The overriding theme of Silent Spring is the powerful—and often negative—effect humans have on the natural world. Carson's main argument is that pesticides have detrimental effects on the environment; she says these are more properly termed "biocides" because their effects are rarely limited to the target pests. DDT is a prime example, but other synthetic pesticides—many of which are subject to bioaccumulation—are scrutinized. Carson accuses the chemical industry of intentionally spreading disinformation and public officials of accepting industry claims uncritically. Most of the book is devoted to pesticides' effects on natural ecosystems, but four chapters detail cases of human pesticide poisoning, cancer, and other illnesses attributed to pesticides. About DDT and cancer, Carson says only:
In laboratory tests on animal subjects, DDT has produced suspicious liver tumors. Scientists of the Food and Drug Administration who reported the discovery of these tumors were uncertain how to classify them, but felt there was some "justification for considering them low grade hepatic cell carcinomas." Dr. Hueper [author of Occupational Tumors and Allied Diseases] now gives DDT the definite rating of a "chemical carcinogen."
Carson predicts increased consequences in the future, especially since targeted pests may develop resistance to pesticides and weakened ecosystems fall prey to unanticipated invasive species. The book closes with a call for a biotic approach to pest control as an alternative to chemical pesticides.
Carson never called for an outright ban on DDT. She said in Silent Spring that even if DDT and other insecticides had no environmental side effects, their indiscriminate overuse was counterproductive because it would create insect resistance to pesticides, making them useless in eliminating the target insect populations:
No responsible person contends that insect-borne disease should be ignored. The question that has now urgently presented itself is whether it is either wise or responsible to attack the problem by methods that are rapidly making it worse. The world has heard much of the triumphant war against disease through the control of insect vectors of infection, but it has heard little of the other side of the story—the defeats, the short-lived triumphs that now strongly support the alarming view that the insect enemy has been made actually stronger by our efforts. Even worse, we may have destroyed our very means of fighting.
Carson also said that "Malaria programmes are threatened by resistance among mosquitoes", and quoted the advice given by the director of Holland's Plant Protection Service: "Practical advice should be 'Spray as little as you possibly can' rather than 'Spray to the limit of your capacity' ... Pressure on the pest population should always be as slight as possible."