The rapidly growing mosquito population may be less because of climate change and more because of urbanization and the slow decay of DDT (dichlorodiphenyltrichloroethane).

Marm Kilpatrick, an associate professor of ecology and evolutionary biology at the University of California Santa Cruz and corresponding author of a new study on mosquito population growth, explained what the growth is due to.

“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,” Kilpatrick said in a statement.

According to long-term datasets from mosquito monitoring programs, mosquito populations have increased as much as ten-fold and the number of mosquito species has increased two-to-four fold over the past five decades in New York, New Jersey and California.

While the potential effects of climate change on the spread of insect-borne diseases is a public health concern, the study found little evidence that the mosquito populations in these areas were responding to changes in temperature or precipitation.

“Precipitation was more important than temperature, but land use was more important than either of those factors,” Kilpatrick said. “The long-term impacts of land use changes on ecosystems are sometimes underappreciated.”

Urbanization is said to lead to increased populations because it changes the species composition in an area, favoring the types of mosquitoes that live near and feed on people and causes other species to decline, including those adapted to wetlands and other natural habitats.

Another factor in mosquito growth is the gradual waning of DDT.

While mosquito control programs continue to help limit mosquito populations, they are not as effective as DDT—an organochlorine chemical used widely as an insecticide.

“Everyone knew DDT was an extremely effective insecticide, but I was surprised by how long-lasting its effects were,” Kilpatrick said. “In some areas, it took 30 to 40 years for mosquito populations to recover.”

Between the 1940’s and 1970’s more than a billion pounds of DDT was used in the U.S. However, it was curtailed in the 1960’s and eventually banned in 1972 because of adverse environmental effects, particularly on birds and other wildlife, as well as potential human health risks.

In New York, New Jersey and California both mosquito abundance and the number of species decreased dramatically during the period where DDT was used, then increased as the amount of DDT in the environment declined.

As recently as 2000 in New York DDT was still detectable in soil cores and researchers found that patterns of DDT use and its concentration in the environment could explain most of the long-term trends in mosquito populations in the Empire State.

The analyses of New Jersey and California showed that urbanization was also an important factor in explaining the long-term population trends of mosquitoes.

According to Kilpatrick, the effects of climate change is expected to be seen at the edges of species’ geographic ranges, as species adapted to warm climates move further north and cold-adapted species retreat from the southern part of their ranges.

This means tropical species like Aedes aegypti would expand their range northward in the United States as temperatures warm and transmit diseases like Zika and dengue into a new segment of the population.

“On the cold edge of a species’ distribution, temperature matters a lot,” Kilpatrick said. “In Washington, D.C., for example, where Aedes aegypti is not common now, it might become more common if the winters get milder.

“Whereas in Florida, urbanization and mosquito control efforts are more likely to be the dominant drivers of mosquito populations,” he added.

The coauthors of the paper include Ilia Rochlin and Dominick Ninivaggi at Suffolk County Vector Control in New York; Ary Faraji at the Salt Lake City Mosquito Abatement District and Christopher Baker at UC Davis.  

The study, which appeared in Nature Communications, can be viewed here.