During the 1970s, after DDT had been banned, the insecticide that replaced it in many agricultural applications, such as the growing of cotton and soybeans, was toxaphene. It is a mixture of hundreds of similar substances, all of which are produced when the hydrocarbon camphene, produced from chemicals extracted from pine trees, is partially chlorinated.

Toxaphene became the most heavily used insecticide (1966-1976) in the United States before restrictions were placed on its use in 1982 and a total ban imposed in 1990. More than 85% of toxaphene use in the United States occurred in the southeastern cotton-growing states, though some was used as a herbicide in the growing of peanuts and soybeans.

Toxaphene is extremely toxic to fish. Indeed, it was first used in the 1950s in North America to rid lakes of undesirable fish. However, it was found to be so persistent that the lakes could not be successfully restocked for years thereafter!

Toxaphene bioaccumulates in fatty tissues and causes cancer in test rodents; consequently it is one of the dirty dozen persistent organic pollutants (Table 10-2) and is now banned in developed countries as well as some developing ones. It still is being deposited in bodies of water remote from its point of usage because of long-range transport by air (see Chapter 12) from countries that still make restricted use of it.

Hexachlorinated Cyclohexane

During World War II, the derivative of cyclohexane having one of the two hydrogens on each carbon replaced by chlorine, namely, 1,2,3,4,5,6-hexachlorocyclohexane, was discovered to be an effective insecticide against a wide variety of insects. In fact, there exist eight isomers having this formula;

camphene camphene they differ only in the relative orientations of the chlorine atoms bonded to different carbons. (The diagrammatic formula below is not intended to illustrate the chlorines' orientations—only their points of attachment.)

(This compound is also known as benzene hexachloride or BHC, not to be confused with hexachlorobenzene)

A commercial mixture of most of the hexachlorocyclohexane isomers was used to control mosquitoes and in agricultural applications after World War II. Its use has been restricted since the 1970s due to its toxicity and tendency to bioaccumulate. Only one of the eight isomers, the gamma isomer, actually kills insects; now sold separately under the name Lindane, it was the active ingredient in several commercial medical preparations used to rid children of lice and scabies and agriculturally to treat seeds and seedlings.

Chlorinated Cyclopentadiene Insecticides

Cyclopentadiene, shown at left below, is an abundant by-product of petroleum refining. As its name implies, there are two double bonds in each molecule. When fully chlorinated (diagram at right), it can be combined with one of several other organic molecules (Diels-Alder reaction) to produce a series of insecticidal compounds with properties, including environmental persistence, that at first made them superficially attractive.

All the cyclodiene pesticides contain the hexachlorinated five-membered ring bonded to at least two other carbon atoms and containing a residue of one double bond.

Most cyclodiene insecticides that were commercially important have now been branded as persistent organic pollutants by the United Nations Environmental Programme and are listed in Table 10-2. They were used to control soil insects, fire ants, cockroaches, termites, grasshoppers, locusts, and

1, 2, 3, 4, 5, 6-hexachlorocyclohexane

other insect pests. In many such applications, their persistence was an advantage since they did not have to be reapplied frequently.

The cyclodiene pesticides, starting with aldrin and dieldrin—which are structurally identical except that the latter corresponds to the former with one of its C=C bonds epoxidized (see Figure 12-2)—arrived on the market in about 1950. Given their persistence, their potential toxicity, their tendency to accumulate in fatty tissues, and the suspicion that dieldrin was causing excess mortality of adult bald eagles, the use of almost all of these compounds has now either been banned or severely restricted in North America and most western European countries. Nonetheless, dieldrin and DDT were the most common POPs still detectable in food in 2002. Some of the compounds are still in use elsewhere (see Table 10-2).

Agricultural uses of dieldrin, mainly to combat soil insects, and its use in buildings to control termites, were largely prohibited in North America by the mid-1980s but continue in many developing countries. Dieldrin was used extensively in tropical countries to control the tsetse fly and is still used in some countries to kill termites. It continues to enter water systems even in developed countries by percolating from waste disposal sites. Danish studies have found a correlation between blood dieldrin levels in women and their increasing risk of developing, and dying from, breast cancer. Dieldrin and another cyclodiene insecticide containing a three-membered epoxide ring, heptachlor epoxide, have been associated with increased risk of non-Hodgkin's lymphoma.

If two perchlorocyclopentadiene molecules are chemically combined, the resultant molecule, known commercially as as mirex, also acts as an insecticide and is particularly effective against the fire ant found in the southeastern United States.

(All 10 carbon atoms are bonded to chlorine, but for clarity the individual chlorine atoms are not shown; only the total is displayed in the formula.) Mirex was also sold as a flame retardant additive for synthetic and natural materials. Most mirex use occurred in the 1960s, although it is still produced and used in China and is used to fight giant termites in parts of Australia. Mirex is classified as a persistent organic pollutant by the U.N. and has been banned in many countries since the mid-1970s.

For the most part, the chlorinated cyclodiene pesticides are chemical products of the past. Their use has been phased out or at least severely restricted because of environmental and human health considerations. The only cyclodiene insecticide still in widespread use is endosulfan, which is discussed in detail in Box 10-1.

mirex mirex

BOX 10-1

The Controversial Insecticide Endosulfan

Endosulfan does not appear on the U.N. list of POPs; indeed, it is one of the few cyelopentadiene pesticides still widely available on the market. Structurally, endosulfan molecules consist of a perchlorinated five-membered ring attached by methylene groups to an 0=S(0—)2 unit.



It is used as an insecticide and an acaricide in agricultural applications, its domestic applications having been phased out—at least in the United States. Both its environmental persistence and its tendency to bioconcentrate are much lower than those of the other cyclodi-enes because it is much more reactive, owing to the presence of the sulfur-oxygen group. In the environment, some of it is converted to the sulfate, which has an additional oxygen doubly bonded to the sulfur, thereby producing S(VI) from S(IV) in the original pesticide. Unfortunately, endosulfan sulfate is just as toxic as endosulfan and is more persistent. The remaining fraction of endosulfan in the environment is converted to the sulfur-free diol in which the —CH2— groups are each bonded to —OH, and which itself undergoes further degradation reactions.

Although endosulfan is classified as mod' erately toxic by the WHO, it is considered to be highly hazardous by the U.S. EPA. It is readily absorbed by the stomach and lungs and through the skin. Although acutely toxic, endosulfan does not persist in mammals because it is degraded into water-soluble compounds and eliminated from the body within a few days or weeks. Because of its short lifetime, the risk from consuming it in drinking water or in food is usually not a concern. Consequently, most of the health concerns about endosulfan relate to its ability to act as an acute poison to workers who handle and apply it in agricultural settings. It is also of environmental concern due to its toxicity to nontarget fish, birds, and other animals. Indeed, there have been massive fish kills in the United States (Alabama) and in other countries in waters that were inadvertently contaminated with the insecticide.

Owing to its toxicity, endosulfan has been banned in many countries and its use restricted in others. The bans followed the accidental deaths of farmworkers and nearby residents as well as its use in committing suicide. For example, it was introduced into Sri Lanka to replace category I insecticides that previously had been used in suicides, but it soon became employed for the same purpose. Since it was banned there in 1998, other category II insecticides that replaced it have been employed instead by would-be suicides.

In addition to its use on some food and grain crops as well as on tea and coffee plantations, endosulfan is also used in the growing of cotton in both developed countries, such as the United States and Australia, and developing countries. For example, endosulfan was introduced late in the twentieth century into French-speaking countries in West Africa when boll worm caterpillars (boll weevils)—which

(continued on p. 434)

BOX 10-1

The Controversial Insecticide Endosulfan (continued)

enter cotton flower buds and bolls and destroy them—became resistant to the lower-toxicity insecticides (pyrethroids) that previously had been employed. Unfortunately, most farmers in certain countries in this region, including Benin, cannot afford the protective clothing— goggles, gloves, and respirators—that should be used when handling and spraying endosulfan. In addition, the cotton farmers sometimes use leftover endosulfan inappropriately as an insecticide on their vegetable crops. Since endosulfan is almost insoluble in water, washing food sprayed with it is largely ineffective.

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