Chlorinated Hydrocarbons
DDT was the first of a long line of insecticides based on hydrocarbons with chlorine atoms replacing some of the hydrogen atoms. Its chemical name is dichloro, diphenyl, trichloroethane (see figure). Some others:
- methoxychlor
- dieldrin (see figure)
- dicofol (Kelthane®)
- endosulfan (Thiodan®, Phaser®)
DDT was introduced during World War II and, along with penicillin and the sulfa drugs, was responsible for the fact that this was the first war in history where trauma killed more people - combatants and noncombatants alike - than infectious disease. DDT is effective against
- vectors of human diseases such as
- malaria and yellow fever (both transmitted by mosquitoes)
- plague (transmitted by fleas)
- many crop pests
Prior to the introduction of DDT, the number of cases of malaria in Ceylon (now Sri Lanka) was more than a million a year. By 1963 the disease had been practically eliminated from the island. However, growing concern about the hazards of DDT led to its abandonment there in the mid-1960s, and soon thereafter malaria became common once again.
DDT was especially effective against malarial mosquitoes because of its persistence - its resistance to breakdown in the environment. One or two sprays a year on the walls of homes kept them free of mosquitoes. But DDT has several serious drawbacks.
Insecticide resistance
As early as 1946, Swedish workers discovered populations of houseflies resistant to DDT. This was quickly followed by many other reports of developing resistance. Other chlorinated hydrocarbons (like dieldrin and methoxychlor) were developed as substitutes, but in time insects developed resistance to these as well.Persistence
DDT is- stable and
- fat soluble.
These properties cause it to accumulate in fat tissue. People who were heavily exposed to DDT (during its manufacture or application) often showed concentrations of DDT in their fat 1000 times higher than that in their blood.
Even these high levels were probably of little harm to the workers. In the early stages of exposure, the blood levels of DDT (and its metabolite DDE) rise rapidly at first and then reach a steady level. From that point on, the body excretes it as fast as it acquires it.
Biomagnification
Although no harmful effects from average exposures to DDT have been seen in humans, DDT and other chlorinated hydrocarbons have been shown to harm other species, such as fishes, earthworms, and robins. The hazard of DDT to nontarget animals is particularly acute for those species living at the top of food chains.
| Link to discussion of how DDT becomes concentrated as it moves up a food chain. |
| Correlation between DDE concentrations in the eggs of Alaskan falcons and hawks and reduction in the thickness of their eggshells (compared with shells collected prior to 1947). DDE is a metabolite of DDT. Data from T. J. Cade, et. al., Science 172:955, 1971. | |||
| Species | Location | Average Concentration of DDE in Eggs (ppm) |
Reduction in Shell Thickness |
|---|---|---|---|
| Peregrine falcon | Alaskan tundra (north slope) | 889 | -21.7% |
| Peregrine falcon | Central Alaska | 673 | -16.8% |
| Peregrine falcon | Aleutian Islands | 167 | -7.5% |
| Rough-legged hawk | Alaskan tundra (north slope) | 22.5 | -3.3% |
| Gyrfalcon | Seward Peninsular, Alaska | 3.88 | 0 |
Another group of nontarget victims of DDT (and other pesticides) are insects that prey upon insect pests; that is, the natural enemies of the pests. Killing these has serious ecological - and economic! - effects.
Once apple growers began controlling pests with DDT, they quickly found their orchards being attacked by scale insects and mites. The reason: DDT had killed off their natural enemies. 
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/I/Insecticides.html