Why US Mosquito Populations Are Dropping and Why Experts Are Worried

Mosquito populations have declined by as much as 70% in Colorado and 50% across parts of the Midwest and Western U.S. this year, according to reporting from inkl. While these numbers suggest a reprieve from insect bites, public health experts warn that drought conditions are paradoxically increasing the transmission risk of the West Nile virus.

The decline in mosquito numbers is attributed to a historic drought driven by climate change, which has also contributed to a destructive wildfire season, according to inkl. In Minnesota and North Dakota, levels have dropped by 50%, while Colorado has seen plunges of up to 70%.

Despite the lower volume of insects, the risk of contracting mosquito-borne illnesses like zika, chikungunya, and dengue remains. Scientists are specifically focused on the West Nile virus, which has reached a 22-year high in U.S. case numbers, according to inkl.

Drought conditions and West Nile virus transmission

The relationship between fewer mosquitoes and higher infection rates is linked to how insects behave during water shortages. Matthew Aliota, a professor at the University of Minnesota who specializes in these kinds of viruses, told The Independent that drought conditions concentrate mosquitoes and birds—the primary hosts of the virus—around shrinking water sources.

This concentration increases the opportunity for the virus to spread between the birds and the mosquitoes, which then pass the virus to humans. According to inkl, 143 West Nile cases have been reported in the U.S. so far this year. Arizona has reported five deaths and accounts for approximately half of those cases, with high numbers also reported in Utah and Colorado.

The Centers for Disease Control and Prevention (CDC) states that severe cases of West Nile virus can result in brain and spinal inflammation, paralysis, tremors, coma, vision loss, and high fever.

Kathleen Walker, a professor and extension specialist at the University of Arizona, told The Independent that these severe symptoms are the tip of the iceberg in terms of the virus activity.

Mosquito population dynamics in the U.S.

While specific regions are seeing sharp declines, the overall mosquito population in the U.S. remains vast. Robert Hancock, a professor at Metropolitan State University Denver who specializes in mosquito behavior, said that there are likely still trillions of mosquitoes in the country.

Hancock noted the explosive nature of their reproduction, stating that one blood meal can give them upwards of 180 eggs.

The impact of environmental changes also varies by species, as more than 200 different types of mosquitoes exist in the U.S. For example, while drought has hindered populations in some states, inkl reports that mosquitoes have remained abundant around the Great Salt Lake in Utah, aided by artificial flooding despite a lack of snowpack.

History and spread of West Nile virus

The West Nile virus was introduced to the United States from the Middle East in 1999 via migrating white storks, according to inkl. The virus spread to all contiguous U.S. states by 2004.

Since its arrival, there have been some 32,000 cases in the U.S. The virus typically causes more than 130 deaths annually in the contiguous United States, according to inkl.