CHAPTER 14. A CHEMIST'S PERSPECTIVE ON ECOLOGY:
THE INTERLOCKING CHEMISTRY OF PLANT AND ANIMAL LIFE
Chemistry and Ecology
Ecology is defined as the study of the interactions of living organisms with each other and with
their environment. Ecological science has assumed increasing importance in recent years as we have
begun to realize that these complex interactions form a fragile web that can be damaged by human
intrusions, perhaps with catastrophic results. From a chemist's perspective, life itself has a molecular
foundation, and we need to understand the molecules of life and the complex web of their reactions if
we are truly to understand life on our planet.
Increasingly, for example, we are beginning to realize that plants are important to us, not only as
agricultural products or as decorative elements in our homes and gardens, but as essential participants
in ensuring our survival on this planet. The life-sustaining chemical reactions that take place within
plants and within animals are dependent upon one another. Understanding these chemical interactions
can give us a greater understanding of the fundamental interdependence of the life forms of our planet.
This chemical knowledge can also shed light on important contemporary issues, like energy sources
and consumption and the pollution of air and water.
Photosynthesis, Our Source of Energy, Food, and Oxygen
As we learned in Chapter 11, plants capture the radiant energy of the sun and transform it into
chemical energy in the process called photosynthesis, described in the equation
6 CO2 + 6 H2O + sunlight ---> C6H12O6 + 6 O2
(14-1)
The equation above is actually a net, or overall, equation which results from a multistep process
involving a series of molecules present in green plants, algae, and photosynthetic bacteria. In these
reaction steps, oxidation-reduction reactions are powered by light energy from the sun. By this process
the abundant compounds carbon dioxide and water are transformed into organic compounds, the
chemicals of living matter, and oxygen is produced as well. Indeed, scientific evidence indicates that our
planet's atmosphere lacked significant amounts of oxygen until plants capable of photosynthesis
evolved.
First, let us examine the photosynthesis product C6H12O6, or glucose. This compound, a simple
sugar, belongs to the class of biological compounds called carbohydrates. The name "carbohydrate"
2
was chosen because their chemical formulas can be thought of as being composed of carbon and water,
since the number of hydrogen molecules is always approximately twice that of the number of oxygen
molecules, just like in the formula of water, H2O. Glucose can be polymerized by plants into longchain carbohydrate molecules like starch and cellulose. The chemical structures of sugars and starches
will be discussed in greater detail in chapter 15, but we are already familiar with their important place in
our diets. In many parts of the world plant material comprises the entire human diet, and nutritionists
now recommend that carbohydrates comprise the greatest portion of our caloric intake. Even when we
use animal protein and fats for food, the energy they contain was originally obtained from the animal's
food ... plant carbohydrates.
When we use carbohydrates as food we are using the energy stored in their chemical bonds
to power our bodies by releasing the energy slowly in a series of biochemical oxidation-reduction
reactions. The net chemical reaction expressing this process, called cell respiration, is as follows:
C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy
(14-2)
Comparing this reaction with (14-1), you will notice it is exactly its reverse! In other words, the process
by which we gain the energy to power our body functions is simply a release of the sun's energy trapped
by photosynthesis, and in doing so we re-create the original starting materials, carbon dioxide and water.
When you "see" your breath on a cold day, it is the condensing water vapor created by this process that
you see. To prove that your breath contains carbon dioxide as well, you can blow through a straw into a
solution of calcium hydroxide. The white solid that forms is calcium carbonate formed as a product of
the reaction of the calcium hydroxide with the carbon dioxide in your breath.
When wood or other plant material is burned, the carbohydrates are oxidized in a combustion
reaction, releasing the energy quickly as heat. The chemical reaction for the burning of biomass is the
same as the net reaction for respiration:
C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy
(14-2)
Burning biomass was man's first source of combustion energy, and remains to this day a useful one.
Biologists are experimenting with new plants that will grow quickly and burn well, providing potentially
more efficient forms of biomass. Fossil fuels are also forms of biomass, chemically changed over the
millennia since they were formed. The energy they provide us is simply, once more, the sun's energy
trapped in chemical bonds by photosynthesis.
The Carbon Cycle and the Oxygen Cycle
In addition to their critical role in energy flow, the reactions of photosynthesis and cell
respiration are an example, as well, of matter cycling in the biosphere. The carbon dioxide you exhale
3
today may well be taken up by a nearby plant and incorporated into its structure as a part of a
carbohydrate molecule. Whatever the fate of that molecule, whether eaten by an animal or perhaps by
another person, or decomposing to become part of the organic content of the soil, its atoms were once
part of your own body. The carbon atoms in your body at this moment may once have belonged to
Cleopatra or Julius Caesar, a rare flower, or a mighty oak tree. The chemical reality is, then, that we are
not only interdependent with other life forms, but share intimately with them in a constant exchange of
atoms. The cycling of carbon and oxygen atoms described by the photosynthesis and cell respiration
equations are called the carbon cycle and the oxygen cycle.
Fig. 14-1. The carbon cycle.
http://www.kidsgeo.com/geography-for-kids/0159-the-carbon-cycle.php
Since the discovery of petroleum as a major fuel source in the nineteenth century, the burning
of fossil fuels has disrupted the natural balance of the carbon cycle. The combustion of long-dead
organic matter in vast quantities produces carbon dioxide faster than it can be utilized by natural
processes. Together with the large-scale destruction of forested areas, this combustion product has led
to a surplus of carbon dioxide that appears to be causing a global warming effect (Chapter 16). The
ocean's microscopic floating plants, known as phytoplankton, are an important source of
photosynthesis, as well as forming the basis of the marine food chain. Pollution which diminishes the
phytoplankton may thus have a major effect on the carbon cycle.
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Fig. 14-2. Scientists have found that respiration by Antarctic birds and mammals may have a
significant effect on the carbon cycle in the ocean in this region, affecting global models of the carbon
cycle which are being made as a part of research on global warming. From Science, 5 July 1991.
The Nitrogen Cycle
In learning about the elements in Chapter 2, we found that the atmosphere is 78% nitrogen gas.
In this form, however, nitrogen is highly unreactive. Nitrogen must be changed into other chemical
forms if it is to be utilized by most living organisms; conversion of nitrogen from the elemental form to
more reactive forms is called nitrogen fixation. The high energy provided by a lightning bolt can
produce nitrogen fixation. The nitrogen and oxygen in the air react in the high temperature of the
lightning bolt to form nitrate ions. The "fixed" nitrogen that comes down with the rainfall in electrical
storms provides only about 10% of the total produced by natural processes. The rest is provided by a
few forms of bacteria and algae, which are able to change nitrogen into the ammonium form, NH4+.
The most important of these is a bacterium which lives in nodules on the roots of a class of plants
called legumes. These plants, which include beans, peas, and lentils, benefit from the nitrogen
compounds formed in their roots.
Animals also require nitrogen. It is a necessary component for protein molecules, for the DNA
molecules which transmit information for protein synthesis, and for some of the vitamins. Nitrogen can
be obtained in the diet in the form of protein from other animals or from plants, with legumes being an
especially good source of plant nitrogen. Animals return nitrogen to the environment in the form of
animal wastes; urine, for example, is high in nitrogen. The decomposing bodies of animals also return
nitrogen to the earth; hence the ancient Native American practice, taught to early settlers, of burying fish
in a planting of corn. Thus there is a natural nitrogen cycle in which nitrogen atoms move through the
natural environment, with plants and animals filling complementary roles.
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http://tbn1.google.com/images?q=tbn:jT_Vg1e5d_h5_M::http://www.kidsgeo.com/images/nitrogencycle.gif
Fig. 14-3. The nitrogen cycle..
Until the twentieth century almost all fixed nitrogen came from natural sources. In the early
1800's the major concentrated sources of nitrogen compounds were deposits of
guano, or bird droppings, at heavily populated nesting areas on the offshore islands of Peru and Chile.
The nitrogen compounds from this source were important both as fertilizer and as a component in the
manufacture of gunpowder and other explosives. The importance of fixed nitrogen is demonstrated by
the fact that a battle was fought over these bird droppings in 1856 when the United States attempted to
declare these islands to be under U.S. control. The over-exploitation of the guano islands and the
slaughter of the birds which produced the guano led to a collapse of the guano trade after about 35
years. After about 1876, when a new process began to be used to purify the sodium nitrate deposits
mined in South America, these deposits became the chief source of the world's fixed nitrogen. The
Nitrate Wars fought by Chile against Peru and Bolivia between 1879 and 1893 resulted in Chilean
control of the desert areas which contained the nitrate deposits. The production facilities were
dominated by the British, despite German efforts to gain control.
With World War I the availability of fixed nitrogen became a critical issue, because nitrogen
compounds are essential both for fertilizer and for the production of explosives. The British navy could
block German access to Chilean nitrate sources, which might have had a major impact on the German
6
war effort. A major German chemical research effort, however, based on the work of Fritz Haber,
succeeded in producing fixed nitrogen in the form of ammonia on a commercial scale by the reaction
of nitrogen with hydrogen:
N2 + 3 H2 ---> 2 NH3
(14-2)
The Haber process, which made fixed nitrogen available in quantity to Germany during World War I,
is still the major means of nitrogen fixation today. The ammonia (NH3) formed in the reaction
undergoes further chemical reactions to make nitrates and other commercially useful types of nitrogen
compounds.
The Phosphorus Cycle
A third element which cycles through a nutrient chain involving both plants and animals is
phosphorus. In the phosphorus chain, shown in Fig. 14-4, animals receive phosphorus by eating plants.
Fig. 14-4. The phosphorus cycle.
As we will learn in Chapter 15, phosphorus is an essential element in our bodies for building
7
molecules for the transmission of genetic information, and protein synthesis; it is essential as well for the
molecules which transfer energy through oxidation-reduction reactions. Plants receive phosphorus
nutrients from the soil, where it originated either as inorganic phosphate ion PO43- from rocks and soils,
or as organic phosphate from animal sources. The phosphorus cycle differs from the carbon and
nitrogen cycles in an important way: it involves no gas molecules. The carbon dioxide of the carbon
cycle can circulate in the atmosphere, and is thus readily transferred from animals to plants. Though
nitrogen gas does not effect a direct transfer between animals and plants, its high concentration in the
atmosphere means that a steady supply will be available for fixation by lightning or by legumes. If,
however, phosphorus is depleted from the soil, it can be replenished only by depositing phosphoruscontaining compounds onto the soil. In a natural ecosystem, animal wastes, deposited in the general
vicinity from which plants are taken for animal food, will complete the phosphorus cycle. If crops are
harvested by man from an area, the natural balance is upset, since the animal wastes created by those
eating the food will not be deposited on the soil where the crops were grown. Depleted of phosphorus,
the soil must be fertilized with phosphorus-rich substances in order to retain its fertility. Disruption of
the phosphorus cycle by man can have unforseen consequences. As we shall see when learning about
water pollution in Chapter 17, phosphorus-rich waste products can act as fertilizers, producing
overgrowths of plant material in water systems and producing unfortunate consequences.
Other Elements as Plant and Animal Nutrients
Carbon, nitrogen, and phosphorus are only a few of the elements required by plants and
animals. Potassium is an essential animal nutrient, and it is so essential for plant growth that most
fertilizers are described by their N-P-K ratios, which give the amounts of nitrogen, phosphorus and
potassium. A fertilizer labelled as "5-10-5" is 5% nitrogen, 10% phosphorus, and 5% potassium.
Nitrogen, phosphorus, and potassium are often referred to as the primary nutrients for plants.
Calcium, magnesium, and sulfur are called secondary plant nutrients. Thhey are all essential in human
nutrition, as well. Calcium and magnesium are seldom deficient in soils, so they are not usually
included in fertilizers. It is easy to understand the necessity for magnesium in plants, as a magnesium
atom sits in the middle of the chlorophyll molecules that are essential for photosynthesis (Fig. 14-5).
Fig. 14-5. Structure of chlorophyll.
Sulfur is essential for the amino acids cysteine and methionine, found in both plants and animals. We
8
will learn about amino acid chemistry in the next chapter on biochemistry. Sulfur compounds can be
odorous, and are responsible for the characteristic smells of onions, garlic, and cabbage. Skunk spray
also contains a compound of sulfur.
Trace elements are those essential for plant or animal life, but only in very small amounts. Like
other mineral nutrients, they are taken up by plants as soluble ions from the soil. The trace elements
needed by plants are iron, manganese, copper, zinc, boron, molybdenum, and cobalt. Not all needs
for these trace elements are well understood, though many have been discovered. Zinc, for example, is
involved in the production of the hormone auxin, which helps stems and leaves to extend in growth.
Human nutrition has been the subject of considerable research, and the list of trace elements
known to be essential for humans is long. Iron, copper, and zinc head the list of trace nutrients, and are
followed by 16 "ultratrace nutrients", most of which are found in amounts of 10 milligrams or less in an
average adult: manganese, molybdenum, chromium, cobalt, vanadium, nickel, cadmium, tin, lead,
lithium. fluorine, iodine, selenium, silicon, arsenic, and boron. It may seem surprising to find toxic
substances like lead, cadmium, and arsenic in a list of essential nutrients, but their presence on the list
illustrates an important principle we shall encounter repeatedly: toxicity is a matter of dosage as well as
the identity of the substance. Occasionally when a new trace nutrient is publicized, there is a popular
rush to obtain the essential nutrient in supplement form on the commercial market; this happened
most recently with selenium. Over-enthusiastic consumption of trace nutrients in pill form can lead to
harmful consequences. How, then, to achieve a dietary balance of essential trace nutrients? Logically,
simply including a variety of plants in the diet should do so.
Pesticides and the Legacy of Silent Spring
A pesticide is defined as a chemical used to kill pests, or unwanted organisms. Pesticides are
further categorized according to the type of pest for which they are designed: insecticides, for example,
kill insects, herbicides kill plants, and fungicides kill fungi. In the past fifty years pesticides have
assumed the role of common household products; ant poison, crabgrass killer, and fungicides in paint
to prevent mold are only a few examples. The extraordinary productivity of modern agriculture is based
partly on the use of pesticides. Yet pesticide residues in food are a source of concern for an increasing
number of consumers, and environmentalists continue to urge caution in introducing toxic substances
into the environment. Are these concerns justified? What are the real issues in pesticide use?
The publication of the book Silent Spring by Rachel Carson in 1962 was a turning point, not
only in attitudes about the use of pesticides, but in public awareness of the importance of
environmental issues. The title exemplifies her writing, which combined scientific facts with an
appealing writing style which has been described as poetic. Explaining the ecological impact of the
heavy use of DDT and other insecticides on the environment, she described the devastating effect on
bird populations in some areas. The end effect of the depopulation of bird species was bleakly
illustrated in Silent Spring by quoting the words of Keats:
9
The sedge is wither'd from the lake
And no birds sing.
The impact of the loss of bird populations went beyond aesthetic considerations. Typically, a bird eats
its weight in insects each day, so birds serve as natural controls on insect populations. To make matters
worse, insects developed tolerance toward DDT over time, as those insects which survived DDT
spraying propagated new generations of DDT-resistant insect strains. It became necessary to apply
higher and higher concentrations of DDT in order to control insect pests. Even a layman could follow
Rachel Carson's argument that the logical conclusion of then-existing policies was that uncontrolled
insect pests might one day dominate the planet, with chemical controls becoming increasingly
ineffectual, and natural controls having been eliminated by these same chemical substances. How did
we find ourselves in such a predicament? The issues involved in pesticide use are complex, and are still
with us today, as government agencies must make decisions about the utilization of newly developed
pesticides, and individual communities must decide whether to authorize widespread insecticide
spraying to control disease-carrying mosquitoes. An EPA study has shown that 92% of all U.S.
households use one or more pesticides to control insects, weeds, fungi, or rodents. The history of
DDT is instructive, demonstrating many of the chemical principles and ecological issues that are still
with us today.
Although DDT is not the only pesticide that was banned as a result of the environmental
awareness that followed the publication of Silent Spring, it is one of the most important ones. A
knowledge of the chemical structure of DDT and some of the chemical principles responsible for its
properties are helpful if we are to understand the issues involved in the use of pesticides.
Fig. 14-6. The chemical structure of DDT, dichlorodiphenyltrichloroethane.
Like most chlorinated hydrocarbons, DDT is an unreactive compound which is quite soluble in
nonpolar substances, but has a very low solubility in water. It does not easily biodegrade, or break down
in the environment. It is highly toxic to insects, but not to mammals. It is inexpensive to produce. All
these properties made it seem ideal for use as an insecticide when the Swiss chemist Paul Muller first
discovered its insecticide properties in 1938.
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From the farmer's perspective, DDT, the first chlorinated hydrocarbon pesticide, and the other
organic chemical pesticides that followed were great improvements over the inorganic poisons like lead
arsenate and calcium arsenate which preceded them. DDT was not toxic to humans, was less
expensive, and, because of its low water solubility and lack of biodegradability, it persisted
in the soil rather than needing frequent applications. The effectiveness of DDT in suppressing insect
pests made possible the use of a wider variety of crops and increased crop yields. Even more important
was the use of DDT to control insects which carried disease. During World War II DDT was used
extensively to kill body lice, which transmitted deadly typhus disease. In previous wars before the
introduction of DDT, more soldiers died from typhus than died in battle. The use of DDT for
mosquito control in tropical regions by the World Health Organization saved millions of lives that
would otherwise have been lost to malaria. In 1948 Muller was awarded the Nobel Prize in recognition
of the great benefits to mankind which had resulted from his discovery of the insecticide properties of
DDT. During the 1950's DDT production continued to increase, going from 37 million pounds in
1953 to 124 million pounds in 1959.
Silent Spring sounded the warning that the chemical properties of DDT had negative
consequences as well as useful ones. Because DDT was soluble in fats and oils but not in water, it was
not excreted from the bodies of mammals, but stored in the body fat. Each bird, therefore, retained in
its body fat the accumulated DDT from all the insects it had ingested. Birds of prey accumulated the
highest concentrations of all, as they accumulated all the DDT stored in the bodies of the birds and
small mammals they ate. This process is called biomagnification, the escalating concentration of
pesticides from soil, water, and air in plants and animals, and, in turn, in the food chains of biological
systems. The U.S. Fish and Wildlife Service began studying the effects of DDT on wildlife soon after
the introduction of DDT in 1943. By the early 1950's, it had been established that dead birds were
commonly found in fields sprayed with more than 5 pounds per acre of DDT. Soon after followed
studies that showed that DDT and the related pesticides DDT, DDD, endrin, aldrin, and diedrin
affected the reproductive success of pheasants and quail, even though there were no other apparent
effects. It had become obvious that large predatory birds like ospreys and eagles were experiencing
serious reproductive problems, and a study of a breeding colony of kestrels, a kind of small falcon,
showed why. When DDT and dieldrin were incorporated into their diets, the shells of the eggs they
laid were so thin that they broke easily, a problem that Rachel Carson had not been aware of when
writing Silent Spring.
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Fig. 14-7.. DDE is a metabolite of DDT and is a very stable compound that accumulated in
the tissues of fishes and birds, moving up the food chain, especially affecting fisheating birds such as the pelicans. The effect was disastrous: by the late 60's
eggshells were so thin that in 1970, only 1 chick hatched out of the breeding
ground of approximately 5,000 nesting sites. If a shell is less than 20% of normal,
the egg will not hatch. In Southern California, shells were 31-51% thinner; even
some shell-less eggs were laid. For an analysis of eggshell thinning, see: Mark
Shields, Brown Pelican, The Birds of North America, No. 609, 2002, Cornell Lab. of
Ornithology, page 15.
Unlike many other pesticides, DDT has not been proven to have serious adverse health effects
in humans. However, data showing that traces of DDT can be found in virtually every tissue of the
human body throughout the population of the U.S. were a factor in the banning of DDT. Pesticides
can be ingested with food, inhaled, or absorbed though the skin, so that not only food, but even house
dust can be a source of pesticide exposure. Stored in the fatty tissue, the DDT is excreted in mother's
milk, another distasteful aspect of the ubiquitous presence of DDT. DDT levels in human tissue have
been monitored worldwide on a regular basis by the World Health Organization and the United
Nation's Environmental Program. Levels in China, Mexico, and India have remained relatively high
because of the continued use of DDT in these countries in agriculture and malaria control. Even in the
United States, where DDT has been limited in use since 1970 and banned since 1972, the pesticide
persists in the environment, though in diminishing amounts, and trace amounts are still found in
humans. Table 14-1 shows how DDT levels, measured as parts per million DDT in fatty tissue, fell in
the years following the restriction and banning of DDT. The table shows that DDT was not evenly
distributed in the population, with both age and race being major factors.
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Table 14-1. Total DDT Equivalent Residues in Human Adipose Tissue from General U.S. Population
by Race (from Silent Spring Revisited, p. 176)
Age
(years)
1970
level
(ppm)
1971
level
(ppm)
1972
level
(ppm)
1973
level
(ppm)
1974
level
(ppm)
0 - 14
4.16
3.32
2.79
2.59
2.15
45 and above
8.01
7.50
7.00
6.63
6.55
Caucasians
15 - 44
Blacks
0 - 14
15 - 44
45 and above
6.89
5.54
10.88
16.56
6.56
7.30
13.92
19.57
6.01
11.32
15.91
5.71
4.91
4.68
3.16
14.11
11.91
9.97
9.18
DDT is only one of a series of organochlorine insecticides, a group which includes chlordane,
aldrin, dieldrin, and lindane; these compounds all contain carbon, hydrogen, and chlorine atoms.
Chlordane, aldrin and dieldrin are all, like DDT, broad-spectrum insecticides, meaning they kill a
wide variety of insects. They are also persistent in the environment like DDT. Unlike DDT, they have
considerable toxicity in mammals. Chlordane, aldrin, and dieldrin are now severely regulated or banned
in the United States. Lindane is less toxic, and is approved for medical use, for example, in treating
infestations of lice in children.
Another class of insecticide currently in use as an alternative to organochlorine compounds is
the organophosphorus compounds; contining phosphorus. Because these compounds are less stable
chemically than the organochlorine compounds, they have largely replaced them as insecticides.
Malathion, for example, has been used in wide-scale spraying for mosquitoes and for the Medfly which
has periodically threatened the California fruit crop. Like other organophosphates, however, malathion
is toxic to mammals and must be used with caution. Parathion has a much higher toxicity. Because of
its cheapness, however, it is in use in some developing countries. Organophosphorus insecticides, like
the organochlorine insecticides, are neurotoxins, meaning that their toxic action disrupts the chemical
transmission of nerve impulses. DDVP, or dichlorvos, has a relatively high toxicity to mammals. It is
the compound used in "pest strips" to be hung in living areas to kill insects; its approval for such use is
currently under review by the Environmental Protective agency.
Carbamates are another class of pesticides similar in action to the organophosphates. Their
toxicity levels vary, and some have been found to be highly specific to certain species. One of the most
widely used of the carbamates is Sevin, or carbaryl, with an oral LD50 in rats of 400 mg/kg (Fig. 14-10).
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Sevin has the disadvantage of being toxic to honeybees. The 1984 chemical disaster in Bhopal, India, in
which more than 2000 people were killed, was the result of an inadvertent release of methyl isocyanate
gas, a chemical intermediate in the production of the carbamate compound Sevin at Union Carbide
plant.
Not all insecticides have their origin in the laboratory. Naturally-occurring poisons have been
used in agriculture for centuries. Both nicotine (from tobacco) and caffeine (from coffee, tea, and cola)
have been used as insecticides. Nicotine used as insecticide must be handled with care, as it is a potent
neurotoxin. One of the oldest classes of natural insecticides are the pyrethrum compounds, or
pyrethroids, which are extracted from the pyrethrum flower, a member of the chrysanthemum family.
Pyrethrum compounds are gaining in popularity as insecticides because they are generally nonpersistent
in the environment and are not highly toxic to mammals.
Table 14-2. Major Types of Insecticides
Insecticide type
Examples
Chlorinated hydrocarbons
DDT, aldrin, dieldrin, lindane
chlordane
Organophospates
Carbamates
Pyrethroids
Malathion, parathion, DDVP,
Diazinon
Sevin, Zineb, maneb, Temik
Pyrethrin compounds from flower extracts,
sometimes chemically modified
Insecticides have their proper uses. They have saved millions of lives and continue to do so. As
Rachel Carson expressed the problem in Silent Spring:
It is not my contention that chemical insecticides must never be used. I do contend that we
have put poisonous and biologically potent chemicals indiscriminately into the hands of persons
largely or wholly ignorant of their potential for harm... I contend, furthermore, that we have
allowed these chemicals to be used with little or no advance investigation of their effect on soil,
water, wildlife, and man himself.
What is the situation of insecticide use today? The Federal Insecticide, Fungicide, and Rodenticide Act
(FIFRA) http://www.epa.gov/lawsregs/laws/fifra.html , administered by the EPA,
required
manufacturers of pesticides to do additional studies regarding the safety of these substances, since many
of them were registered decades ago under inadequate standards. Reregistration of nearly 215,000
pesticide products based on about 600 active ingredients were completed by 1997. Still, millions of
pounds of pesticides are used annually, with home and garden use representing a significant fraction
(Table 14-3).
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Table 14c (2 of 2)
U.S. Annual Volume of Pesticide Usage,
Home and Garden Sector
By Type, 1989-1995 (Millions of pounds of active ingredient)
Pesticide
Type
Herbicides
Insecticides
Fungicides
Other
Conv.
Other
Chems.
Total
1989 1990 1991 1992 1993 1994 1995
44
19
13
46
19
10
46
18
9
46
18
8
46
18
8
46
18
8
47
17
8
2
2
2
2
2
2
2
68
66
65
64
62
61
59
146
143
140
138
136
135
133
Source: EPA/OPP/BEAD estimates, 3/97.
Each year in the U.S. an estimated average of 45,000 pesticide poisonings of humans occur, of which
about 50 are fatal. Accurate data on human pesticide poisonings are still not available, over 20 years
after the publication of Silent Spring. An EPA study has shown that pesticides used in the home are
applied at much higher levels than in commercial farming, and that each year more than 250,000
Americans become sick because of pesticides used in the home. Pesticide poisoning in developing
nations is thought to be a serious issue, though solids statistics are hard to come by.
"Most estimates concerning the extent of acute pesticide poisoning have been based on
data from hospital admissions which would include only the more serious cases. The latest
estimate by a WHO task group indicates that there may be 1 million serious unintentional
poisonings each year and in addition 2 million people hospitalized for suicide attempts
with pesticides. This necessarily reflects only a fraction of the real problem. On the basis
of a survey of self-reported minor poisoning carried out in the Asian region, it is estimated
that there could be as many as 25 million agricultural workers in the developing world
suffering an episode of poisoning each year." (Jeyaratnam J., 1990).
What of Rachel Carson's warnings that DDT had brought on the "age of resistance" through the
development of pesticide-resistant species, and that chemical treatment was a treadmill that, once
started, could not easily be stopped?
From the 1940's to the present, crop losses due to insects have doubled, from 7% to about 13%, even
though the use of insecticides has increased tenfold. These losses have been attributed to a variety of
15
causes. Pesticide-resistant insect pest species, as we have seen, have greatly increased in number. At the
same time, the natural enemies of certain pests have been destroyed. Since the introduction of pest
controls to modern farming, crop varieties have been used which are less inherently resistant to pests,
and crops are raised in areas once useless for agriculture because of the prevalence of insect pests.
Farming practices which discourage pests, such as crop rotation, frequent tilling of the soil, and
destruction of infected plant residues, have been cut back in favor of increased use of chemicals. Some
crops are made more susceptible to insect attack because of the effect of insecticides on plant
physiology. Finally, consumers have come to expect a degree of cosmetic perfection in fruits and
vegetables which is achievable only with increasing levels of pesticide use; the rejection of imperfect
products results in additional crop loss.
Insecticide Alternatives: Natural Chemical Control
For some insect pests, alternatives to toxic chemicals are available; these fall into the general
category of natural chemical control. These are chemicals which have been identified as having an
important role in the insect's life cycle. Those which have been so far synthesized by chemists and used
commercially are of two types, insect hormones and insect pheromones.
Hormones are chemicals produced by an organism which provide signals that control
developmental processes and metabolic functions. Juvenile hormones in insects control the growth of
the larval stage. They are switched off when it is time for the insect to progress to the adult stage. For
insects like mosquitoes that are pests in the adult stage, but not in the larval stage, applying synthetically
produced juvenile hormone is useful because it causes the insect to remain in the larval stage.
Pheromones are sex attractants usually released by a female to lure a male of the same species
for mating. First discovered in insects, they were then reported in fish, crustaceans, and mammals,
including dogs, cattle, and primates. Their role in human courtship and reproduction is currently a
subject of research, but perfumes commercials to the contrary, this research is still in early stages. So
far, sex pheromones have been discovered for about forty insect species. Two commercially successful
applications for pheromones in insect control are the Japanese beetle and gypsy moth traps, in which
the insect pheromone is placed inside a trap, luring virtually all male insects in the vicinity and limiting
further reproduction of the species in the area.
One of the great advantages of the use of pheromones is that a very tiny amount of the chemical is
effective. The plastic traps used to control Japanese beetles contain only 1 milligram of the Japanese
beetle pheromone, first isolated and synthesized in the late 1970's. Tests have shown that one picogram
(1 x 10-12 g) of the gypsy moth pheromone is sufficient to serve as an attractant. The extreme
effectiveness of minute quantities of this substance is demonstrated in the humorous predicament
described in a research chemist's story, reported in the NEACT Journal:
In 1982 I occupied an office adjacent to a laboratory where a team of chemists was busily
engaged in the synthesis of [gypsy moth pheromone]. Within a few weeks I noticed gypsy
16
moths feverishly beating against the window screen adjacent to my bed when I retired in the
evening. A few days after I sold my car to a neighbor, he complained that he purchased the
largest gypsy moth trap in existence. Evidently trace amounts of the pheromone from my
hands, hair, or clothing clung to the upholstery and attracted moths as soon as the windows
were opened.
When I attended a meeting at MIT, I was the only person on campus surrounded by a cluster
of moths. A month later in the Ming Tombs in China I was the Pied Piper for moths to the
underground chambers. The next day I visited The Wall north of Beijing on a hot summery
Sunday afternoon in July. The walkway on top of The Wall was jammed with many Chinese
enjoying a beautiful weekend. When I saw what appeared to be moths in the distance, I raised
my sweaty palm. Within moments a stream of moths landed on my outstretched hand. They
did not approach anyone else. I shall never forget the looks of consternation on the hordes of
people at that moment.
Natural chemical controls must be developed, painstakingly, often requiring decades of
research. Developing a natural control involves first of all a detailed knowledge of the life cycle of the
specific insect pest to be controlled. Then the chemicals involved in the life cycle, for example the
hormone or pheromone, must be chemically isolated and its chemical structure determined, not always
an easy task. Finally, a method of chemical synthesis must be devised so that the desired chemical can
be manufactured in the quantities needed for insect control. The rewards for developing natural
chemical controls, however, are great, because the control is specific for the undesired species, a much
preferable solution to spreading wide-spectrum poisons in the environment.
Herbicides
Herbicides are chemicals that kill or otherwise interfere with the growth of plants; they
represent about two-thirds of total pesticide sales. In modern agriculture herbicides have virtually
replaced the traditional mechanical means of weed control like tilling and hoeing. Greater than 90% of
all corn, cotton, soybean, and peanut acreage is treated with herbicides each year. Table 14-4 lists some
of the major herbicides by their mode of action.
17
Table 14-4. Major Types of Herbicides
Herbicide Type
Examples
Mode of Action
Contact
Atrazine, paraquat
Systemic
2,4-D, 2,4,5-T
Kills foliage by stopping
photosynthesis
Soil sterilants
Treflan, Dymid, Dowpon,
Sutan
Growth hormones cause plants
to overgrow, swell, and die
Kills soil microorganisms
necessary for plant growth;
most also act as systemic
herbicides
Contact herbicides act by coming into contact with the plant foliage. One of the contact
herbicides is paraquat, which figured in the national news when it was used for wide-spread spraying of
fields of marijuana and poppies, largely in Central and South America. Paraquat and related
compounds are moderately toxic. Small amounts can cause skin irritation as well as nose bleeding if
inhaled, but can be rapidly eliminated from the body. Heavy exposure, however, can lead to
proliferation of lung cells and irreversible respiratory failure.
Systemic herbicides are actually growth hormones, which cause the death of plants by causing
unnatural growth patterns in plant cells. Sometimes systemic herbicides can be used as selective
herbicides, killing one type of plant, but not another. Narrow-leaved plants like grass and corn, for
example, can be affected differently from broad-leaved plants. Timing is an important variable in
growing plants, and is particularly important in herbicide application. Preplant herbicides are applied
before the crop is planted. Preemergence herbicides are used after the crop seeds are planted, but
before the plants emerge from the soil. Postemergence herbicides are applied after the plants have
begun to grow.
Herbicides have been generally regarded as less harmful to humans than insecticides, for their
toxicity as measured in LD50 values of animal studies is low. Widespread spraying of Agent Orange, a
mixture of 2,4,5-T and 2,4-D, as a defoliant in the Vietnam War was implicated in a high rate of birth
defects in the children of the exposed Vietnamese population and in the soldiers who performed aerial
spraying of the compounds. Compounds called dioxins were found to be present in low levels in 2,4,5T as contaminants, and were found to cause birth defects in laboratory animals. 2,4,5-T has been
withdrawn from the market.
Agent Orange is now recognized as a cause of service-related disability in Vietnam veterans as
described by the US Office of Veteran’s Affairs:
Diseases Associated With Exposure to Agent Orange
18
These are the diseases which VA currently presumes resulted from exposure to herbicides
like Agent Orange. The law requires that some of these diseases be at least 10% disabling
under VA’s rating regulations within a deadline that began to run the day you left Vietnam.
If there is a deadline, it is listed in parentheses after the name of the disease.
•
Chloracne or other acneform disease consistent with chloracne. (Must occur within
one year of exposure to Agent Orange).
•
Chronic Lymphocytic Leukemia
•
Diabetes Mellitus, Type II
•
Hodgkin’s disease.
•
Multiple myeloma.
•
Non-Hodgkin’s lymphoma.
•
Acute and subacute peripheral neuropathy. (For purposes of this section, the term
acute and subacute peripheral neuropathy means temporary peripheral neuropathy
that appears within weeks or months of exposure to an herbicide agent and resolves
within two years of the date of onset.)
•
Porphyria cutanea tarda. (Must occur within one year of exposure to Agent Orange).
•
Prostate cancer.
•
Respiratory cancers (cancer of the lung, bronchus, larynx, or trachea).
•
Soft-tissue sarcoma (other than osteosarcoma, chondrosarcoma, Kaposi’s sarcoma,
or mesothelioma).
Integrated Pest Management
The method of pest management advocated by most environmentalists today is called
integrated pest management, or IPM. Integrated pest management utilizes the full spectrum of pest
control measures, including, in some cases, pesticides. Each measure is undertaken, however, only after
a careful analysis of the ecological system as a whole, and of all the possible options for maintaining the
maximum health of the desired species. The proper use of integrated pest management requires a
sophisticated knowledge of all the issues involved in pest control. For this reason it is more complicated
to use than standard agricultural practices which prescribe the use of chemicals on a regular basis to
maintain crop health. It is usually more expensive because it requires "more eyes per acre" to analyze all
the factors contributing to a pest problem. Only with some form of integrated pest management,
however, are chemical pesticides used most safely and most effectively.
19
A good example of the use of integrated pest management is the U.S. program to protect fruit
crops from the Mediterranean fruit fly, or "Medfly." The Medfly lays its eggs on unripe fruits and
vegetables growing in the fields, and its maggots feed upon the fruit both before and after harvest. So far
the Medfly has not become established in the U.S.; if it does, millions of dollars of damage to the fruit
harvest will result annually. Medfly prevention relies first of all on vigilance at all possible points of entry
from other countries. Imported produce is inspected rigorously, and suspect shipments are fumigated.
Traps baited with pheromones are maintained, and are monitored regularly for the presence of
Medflies. If Medflies are found, sterile male Medflies are released in the area in addition to the
pheromone traps, and all fruit is removed from the area to deprive the Medflies of a place to lay eggs,
even if this means removing unripe fruit from trees. Finally, if a Medfly epidemic appears to threaten,
Malathion insecticide is used in combination with pheromone bait to increase its effectiveness. So far,
IPM has proven successful in combatting the Medfly, and its last resort, Malathion, has been needed
only a few times.
"The Perfect Lawn": Agricultural Management Issues in Microcosm
Increasingly, agriculture is centered in large-scale growing areas removed from the everyday
experience of most people. Few, however, escape at least some contact with that ubiquitous symbol of
home ownership, the lawn. A wide variety of pesticides: insecticides, herbicides, and fungicides, are
directed at these small patches of land; lawn pesticides support a $1.5- billion industry. Regular
"preventive" spraying of insecticides, herbicides, and fungicides, whether or not a pest is currently
present, is a standard practice in the lawn industry. Whether applied by homeowners or by lawn-care
professionals, the lawn pesticide industry has come under fire as an example of overuse of toxic
chemicals.
Chemlawn, the nation's largest lawn-care company, discontinued the use of the herbicide 2,4-D
after a 1986 study by the national Cancer Institute showed that Kansas wheat farmers who frequently
applied the substance experienced a marked increase in the incidence of non-Hodgkins lymphoma. In
1991 the National Cancer Institute published the results of a study which showed that dogs whose
owners use 2,4-D on their lawns are up to twice as likely as other dogs to develop lymphatic cancer.
The report suggested further studies of the possible effect on owners. Nevertheless, the product
continues to be marketed and used extensively as a herbicide for home use.
Are there alternatives to the regular applications of neurotoxins and possible carcinogens to the
home property in maintaining an acceptable lawn? Just as in large-scale agriculture, part of the solution
to this problem lies in good agricultural practice, and the issues involved exemplify those which are
currently under study in agricultural management worldwide:
Selection of appropriate plant material. If sun-loving plants like bluegrass are placed in shady areas, for
example, they will be weak and stressed, vulnerable to pests. Worldwide, one of the causes of
increased losses of crops to pests is the use of crops in marginally appropriate land. In the case
of a lawn, choosing appropriate strains of grass for the local environment is important. The
homeowner may have to face the fact that, because of local climate conditions or competing
20
plant materials like large trees, the site is inappropriate for any type of grass, and alternative
plant materials should be found.
Water and fertilizer management. Plants given too little water and fertilizer may be weakened. On the
other hand, too-frequent applications of fertilizer can cause weak, shallow-rooted plants.
The consequences of using monocultures. Large-scale agriculture produces higher profits by planting
large blocks of one kind of crop: corn and cotton are examples of crops grown over large areas
in this way. Such growing practices render the crops more vulnerable to pests, as a pest
outbreak can multiply rapidly. Much of the heavy use of pesticides today in agriculture is a
consequence of the decision to maintain monocultures. In the home lawn, seed products which
rely on one or two strains of grass to produce a perfectly uniform turf produce a disease-prone
monoculture which requires heavy use of pesticides for its maintenance. A variety of types of
grasses is preferable, and varying lawn areas with other kinds of plantings is better still.
The use of integrated pest management. Rather than applying crabgrass killer on a regular basis, the
homeowner can have the lawn kept at least three inches high, shading the soil and depriving
crabgrass seedlings of the full sun they need. Small patches of dandelions and other weeds can
be watched for, and weeded by hand before they go to seed, propagating themselves in the
lawn. Timing fertilizer applications for the late fall allows the lawn grasses to grow strong roots in
the cool weather when crabgrass does not compete for light and nutrients. A sturdy, wellplanned, well-cultivated lawn should not be susceptible to pest invasions. If a dieoff does begin
to occur, the cause of the problem should be determined right away with the help of a county
agent or other horticultural expert. Whenever possible, natural controls should be used, though
these seldom give the satisfaction of an immediate kill. Using pheromone traps for Japanese
beetles, for example, will prevent or diminish the damage that the next generation of beetle
grubs will do to the lawn roots. Finally, if a pesticide is to be used, it should be chosen correctly
and applied only in necessary quantities. Before the pesticide is applied, a risk-benefit analysis
should be made: what are the possible risks of the pesticide use, and are they justified by the
possible benefits? If the pesticide is used, the instructions supplied with it should be read
carefully and followed explicitly. They will probably include a prohibition on the use of the
lawn by humans or pets for a specified period of time.
As an analysis of even the smallest patch of lawn shows, the issues involved in dealing with any
living system are complex. The problems we create for ourselves in trying to manage the environment
arise not because we are using chemicals; the grasses, plants, and even the insects we think of as pests
are all themselves made up of chemicals. Rather, we need a fuller understanding of the complex
interactions taking place in each part of our natural environment and of how these interactions operate
on the molecular level if we are to intervene without ultimately causing harm.
21
DECODING CHEMISTRY: GARDEN FERTILIZERS
A 50-pound bag of "Premium Garden Fertilizer" is labelled "510-10" and lists the ingredients ammonium phosphate, muriate of
potash, urea. What do the numbers mean, and how do they relate
to the ingredients list?
The numbers 5-10-10 are the N-P-K ratio, or the relative
amounts of the elements nitrogen, phosphorus, and potassium. In
this case, the fertilizer contains 5% nitrogen, 10% phosphorus,
and 10% potassium. Ammonium phosphate is an ionic compound, or
salt; the chemical formulas of the ammonium ion, NH4+, and the
phosphate ion, PO43-, were given in Chapter 5, and the formula
for ammonium phosphate is (NH4)3PO4. Looking at the formula for
ammonium phosphate, we see that it is a source of the essential
plant nutrient elements nitrogen and phosphorus. "Muriate of
potash" combines the terms "potash," a common industrial name
referring to potassium compounds, and "muriate," the industrial
name for chloride; hence, muriate of potash is potassium
chloride, a source of the nutrient element potassium. Urea is
an organic compound containing the element nitrogen; you may
recognize it as the organic compound synthesized by Wohler from
inorganic starting materials.
22
CONCEPTS TO UNDERSTAND FROM CHAPTER 14
Ecology is the study of the interactions of living organism with each other and with their environment.
Plants capture the radiant energy of the sun and transform it into chemical energy in the process called
photosynthesis, described in the equation
6 CO2 + 6 H2O + sunlight ---> C6H12O6 + 6 O2 (14-1)
The equation above is actually a net, or overall, equation which results from a multistep process
involving a series of molecules present in green plants, algae, and photosynthetic bacteria. In these
reaction steps, oxidation-reduction reactions are powered by light energy from the sun. By this process
the abundant compounds carbon dioxide and water are transformed into organic compounds, the
chemicals of living matter.
The photosynthesis product C6H12O6, or glucose, is a simple sugar, and belongs to the class of biological
compounds called carbohydrates. Glucose can be polymerized by plants into long-chain carbohydrate
molecules like starch and cellulose.
When we use carbohydrates as food we are using the energy stored in their chemical bonds to power
our bodies by releasing the energy slowly in a series of biochemical oxidation-reduction reactions. The
net chemical reaction for this process, called cell respiration, is :
C6H12O6 + 6 O2 ---> 6 CO2 + 6 H2O + energy
(14-2)
The products for the net reaction for cell respiration are exactly the same as the reactants for the net
equation for photosynthesis. The reactants for the cell respiration net equation are the same as the
products of photosynthesis. Hence, the process by which we gain the energy to power our body
functions is simply a release of the sun's energy trapped by photosynthesis, and in doing so we re-create
the original starting materials, carbon dioxide and water.
The carbon and oxygen cycles shown in Fig. 14-1, the nitrogen cycle in Fig. 14-3, and the phosphorus
cycle in Fig. 14-4 are important examples of matter cycling in the biosphere.
Since the discovery of petroleum as a major fuel source in the nineteenth century, the burning of fossil
fuels has disrupted the natural balance of the carbon cycle by producing carbon dioxide faster than it
can be utilized by natural processes, thus causing a global warming effect.
Conversion of nitrogen from the unreactive elemental form to more reactive nitrogen compounds is
called nitrogen fixation. Lightning bolts and bacteria on the roots of legumes are natural sources of
nitrogen fixation.
23
The Haber process, which made fixed nitrogen available in quantity to Germany during World War I,
is still the major means of nitrogen fixation today. The equation for this process is:
N2 + 3 H2 ---> 2 NH3
(14-2)
Nitrogen, phosphorus, and potassium are the primary plant nutrients. Fertilizers are usually labelled
with their N-P-K content.
A pesticide is a chemical used to kill pests, or unwanted organisms. Examples of pesticides are
insecticides, which kill insects, herbicides, which kill plants, and fungicides, which kill fungi.
The publication of the book Silent Spring by Rachel Carson in 1962 was a turning point, not only in
attitudes about the use of pesticides, but in public awareness of the importance of environmental
issues.
DDT, one of the first organochlorine insecticides, is oil-soluble and unreactive. Hence it is persistent in
the environment, concentrating in the fatty tissues of organisms that ingest it.
Biomagnification is the escalating concentration of pesticides from soil. water, and air in plants and
animals and, in turn, in the food chains of biological systems.
Neurotoxins are toxic substances which act by disrupting the chemical transmission of nerve impulses.
Important classes of insecticides are organochlorine compounds, organophosphorus compounds,
carbamates, and pyrethroids. Examples of each are given in Table 14-2.
The development of pesticide-resistant species over time results in the need for ever-increasing
amounts of pesticide required per application. This has been called the pesticide treadmill. The
elimination or depopulation of natural predators because of pesticide use also contributes to this effect.
Natural chemical controls are chemicals which have an important role in an insect's life cycle and
which can be used to control insect populations. Insect juvenile hormones and insect pheromones
have been used as natural chemical controls.
Pheromones are sex attractants, usually released by a female to lure a male of the same species for
mating.
Integrated pest management is a method of pest control which considers the overall ecosytem,
utilizing all necessary methods of pest management, but emphasizing careful monitoring of the
ecosystem and the use of natural controls.
24
25
Name _______________________________
Date _____________________
PROBLEMS TO SOLVE USING CONCEPTS, FACTS, AND DECODING SKILLS
FROM CHAPTER 14
1. True or false? Briefly, explain your answer in each case.
a. Rachel Carson advocated the boycotting of all pesticides.
b. Homeowners apply pesticides at lower levels than large commercial growers.
c. Integrated pest management never involves the use of pesticides.
d. Toxicity is the only concern about pesticides.
e. Calcium is essential for human nutrition.
f. Calcium is essential for plant nutrition.
g. Arsenic is essential for human nutrition.
h. Essential nutrients should be eaten in the greatest possible quantities.
i. Malathion is a neurotoxin.
j. Nicotine is a neurotoxin.
k. 2,4,5-T is a neurotoxin.
2. Write the equation for photosynthesis. Write the name of each chemical substance under its
chemical formula in the equation.
3. Write the equation for cell respiration. Write the name of each chemical substance under its
chemical formula in the equation.
26
4. Explain the relationship between the equation for photosynthesis and the equation for cell
respiration.
5. Write the chemical equation for the Haber reaction. Write the name of each chemical substance
under its chemical formula in the equation.
6. In your own words explain the historical significance of the Haber reaction.
7. In your own words explain the historical significance of the guano deposits off the South American
coast. Why did these islands lose their significance?
8. Explain how large-scale burning of fossil fuels can disrupt the carbon cycle. Use appropriate chemical
equations in your explanation.
9. Explain how destruction of large areas of rain forest can disrupt the carbon cycle. Use appropriate
chemical equations in your explanation.
27
10. What is FIFRA?
11. What is IPM? How does it work?
12. Describe the trend in crop losses due to insects from the 1940's to the present. What was the trend
in pesticide use during this time? Describe what you think are the important factors linking these two
trends.
13. How are juvenile hormones used to control mosquitoes? Would they be useful components in a
mosquito spray before an outdoor cookout?
14. How are pheromones used to control Japanese beetles? Have you ever observed them in use?
15. Are lawn grasses used on your home property? If so, can you think of improvements to the way
they are maintained?
28
16. DECODING CHEMISTRY
Match each chemical name with the proper descriptive term.
DDT
A. Found in chlorophyll
Glucose
B. Banned herbicide
Phosphorus
C. Natural toxin
Legume
D. Responsible for garlic odor
Magnesium
E. Used to make fertilizer, explosives
Sulfur
F. Plant with nitrogen-fixing bacteria
2,4,5-T
G. Organophosphate insecticide
Ammonia
H. Persistent pesticide
Nicotine
I. Primary plant nutrient
Paraquat
J. Contact herbicide
Malathion
K. Carbohydrate
17. In your opinion, what is the significance of Rachel Carson's book Silent Spring today?
18. DECODING CHEMISTRY
The label on the container of a water-soluble plant food lists prominently the numbers 23-19-17. What
do these numbers mean?
29
19. DECODING CHEMISTRY
In another portion of the label described in the problem above is found the statement "Primary
nutrients from urea, ammonium phosphate, and potassium nitrate." Using your knowledge of chemical
formulas gained from other chapters in this book, tell:
a. Which of these compounds are sources of nitrogen?
b. Which of these compounds are sources of phosphorus?
c. Which of these compounds are sources of potassium?
20. Look at the structure of DDT in the chapter. Can you explain why it is not soluble in water? Why
is this fact important in the buildup of DDT in the bodies of humans, birds, and anumals?
21. a. What is nitrogen fixation?
b. Name some natural sources of nitrogen fixation.
c. What is the chief means of nitrogen fixation today?
d. Name some important commercial uses for nitrogen compounds.