Is pesticide the answer to the Zika outbreak?

The chequered history of chemical quick fixes suggests that extreme caution should be taken

In Europe the story of Zika virus has been dominated by golfers and the Olympics. But where Zika is a more immediate health threat, in North and South America, the story is of how to contain the mosquito-borne disease.

Among the many suggestions is an increasingly loud call for the application of pesticides to kill the mosquitoes. These suggestions have been met with equally loud voices against pesticide use and even street protests against the application of the chemical Naled in Puerto Rico. Behind these concerns is an understanding of the chequered history of chemical quick fixes. Much of the wariness expressed by the public and by scientists about pesticide application can be attributed to one woman.

In 1962 the American science writer Rachel Carson published what would be her lasting legacy, Silent Spring. It would not be an overstatement to say that this book changed the course of history in ways that even the author would not have dared to hope. The book is a 368-page indictment of the indiscriminate use of chemical pesticides and herbicides. Drawing on the work of scientists, Carson built a convincing argument about the impact of the chemicals in wide use in the United States after the second World War, and her work was to influence the decision to ban DDT in the US.

Carson was one of the founders of the American environmental movement. She reminded people that they were not separate from the environment or free to control it without consequences for themselves. Silent Spring argues that we are woven into an environmental web that has no clear boundaries or time limit. Chemicals sprayed in a single place and time were spread well beyond that place and lasted within the tissues of people, plants, and animals for many years.


Carson is rarely referred to in the present debates over how to control Zika, but her ghost is hovering in the background. The widely expressed distrust of pesticides is at least partly a legacy of Silent Spring. Not everyone agreed with Carson's arguments, and she was even accused of sensationalism, but her scepticism about the views of government experts, agribusiness and industry was at least as influential as her ideas about DDT.

Carson's scepticism was influenced by her historical circumstances. She began her career in the shadow of the atomic bomb, which she viewed as a profoundly disturbing misuse of science and technology. She felt deeply that humans needed to seriously consider the more-than-immediate consequences of their actions on people and on the environment. In Silent Spring she expressed doubt that even the highest offices in the land could be unquestioningly trusted. This scepticism brings us back to Zika and to Puerto Rico.

Zika virus is currently spreading at an alarming rate in Puerto Rico. Yet the announcement by the Centres for Disease Control and Prevention of a plan to halt the spread of Zika by spraying mosquitoes with Naled was greeted by popular protests and by scepticism from scientific and medical officials. Puerto Ricans have their own historical reasons for mistrust of chemicals. The island was used as a test site for the herbicide Agent Orange, which gained notoriety during the Vietnam War.

Severe outbreak

Naled has previously been used in Puerto Rico to kill mosquitoes in an effort to control the transmission of dengue fever. Although this might be an effective short-term solution to a severe outbreak, Puerto Ricans probably have reason to be concerned about long-term effects.

Naled, banned for use in the European Union, is toxic to a wide range of insects and animals, including people. Marchers in Puerto Rico carried banners depicting frogs and puppets in the shape of bees.

Instead of seeing the killing of nonhuman organisms as a necessary side effect of protecting humans, they were seeing this as a warning that the insecticide plan had risks. And it does have risks: the question is whether they are worth taking or not. Carson’s work helped to make it commonplace to link risks to the environment with risks to people and to suggest – for better or worse – that government scientists are not always right.

Juliana Adelman lectures in history at St Patrick’s College, Drumcondra