A surprising new study finds that mosquito populations have exploded in parts of the U.S. but not because of a warming climate. Instead, the study finds, growing cities and the ban on the insecticide DDT may be responsible.
The trend doesn't bode well for the spread of new diseases — not just Zika virus, but West Nile virus, dengue virus, chikungunya and others, the team at the University of California, Santa Cruz found.
They tracked mosquito populations in New York, New Jersey and California.
"Mosquito populations have increased as much as tenfold, and mosquito communities have become two- to fourfold richer over the last five decades," A. Marm Kilpatrick and colleagues wrote in their report, published in Nature Communications.
"These increases are correlated with the decay in residual environmental DDT concentrations and growing human populations, but not with temperature."
Related: A list of mosquito-borne killers
DDT was first used in the U.S. in 1945 to fight mosquitoes that carried diseases such as malaria. It was most heavily used in the 1950s and '60s, but use was phased out when it became clear the chemical was not just killing insects, but affecting birds and other wildlife.
Most notoriously, it thinned eggshells and caused precipitous declines in populations of bald eagles, ospreys and songbirds.
DDT for agricultural use was banned in the U.S. in 1972.
Related: DDT Raises Breast Cancer Rates
It's clear why cleaning up pesticides might allow mosquitoes to thrive, but less clear why urbanization would.
The connection is that some of the most dangerous species of mosquitoes, such as the Aedes aegyptis species that carries Zika, dengue, yellow fever and chikungunya, prefer living in and around buildings.
"Urbanization results in increased impermeable surfaces (for example, pavement) associated with buildings and roads, and decreases in forest cover, wetlands and other natural habitats," Kilpatrick's team wrote.
This changes where mosquitoes lay their eggs and what animals and plants they feed on. They may displace species that depend on natural habitats in preference to the urban pest mosquitoes like Aedes species, the team said.
"The increase in species richness with urbanization likely reflects expansion of habitat for mosquito species associated with suburban environments and man-made wetlands," the team wrote.
In the past 10 years, epidemics of dengue and chikungunya have swept through Latin America and the Caribbean. Dengue's also been seen in Hawaii. Now Zika's doing the same thing, and has also hit the U.S. states of Florida and Texas and the territories of Puerto Rico and the U.S. Virgin Islands.
Zika virus is causing small epidemics of terrible birth defects wherever it turns up. It's also causing severe cases of the paralyzing Guillain-Barré syndrome, and it has turned out to be transmitted by sex. Zika stays in semen for months and may cause invisible brain damage in babies and perhaps even adults.
West Nile virus is carried by different species of mosquitoes and has spread across all 48 contiguous states since it showed up in the U.S. for the first time 16 years ago. It has infected hundreds of thousands of people, causing severe illness in about 40,000 and killing more than 1,600.