In recent years, (basically post-World War II) chemical pesticides have become the most important consciously-applied form of pest management. This is a generalization of course; for some crops in some areas, alternative forms of pest control are still used heavily, such as the burning of the grass fields that we experience in late summer and fall in the Willamette Valley.
The "first generation" pesticides were largely highly toxic compounds, such as arsenic and hydrogen cyanide. Their use was largely abandoned because they were either too ineffective or too toxic.
The "second generation" pesticides largely included synthetic organic compounds. ('Synthetic' here means made by humans -- not naturally occurring, while 'organic' means carbon containing, not to be confused with the popular use of "organic" as in "organic farming".)
The first important synthetic organic pesticide was a chlorinated hydrocarboon (or organochlorine): dichlorodiphenyltrichloroethane or DDT. DDT was discovered in 1939 by a Swiss chemist Paul Muller. In its early days, it was hailed as a miracle for a number of reasons:
It was toxic to wide range of insect pests ("broad spectrum") yet appeared to have low toxicity to mammals.
It was persistent (didn't break down rapidly in the environment) so that it didn't have to be reapplied often.
It was not water soluble (insoluble), so didn't get washed off by rains.
It was inexpensive and easy to apply
It was so effective at killing pests and thus boosting crop yields and was so inexpensive to make that its use quickly spread over the globe. In 1948, Muller was awarded the Nobel Prize in Mecine or Physiology for its discovery. It was used for many non-agricultural applications as well. For example, it was used to delouse soldiers in WWII, and, until the 1960's to control mosquitoes in residential areas of the US. I can remember as a child, in a small town in Minnesota, the excitingly eerie sight and sound of the "mosquito truck" driving up and down the streets at night, producing a gentle mist behind it....We would often go out and play to be near it!
The magic of DDT seemed to spread. Yields increased on treated crops, diseases such as malaria were brought under control as never before...(incidentally, malaria is now is on the increase again as the vectoring insects develop resistance to chemical pesticides). It all seemed so wonderful -- people could cheaply and easily control so many pests!
Then, things began to temper the enthusiasm for pesticides. Notable among these was the publication of Rachel Carson's best selling book "Silent Spring," which was published in 1962. She (a scientist) issued grave warnings about pesticides, and predicted massive destruction of the planet's fragile ecosystems unless more was done to halt what she called the "rain of chemicals." In retrospect, this book really launched the environmental movement.
She was focusing on the chlorinated hydrocarbons, such as DDT, and pointed to evidence linking them to death of nontarget creatures (organisms other than those that the pesticide is intended to kill), such as birds. She argued that the death of nontargets occurred via two basic ways:
(1) Direct toxicity. It was discovered that DDT was toxic to fish (especially juveniles) and crabs, not only to insects.
(2) Indirect toxicity, related to its persistence. (It's persistence came in part from its insolubility, from the fact that it was a synthetic, recently introduced compound that microconsumers, such as bacteria, lacked enzymes capable of degrading -- basically they hadn't evolved to use it as an energy source, as well as from other features of its chemistry.)
She reported that insect and worm eating birds were dying in areas where pesticides had been aerially applied (hence her title, "Silent Spring."
The pesticide manufacturers said that the minute amounts found in the environment couldn't possibly be killing them. However, some experimental work demonstrated that even small amounts of some of the pesticides could affect the survival and reproduction of some species. More important, research demonstrated that, although concentrations were very low in the soil, atmosphere and water, concentrations were higher in plants, higher still in herbivores, and still higher as one moved up the food chain.
The indirect toxicity related to two principles :
(1) Bioconcentration – the tendency for a compound to accumulate in an organisms's tissues (especially in fatty tissues for fat soluble organochlorines such as DDT) and
(2) Biomagnification. – an increase in concentration up the food chain.
(These terms are sloppily used; sometimes "bioaccumulation" is also used to mean either of these, and people often use all of these terms interchangeably.)
Because DDT was (is) persistent, there was abundant opportunity for it to be taken up from the environment by organisms. For example, in the estuarine ecosystem next to Long Island Sound, the following concentrations of DDT were found:
In water = 3 ppt (0.000003 ppm)
In zooplankton = 0.04 ppm (bioconcentration and biomagnification from eating plants)
In minnows = 0.5 ppm (bioconcentration + biomagnification) (Because of the inefficiency of energy transfer, each minnow has to eat lots of zooplankton, and so acquires quite a burden from them.)
In large fish = 2.0 ppm
In ospreys (fish eating birds) = 25.0 ppm
Thus, concentrations had increased 10 million times up this progression, largely because of biomagnification (differential uptake and secretion may also be involved). These concentrations were not always directly lethal to the highest order carnivores, but did impair their reproduction. DDT (actually, its breakdown product DDE) reduced the deposition of calcium in eggshells. The birds thus produced thinner shell that cracked more readily during incubation.
The populations of many predatory populations (the highest order carnivores), such as bald eagles and brown pelicans were nearly eliminated. The peregrine falcon disappeared in the eastern US as a result of reproductive failures by the 1960's.
DDT (and DDE, a breakdown products from DDT) also appeared in the fatty tissues of seals and Eskimos, far from any area of use, indicating that, because of its persistence, it was being transported for long distances in the atmosphere and by marine mammals and fishes. It also showed up in human breast milk at remarkably high concentrations -- so high that the milk couldn't legally be sold through interstate commerce if it were cow's milk! DDE is one of the most widespread contaminants in human milk around the world.
When you think about it, human breast fed babies are way up there on the food chain, and are thus very susceptible to the effects of biomagnification and bioconcentration. For persistent compounds like DDT (or other persistent compounds, such as dioxins or PCB's -- see "POPs," below) human milk is the most contaminated of all human foods. Typically, concentrations of organochlorines (such as DDT) in human milk are 10 - 20 times higher than in cow's milk, and prevailing levels are often greater than those allowed in commercial food stuffs. While many mothers (including me) breast feed (or fed, depending on our ages!) partly because we feel that it is healthiest for our babies, we may be only partly right, at least in so far as concerns about persistent organic pollutants are concerned. Formula fed infants receive far less exposure, as they aren't eating as high on the food chain as are our breast fed babies. Further, because these compounds are so persistent, the effects of having been breast fed persist. A study of 3 1/2 year old Dutch children compared those that had been breast fed for at least 6 weeks with those who had not been breast fed, and found that the breast fed children had nearly 4 times more PCB's in their blood stream than the formula fed children...(See below, however, about the recently negotiated international treaty regulating these persistent compounds...there is hope!)
I do not want, however, to discourage prospective nursing mothers! There are many benefits associated with breast feeding -- nutritional, immunologic, developmental, psychological, social, economic, and environmental. These must be weighed against the possible risks of contamination, described above. See the following link if you want to read more about this complex issue: http://www.nrdc.org/breastmilk/formula.asp
A student in BI 301 in 2002 with interests in killer whales (orcas) pointed out that these same kinds of concerns about the accumulation of persistent organic pollutants (specifically, PCB's and organochlorines, such as DDT) apply to their situation as well -- see http://www.orcafree.org/danger_toxins.html if you want more information. Similaraly, an article from August 2005 in the journal, Science, indicated that concentrations of DDT and its residues are quite high in sperm whales (which are meat-eaters) across the Pacific Ocean.
(While researchers used to think that DDT was not toxic to mammals, it is increasingly believed that, at high concentrations ("acute" exposures) and for chronic, long-term exposures, it may be involved with various disorders including in the liver, kidnesy, immune system, nervous sytem, reproductive systems, and, potentially, some cancers; see http://pmep.cce.cornell.edu/profiles/extoxnet/carbaryl-dicrotophos/ddt-ext.html for a useful review.) Studies of possible relationships between DDT exposure and breast cancer have led to various results. One reported in 2007, however, provided fairly strong evidence that a link is there IF the woman was exposed to DDT when she was young (UC Berkeley study, reported in Corvallis G-T 9/30/07). The investigators obtained blood samples from women who were in their 20's during 1959 - 1967, when DDT was being used heavily in the US. The women in the top 1/3 in terms of blood concentrations were five times more likely to develop breast cancer later in life than were women with lower blood concentrations. The relationship was particularly strong for those women who were exposed before the age of 14. This is a very large increase in risk -- most other known risk factors increase chances of cancer development "only" by 50% to 100%. Further, the investigators found no relationship between cancer and blood concentrations in women in were born before 1931, who would have been older by the time DDT use began (~ 1945 and then banned in 1972), strengthening the indication that exposure when young is important.
These concerns, and the resulting public outcry prompted the US Environmental Protection Agency (EPA) to cancel the registration of DDT in the US in 1972. (All pesticides used in the US must be registered with the federal government, so this cancellation effectively pulled DDT off of the US market.) (Its use is still allowed in special cases, as in controlling vectors of human diseases.) Manufacture of DDT in the US did continue for export until the late 1970's. Currently there is no manufacturing of DDT in the US.
DDT is still, however, widely used in less developed countries. And, ironically, (but all too typically), when the last DDT manufacturing plant in the US was dismantled in 1983, it was sold to Indonesia, where it continued to manufacture DDT until Indonesia ratified the Stockholm Convention on Persistent Organic Pollutants in 2009; as I understand it, Indonesia is no longer manufacturing DDT.
Since the time of the US ban on DDT, populations of many of these predatory birds have recovered remarkably. (However, for information suggesting that many pesticides still used today in the US are toxic to birds, resulting in the deaths of thousands per year [perhaps even millions], even when used in the recommended way, see Audubon magazine from Jan/Feb. 1997, pages 28-35.)
By 2001, over 100 nations had signed an international treaty intended to phase out completely Persistent Organic Pollutants ("POP's"), including DDT. This is referred to as the Stockholm Conventionon Persistent Organic Pollutants. As of 2011, 176 nations were parties to this convention, which went into force in 2004. As of 2012, the US is not a party to this convention.
(Twenty five developing countries, however, got exceptions allowing them to keep using DDT, as it is the cheapest and most accessible defense againt malaria-transmitting mosquitoes. This whole malaria and DDT issue is really tough -- between 1 and 3 million people die each year from malaria -- a death toll comparable to that resulting from AIDS. In some cases, DDT is still effective against the disease-bearing mosquitoes, and many people feel that the benefits from continuing to use it for this purpose outweigh the risks. Resistance problems seem likely, ultimately, to force a change in control approaches in any case. There is considerable success with the use of bednets -- mosquito netting that is treated with an insecticide and suspended over a person's bed, protecting them during the evening and night when the mosquitoes are most active -- coupled with more effective dissemination of anti-malaria drugs, and measures such as filling chinks in walls that fill with water and provide mosquito breeding grounds.)
These POP's are generally fat-soluble (as is DDT), and are also very dispersable (occur in forms that are able to travel over great distances). The list of POP's covered by the treaty includes nine pesticides (Aldrin, Chlordane, DDT, Dieldrin, Endrin, Heptachlor, Hexachlorobenzene, Mirex and Toxaphene) as well as PCB's, furans, and dioxins. Dioxins are a by-product of some types of paper processing, and are produced in other processes as well. You may remember hearing about "agent orange" -- a defoliating agent used during the Viet Nam war? It has been implicated in causing a diversity of health problems, and dioxins are causal agents for several of these problems. Dioxins aren't part of the active ingredients of agent orange (which are 2,4,5-T and 2,4-D), but are byproducts of their production. The EPA wrote, in 1985, that dioxins are "the most potent carcinogen ever tested in laboratory animals." Many more POP's are likely to be added to the list of globally-banned compounds, but ecological/epidemiological studies linking them to human or ecosystem injury have not been completed.
See http://www.thegreenguide.com for reports on hormone mimics (hormone altering chemicals) in products of all types. Reports include phthalate plasticizers, PCB's (polychlorinated biphenyls), bisphenol A (BPA; found in polycarbonate plastics [recycling code #7], including "hard" plastic water bottles and other products) dioxins, etc.
The atmosphere is not the only means by which POP's get moved around: Arctic seabirds and migrotory fish species, such as salmon, transport these compounds from the ocean to the land. For example, many pelagic seabirds have nesting colonies near inland lakes in the Arctic (Science July 15, 2005).
Another interesting accomplishment of this treaty was acceptence of the " precautionary principle ," which reverses the traditional regulatory burden of proof. Instead of assuming that a compound is harless unless it can be shown to be dangerous, manufacturers must avoid production of likely POP's even if they are not actually proved to be POP's -- that is, they are assumed to be "guilty" of causing harm until they are proved to be otherwise. Another way of stating the precautionary principle is to say that, if a technology or activity may pose a threat or harm, measures to anticipate and prevent that harm are warranted. This concept is probably also familiar to you as, "Better safe than sorry" "Look before you leap!" "An ounce of prevention is worth a pound of cure" and other similar adages.
The chemical industry responded to the concern over DDT and its relatives with new classes of pesticides, which are less persistent than DDT and the other organochlorines, but which are generally more water soluble (with consequent potential for contaminating surface and ground water) and are often also more acutely toxic.
Their acute toxicity is demonstrated in the minute amounts of them that are effective:
In 1945, DDT was typically applied at a rate of 2 kg/ha
Today, similar levels of control are obtained with aldicarb and pyrethroids at 0.1 and 0.05 kg/ha (50 g/ha!) respectively. That is, they are over 10 times more effective (or toxic), and as much as 40 times more so!
A solution to one problem can create new problems – we will see this over and over!
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Page maintained by Patricia Muir at Oregon State University. Last updated Oct. 22, 2012.