The Effectiveness of DDT as Insecticide

DDT is an acronym for dichlorodiphenyl trichloroethane, but should be named more precisely as 2,2-bis(p-chlorophenyl)-l,l,l-trichloroethane (see Fig. 16.1 for its structure). A German chemist, Othmar Zeidler synthesized this compound in 1874 while he was pursuing his PhD. And it had remained as that, a new synthesized compound, for a quite while. Carl Muller of J. R. Geigy (a Swiss Pharmaceutical company, now Chiba-Geigy) discovered in his pursuit of insecticides that the compound synthesized by O. Zeidler was extremely toxic to houseflies. Numerous tests were conducted, and the compound DDT was found to be an excellent insecticide. Besides it is cheap to make. This was the time when the World War II was raging. DDT was then used to control lice on soldiers on the front. In earlier wars, more soldiers died of typhus (bom by louse) than by bullets. The WWII was really the first war in history where more soldiers died actually from bullets than the louse-bom disease, thanks to DDT. DDT was then considered to be a savior to control many kinds of harmful pests. C. Muller was awarded a Nobel prize in 1948. 

A large amount of DDT was sprayed on the field and everywhere. In 1960s more than 3,000 tons of DDT was sprayed in Asia alone to control malaria (borne by mosquito). Its effectiveness is illustrated by the following example. There were 2.5 million cases of malaria in Sri Lanka inl948 when DDT was not used. The number of cases of malaria was dramatically reduced to only 31 cases in 1962 after DDT had been extensively sprayed on homes since 1948. The number of cases returned to around two million within 5 years after the spraying of DDT on homes had been banned in 1964. 

It was estimated that about 1.5 lb (0.7 kg) of DDT had been applied for each acre of the world s arable land by 1964. Let us look at an example of the effects of DDT on a crop. It is a set of data on a cotton field in a Pemvian valley (Cannette Valley). The average yield of cotton there was 406 Ib/acre in 1943 (before DDT was 

DDT kills insects such as flies and worms. How does it do that Then would it not be harmful to also humans and other animals DDT is a stable, relatively non-polar organic compound. So it has similar characters to fats and oils. As such, it dissolves well in fat and oil. Organisms of any kind are made of cells, and the wrapping (membrane) of the cells are essentially fat chemically phospholipids (Fig. 1.5). DDT therefore sticks well to the membrane and goes through it readily and eventually is stored in fat tissues. 

When applied to small organisms such as flies and worms, DDT goes to fatty tissues, particularly those in neuronal tissues. It kills an insect by disrupting the neuronal system, often through convulsion and suffocation. This is an acute effect. 

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