly aquatic ones.
Importance of the Impacts
Probably of greatest importance were the drastic population decreases
over large regions or continents of bird species at the top of their food
chains. Some of these decreases such as those of the osprey, brown
pelican, peregrine falcon, and bald eagle resulted in the extirpation of
the species over vast areas of the United States and were grounds for
legitimate concern over total extinction of these species.
Boundaries of the Problem
Effects of DDT in the ecosystem were at first perceived to be local and
restricted to areas where DDT was applied, but findings from studies of
DDT kept expanding the boundaries of the problem. Eventually, it was
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SELECTED CASE STUDIES
recognized that DDT residues appeared throughout the world. Studies
were conducted, often cooperatively with scientists in other nations, to
elucidate the problems wherever they existed, e.g., raptor decline in Eu-
rope and residue appearance in the Arctic and Antarctic and in marine
environments throughout the Northern Hemisphere. Actions by the United
States to resolve the problems, however, were restricted to the control of
DDT within the United States. In fact, because DDT manufactured in the
United States was superior to others, manufacture for use in international
health programs was permitted after the ban on use in the United States
took effect. For several years, unfortunately, the manufacturer was less
adept at controlling wastes from the manufacturing process than at main-
taining purity of the product, so considerable waste DDT and related
products continued to be emptied into the Pacific Ocean, with detrimental
effects on the brown pelican and other species.
Study Strategy
Because of the nature of the problem, there was no formal study strategy.
Studies simply built on earlier studies. The earliest studies were carried
out largely by government scientists, especially fish and wildlife biolo-
gists. As results were disseminated, more people became interested in the
problem, and more academic scientists began to participate. Extensive,
informal cooperation developed among scientists in the United States,
Canada, and Great Britain. Results of field and laboratory studies were
exchanged, and these results were often used in shaping the work of other
investigators. In addition, more formal cooperation was arranged through
such international agencies as the North Atlantic Treaty Organization, the
Organisation for Economic Co-operation and Development (OECD), and
various bodies of the United Nations.
Monitoring
Measurements of eggshell thickness were made in many areas of North
America and Europe by using eggs of predatory birds in museums and in
nests. Thinning of 20% or more was found in many species (see Cooke,
1973, for a review of eggshell thinning). The thinning seems to be caused
only by DDE, inasmuch as experiments with other chlorinated hydrocar-
bons showed thinning only when the other compounds were at nearly toxic
concentrations, and it disappeared as soon as signs of toxicity diminished
(Haegele and Tucker, 19741. In the years after cancellation of DDT use
in the United States and after elimination of industrial discharge by Mon-
trose Chemical Company's DDT manufacturing plant, residues in many
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ENVIRONMENTAL EFFECTS OF DDT
367
aquatic species began to decrease, and field studies of predatory birds
showed increasing shell thickness and improved reproductive success (U.S.
EPA, 1975).
Extensive programs for monitoring pesticide residues, including those
of DDT and its metabolites, began in the United States during the 1960s
under the sponsorship of the Federal Committee on Pest Control. Results
of these programs in which sampling and analytical methods were stan-
dardized and which covered human food and animal feeds, air, freshwater,
estuarine areas, soils, and selected members of the biota were published
in the Pesticide Monitoring Journal. Less extensive results were available
through a cooperative program sponsored by OECD that provided com-
parable data from Canada, the United States, and several European coun-
tries.
It was in the course of these monitoring studies involving marine species
that substances interfering with DDT residue analyses were identified as
polychlorinated biphenyls (PCBs). This discovery, by the Swedish chemist
Jensen (1966), laid the groundwork for extensive studies of what has
turned out to be the most severe nonpesticide chemical contamination
problem in the world. Enactment of toxic-substance control legislation in
the United States was probably a result of studies on PCBs.
Effects of DDT and its metabolites undoubtedly are aggravated by other
hydrocarbon residues. Hardly any wild species on which residue analyses
have been completed contain only DDT residues, and laboratory studies
have shown that many of the other compounds are at least as toxic as
DDT. The interaction of these residues is poorly understood.
SOURCES OF KNOWLEDGE AND UNDERSTANDING
Generally Accepted Ecological Facts
Studies of acute toxicity of DDT to birds and fish were undertaken with
bioassay methods devised by pollution control biologists and toxicologists.
Data on mammals, at least laboratory rats and mice, were available and,
in the absence of specific tests on wild mammals, were used to predict
effects on wild mammals. Feeding studies were conducted on pheasants
and bobwhite quail, which were available because they were routinely
raised for release to the wild. Later, to answer specific questions, studies
were carried out on species not routinely kept in captivity. Such studies
usually required prior development of appropriate husbandry. The captive
birds studied eventually included bald eagles, kestrels, cowbirds, mallards,
pigeons, and others. Similarly, trout and goldfish were exposed, and LCsoS
for 24-48 hours were determined. The aquatic organisms studied included
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SELECTED CASE STUDIES
other species of fish and a number of invertebrates. Relatively elaborate
exposure systems for aquatic species were devised to overcome the prob-
lems of DDT adsorption on surfaces and loss to the air through codistil-
lation.
On the basis of these laboratory-determined estimates of toxicity, nat-
uralists observed field applications of DDT for control of forest insects.
In addition to observations of deaths of birds and aquatic organisms,
analytical chemistry was used to measure DDT residues in organisms from
treated areas. General knowlege of food chains, coupled with residue
analyses, led to predictions of effects on organisms at higher trophic levels,
which were indeed observed.
An iterative process of field observation, chemical analysis, laboratory
bioassay, and field experimentation was followed, and a reasonable un-
derstanding of the movement, concentration, and effects of DDT began
to emerge. The ecological theory of food chains and cycling of nutrients
(substituting "toxicants" for "nutrients") was used. In the case of de-
creasing populations of predatory birds, a hypothesis of pesticide causation
emerged. So, too, did a series of hypotheses on sublethal effects operating
through behavioral changes e.g., increased stress-and selection of un-
desirable temperatures.
Some results of field studies of effects on bird populations were sur-
prising. In some environments, few dead birds were found, although many
should have been, because residue in or on prey insects exceeded the
predicted LCso for birds. At the same time, populations of birds did not
decrease noticeably. Two kinds of studies were carried out: one to deter-
mine disappearance of bird carcasses, the other to determine repopulation.
It was hypothesized that only a small fraction of bird carcasses present
were found and that, at least in areas of a few to hundreds of acres in
relatively homogeneous habitat during the early nesting season, repopu-
lation would take place very rapidly. Studies carried out at Patuxent Wild-
life Research Center in Laurel, Maryland, confirmed both hypotheses.
Some 30-90% of the small-bird carcasses spread along a powerline
through a wooded area were not found by trained observers 4 days later;
some of the carcasses were simply not seen, but many were later deter-
mined to have been buried by beetles or carried away by scavengers.
Where 44% of the singing males of nine species of songbirds on a 100-
acre tract of bottomland forest were removed by mist netting, the removal
was estimated at only one-fourth of the actual removal. Banding data
showed that an influx of new birds,which took place immediately after
the 3-day removal period, made it impossible to measure accurately the
number of birds removed (U.S. FWS, 19631.
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ENVIRONMENTAL EFFECTS OF DDT
Theory and General Principles of Ecology
369
Knowledge of trophic interactions and food webs was an inherent part
of the studies of accumulation of DDD at Clear Lake, California, and of
the studies of robin mortality after DDT spraying for Dutch elm disease
control. Mortality of bald eagles and other predators was shown to be
caused by DDT (and other pesticides) accumulated through the food webs
of which these predators were a part. Eggshell thinning also was caused
by food-chain accumulations of DDE, which was formed from ODT by
organisms lower in the food chain. The importance of the eggshell thinning
and consequent reduced reproductive success was understandable only in
terms of population dynamics. The observations of distribution of DDT
in field studies and monitoring led Metcalf (1972) to devise microcosms
in which radioactively labeled substances could be followed as they were
partitioned into different environmental compartments.
The Value of Experiments
Studies in the 1940s involved treatment of experimental areas with
amounts of DDT similar to those used in actual forest insect control
programs. It was hoped that the use of control and experimental areas and
the study of the areas before and at various periods after treatment would
make it possible to detect changes in the nontarget biota, including re-
population if depopulation occurred. These studies were helpful in plan-
ning large-scale studies.
When first introduced in the United States, DDT was believed to be
harmless to people and all other valued organisms. Therefore, the regis-
tration permitted use in concentrations high enough to be effective. Tol-
erances in food were based on the amount that would be present as a result
of the approved uses. In the 1950s, DDT residues were discovered in
cows' milk, so applications to animal feed crops were reduced to the point
where the residues were not detectable in milk (i.e., to below about 1
ppm at that time).
Expert Judgment
Probably the most outstanding example of the role of expert judgment
was that played by trained ornithologists (especially Hickey), who first
noted the thinning of eggshells and hypothesized that it was caused by
pesticides. In 1965, Hickey had convened an international conference to
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SELECTED CASE STUDIES
bring together what was known about decreasing peregrine falcon popu-
lations (Hickey, 1969). Hickey suggested that pesticide residues were
probably responsible for reduced reproduction, primarily through egg de-
struction and egg-eating, but there was no mention of shell thinning. In
the next several years, the hypothesis of eggshell thinning was formulated.
Another pertinent example of expert judgment was provided by Rachel
Carson, a marine biologist, who saw and forcefully exposed the effects
of widespread pesticide use on the functioning of the natural environment
(Carson, 1962~.
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Adams, L., M. G. Hanavan, N. W. Hosley, and D. W. Johnston. 1949. The effects on
fish, birds, and mammals of DDT used in the control of forest insects in Idaho and
Wyoming. J. Wildl. Manage. 13:245-254.
Anas, R. E., and A. J Wilson. 1970. Residues in fish, wildlife, and estuaries. Organo-
chlorine pesticides in fur seals. Pestic. Monit. J. 3:198-200.
Anderson, D. W., and J. J. Hickey. 1972. Eggshell changes in certain North American
birds. Pp. 514-540 in Proceedings of the 15th International Ornithological Congress,
Symposium on Chemical Pollutants. E. J. Brill, Leyden, Neth.
Barker, R. J. 1958. Notes on some ecological effects of DDT sprayed on elms. J. Wildl.
Manage. 22:269-274.
Boyd, C. E., B. Vinson, and D. E. Ferguson. 1963. Possible DDT resistance in two species
of frogs. Copeia 1963:426-429.
Brown, A. W. A., and R. Pal. 1971. Insecticide Resistance in Arthropods. World Health
Organization, Geneva.
Buckley, J. L. 1979. Nontarget effects of pesticides in the environment. Pp. 73-81 in
Pesticides: Their Contemporary Roles in Agriculture, Health and the Environment. Hu-
mana, Clifton, N.J.
Burdick, G. E., E. J. Harris, H. J. Dean, T. M. Walker, J. Skea, and D. Colby. 1964.
The accumulation of DDT in lake trout and the effect on reproduction. Trans. Am. Fish.
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Carson, R. 1962. Silent Spring. Houghton Mifflin, Boston.
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River System. Trans. Am. Fish. Soc. 90:239-251.
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maintained on ad libitum or control-feeding regimens. Arch. Environ. Contam. Toxicol.
2:222-232.
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Environ. Pollut. 4:53-57.
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Committee Comment;
The wide use of DDT after World War II for the control of insects that
caused public health problems or were agricultural and forest pests created
a global environmental problem. Scientists and regulators were unprepared
to deal with this problem. Before 1947, the sole concern of regulatory
processes for pesticides was protection of purchasers from fraud. The
passage of the Federal Insecticide, Fungicide, and Rodenticide Act in
1947 permitted attention to the possible effects of a pesticide on nontarget
organisms other than people and domestic animals; but in practice, if the
proponent of a use certified that the product was both effective and safe,
registration was granted. The regulatory agencies the Food and Drug
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SELECTED CASE STUDIES
Administration (FDA) and the U. S. Department of Agriculture (USDA)-
required additional information from the proponent; until they were sat-
isfied, they could withhold registration (USDA) or a tolerance (FDA).
However, a proponent could obtain a "protest registration," shifting the
burden of proof from the proponent to the government, which had to
disprove the proponent's claim. Furthermore, during this period, few on
the staff of the pesticide registration office were knowledgeable about fish
and wildlife. Little concern for ecological problems of any kind was
evident in the regulatory agencies.
Scientists were little better prepared to deal with the problems posed
by DDT. Toxicologists believed that acute toxic effects of the chemicals
were the central issues of concern and that the metabolism of toxic sub-
stances invariably resulted in decreased toxicity. The possibility that me-
tabolites of DDT might be as toxic as or even more toxic than DDT itself
was not considered initially.
Ecological investigations were similarly narrow. At first, ecologists
concentrated their attention on field observations of areas that had been
directly treated with DDT. These studies revealed much about acute tox-
icity of DDT to different organisms and rates of recolonization of treated
areas. But ecologists were not prepared for the striking phenomena of
movement of DDT far from the sites of its application or for the remarkable
bioconcentration of DDT that resulted in reproductive failure of top car-
nivores. These ideas are now generally accepted, but in the late 1940s
and l950s were not. As a result, the first data indicating that DDT was
globally distributed and that concentrations were extremely high in the
tissues of organisms high on food chains were not believed.
The history of research to determine the results of the massive use of
DDT and of how those results were achieved reveals strikingly how the
state of science at any time constrains the questions that are likely to be
asked and the range of answers that are considered reasonable. Had con-
cepts of the trophic-dynamic organization of ecological communities been
developed before 1950, progress in understanding the roots of the problems
caused by DDT might have been much more rapid. Lindemann (1942)
had published his pioneering paper on trophic-dynamic ecology, but this
perspective had yet to become a part of the thinking patterns of ecologists
at the end of World War II. Indeed, as this case study demonstrates, efforts
to determine the causes of the effects of DDT were a major stimulus to
the development of this field of ecology. Persistence of organic chemicals
in their original form, or as toxic metabolites, is now routinely measured
as an indicator of likelihood of ecological effects. Bioaccumulation of
lipid-soluble chemicals to 104 or 1Os times the concentrations present in
the physical environment is now expected as a normal part of food-chain
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ENVIRONMENTAL EFFECTS OF DDT
373
dynamics. Spread of materials far beyond the sites of their application is
now considered characteristic of many chemicals in ecological systems.
Toxicology has likewise been influenced by research on the effects of
DDT. That metabolites of toxins can be toxic is now commonly accepted.
Efforts are no longer directed exclusively or even primarily to determining
lethal doses of toxins. Much more attention is now paid to sublethal effects
that might affect behavior, survival, and population dynamics, because
investigators recognize that severe decreases in animal populations can
occur in nature without massive direct mortality of adults. As a result,
although it can be argued that one "DDT" was inevitable, it is equally
clear that another "DDT" would be inexcusable.
Striking though these advances are, it is clear that the basic problems
encountered in dealing with DDT can express themselves in the future.
Metabolites, transport, and bioaccumulation are now routinely investi-
gated, but we still know almost nothing about the effects of toxicants in
mixtures. Nearly all toxicity studies still deal with chemicals one at a
time. In nature, however, organisms encounter potentially toxic chemicals
in complex mixtures. How these chemicals interact in the environment
and in the bodies of animals to produce different and unexpected effects
is still generally unknown. As long as this is true, effects that are unex-
pected and difficult to believe or explain are likely. The general recognition
that complex mixtures of toxicants require concerted efforts means that
the speed with which scientists and regulators can respond to the surprises
of the future should be greater than was the case with DDT. Nonetheless,
the major lesson of the DDT story would be lost if it did not heighten our
awareness of the current state of ignorance about important processes that
affect the responses of individual organisms, populations, and ecological
communities to toxic materials introduced into the environment.
The real basis for banning of DDT was its ecological effects, rather
than its effects on human health. Even today, the evidence on human
health effects of DDT is inconclusive, whereas the data on adverse eco-
logical effects are vast and convincing. Also, the decision to ban DDT,
rather than to restrict its use, was based on the conclusion that there was
no way to control its movement once it was released into the environment.
The decision was made more palatable politically by the rapid evolution
of resistance to DDT among crop pests. It is doubtful that the ban would
have been proposed or sustained if DDT had not decreased substantially
in its effectiveness as a result of evolved resistance. The agreement to
allow continued manufacture of high-grade DDT for export for use in
public health vector control programs also made the ban politically more
acceptable.
The assessment of the environmental effects of DDT is a good example
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374
of the interplay between laboratory analyses, field tests, and conceptual
developments in both ecology and toxicology. The constraints imposed
by the state of both sciences are clear in retrospect. We hope that our
awareness of those limits and of the deficiencies in knowledge will make
scientists and regulators more alert to evidence that does not fit into our
current picture of how the world works.
Reference
Lindemann, R. L. 1942. The trophic-dynamic concept of ecology. Ecology 23:399-418.
SELECTED CASE STUDIES
Representative terms from entire chapter:
control programs
