The search for an effective topical insecticide led to the development of DDT by Paul Muller. It was such as success that Muller was awarded the 1948 Nobel Prize in Physiology or Medicine.
DDT saved millions of lives and saved many millions more from the suffering that accompanies diseases such as malaria and dengue fever.
But even Muller recognized that scientists were playing a game where they didn't know all the rules. For that matter, we still don't but we have a much better understanding of the rules now.
The United States annual production of DDT increased from 10 million pounds in 1944 to a peak of 188 million pounds in 1962-1963. And as Muller had hoped, the success of DDT promoted research into finding other comparable synthetic insecticides based on chlorinated hydrocarbons, resulting in chlordane, toxaphene, and aldrin.
Popular substitutes for DDT included the organophosphates, such as parathion, and the carbamates. Unfortunately, the latter compounds are also high toxic to humans and other mammals. Unlike DDT, parathion and aldicarb have been responsible for the poisoning of numerous farm workers.
It is for this reason that DDT remained the insecticide of choice. It was saving human lives with no apparent significant impact on mammals. Not at first.
However, by 1962, it was readily apparent that DDT was playing havoc with wildlife populations, particularly with birds particularly those at the top of the food pyramid. This was the Mr. Hyde side of its existence - the potential to do harm while saving lives.
As early as 1948, there were also several insects such as bees, cotton parasites, and grasshoppers that had develop resistance to the compound. It is an example of evolution in action where, due to natural genetic variations, some insects survive exposure to an insecticide. These insects form the breeding population for the next generation and the genes that allow for survival get passed along or amplified in the population.
By the early 1950s, DDT used against flies in cow barns was showing up in milk. Fish-eating birds of prey were producing fewer or no young. DDT was showing up in the tissue of dead birds that nested in Dutch Elm trees and the nestling survival rates had plunged to only 44 percent.
Even some of DDT's beneficial effects turned out to be counterproductive. It actually encouraged some insects, such as citrus-fruit pest, by killing both the insects and their natural predators. Pest populations recover much faster than predator populations.
In killing all of the insects birds and other small creatures were left without a food supply and either had to relocate or starve. In many cases, insectivorous bird populations plummeted not from the toxicity of DDT but the absence of their favorite meal.
In the public's eye, Rachel Carson's 1962 book Silent Spring was a wake-up call to the hazards of utilizing insecticides. Its premise was that one day soon, the hedgerows of New England would be quieted as all of the song birds had died or vanished. This echoed a much earlier concern.
In 1945, in a New Republic article, J.K. Terres wrote: "The sun arose on a forest of great silence - the silence of total death. Not a bird call broke the ominous quiet." Around the same time, the Food and Drug Administration recognized that DDT accumulates in the fatty tissue of animals and as a consequence, it was "extremely likely that the potential hazard of DDT has been underestimated." Even the New York Times published articles warning of the potential hazards.
In the end, it took more than a decade for scientists to prove the harmful effects of the insecticide. Why so long? There are many reasons, not the least of which is that the instruments that allow scientists to detect trace amounts of an insecticide in food or fat had not been invented.
However, it was mostly because DDT did not manifest as an acute toxic substance in humans. Scientists were accustomed to working with insecticides such as lead arsenates that were immediate central nervous system toxins. They did not recognize the low-level hormonal imbalances that arose with DDT.
There was also little funding for such work. In the mid-1960s, government had no authority to regulate chemicals and it wasn't in the best interests of industry to discover the truth. Further, many scientists in the U.S. Public Health Service and the Department of Agriculture thought DDT was safe if used properly.
Carson's book spurred an impassioned public debate - one that still rages. Every year, malaria infects half a billion people and kills up to 2.7 million, most young children and pregnant women in Africa. DDT is sprayed in houses to keep the mosquitoes at bay because it works.
But the warning of a silent spring, when the birds did not return, was enough to convince North America and Europe to ban the substance from general commercial use.
