Is It for the U.S. to Revisit Its Ban on DDT?

Populations of mosquitoes thriving without the insecticide could spread diseases like Zika and dengue – and global warming hasn't even kicked in yet.

An aedes aegypti mosquito, which transmits Zika, in Costa Rica, Feb. 2016.
Juan Carlos Ulate/Reuters

The ban by many countries in the 1970s and '80s on the use of the insecticide DDT may have saved many species of wild animals from extinction, but it also has led to an explosion in mosquito populations. Among them are species that spread such diseases as Zika, Dengue fever and West Nile Fever.

Research published last week in the prestigious scientific journal Nature Communications demonstrates how the mosquito population has grown as much as 10-fold in some places, in tandem with “the decay in residual environmental DDT (dichlorodiphenyltrichloroethane) concentrations and growing human populations.”  

The paper, entitled “Anthropogenic impacts on mosquito populations in North America over the past century,” focused on the East and West coasts, and tried to determine the cause of the uptick in their numbers. The scientists relied on data from mosquitoes that have been systematically trapped, over a period of some 80 years in California, and 60 years in New York and New Jersey. Researchers also studied DDT residue in the ground and in humans in the same regions.

What the authors of the study did not find was a correlation between rising global temperatures and the spread of mosquitoes and the diseases they carry. However, they revealed that growing urbanization is a definite factor in the growth of the mosquito populations: Urbanization favors those species that live near/feed on people and spread disease – as opposed to the species that typically proliferate in other, natural habitats such as wetlands.

DDT was banned in the United States in 1972 after it was found to have caused serious damage to various species of animals and birds, including the bald eagle, the national bird and symbol of the U.S. Mosquito populations have grown continuously ever since.

“At first glance, recent increases in mosquito populations appear to be linked to rising temperatures from climate change, but careful analyses of data over the past century show that it’s actually recovery from the effects of DDT,” according to study co-author Marm Kilpatrick, associate professor of ecology and evolutionary biology at the University of California-Santa Cruz.

Kilpatrick says, however, that the effects of climate change are expected to be seen in changes in the geographical areas in which the species’ thrive: that is, those typically adapted to warm climates will move further north and those preferring cold will retreat from the southern parts of their ranges. So, the scientists argue, a tropical species like Aedes aegypti, which transmits Zika, dengue and other diseases, could expand its range northward in the U.S. as temperatures warm.

The return of mosquito populations in the New York area took much longer than elsewhere, because of the especially high residual concentrations of DDT, a result of extremely high usage of the insecticide.

Another interesting side effect of the exploding insect populations, scientists note, has been the growth in populations of insectivore bats, apparently due to the greater availability of their sources of food.