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When will the risk for Zika-carrying mosquitoes subside in the US?

While mosquitoes are typically seen as a summertime pests in much of the country, continued warmth could mean that the insects hang around longer than usual into the fall season.

According to Dr. Jim Fredericks, chief entomologist for the National Pest Management Association, as long it remains warm and humid, mosquitoes will remain a concern.

"In general, warm, wet weather equals more mosquitoes," Fredericks said.

A Miami-Dade County mosquito control worker sprays around a home in the Wynwood area of Miami on Monday, Aug. 1, 2016.(AP Photo/Alan Diaz)

A recent report on summer 2016 weather from the National Oceanic and Atmospheric Administration (NOAA) said that every state in the contiguous U.S. saw temperatures above average this summer and 29 states were much warmer than average. The summer U.S. temperature was 73.5 F, 2.1 degrees above average and the fifth highest on record.

The World Health Organization (WHO) said on Sept. 2 that the mosquito-borne Zika virus remains a Public Health Emergency of International Concern. Travel advisories for pregnant women remain in effect for countries around the world where Zika has been reported, including two locations in Miami-Dade County, Florida.

Both mosquitoes that are known to carry the Zika virus, Aedes aegypti and the Aedes albopictus (also known as the Asian tiger mosquito), are subtropical. Maps from the Centers for Disease Control and Prevention (CDC) show the estimated range for the Aedes aegypti is distributed more across the South and up the Atlantic coast. The Asian tiger mosquito is said to have a higher cold tolerance and can be found as far north as the Upper Midwest and New England.

Standing water associated with flooding is another important factor in keeping mosquitoes around because it becomes breeding territory. When a storm system delivered over 30 inches of rain to parts of Louisiana in mid-August, conditions were conducive for mosquitoes to thrive. A large number will likely remain as the weather stays warm, Fredericks said.

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Temperatures between 85-95 are ideal for mosquitoes, according to Fredericks. When conditions get too cold, that's typically when the number of mosquitos will begin to dwindle as they lose the ability to fly and their body processes are slowed.

"[Mosquitoes] will continue to be active until temperatures are consistently falling below 50 degrees Fahrenheit," Fredericks said.

AccuWeather Expert Long-Range Forecaster Paul Pastelok said daytime highs will not be below 50 until mid-November and beyond in the East. Nighttime lows, however, may be more consistently below 50 starting later in October, he added.

In the South, nighttime lows might not fall consistently below 50 until mid-to- late-November, while it might be early October for the northern Plains and mid-October for the central Plains, Pastelok said.

AccuWeather Long-Range Meteorologist Ed Vallee added that the extreme heat in the East has likely subsided.

In South Florida, where the first cases of local mosquito-borne Zika transmission occurred in the U.S., conditions remain mild during the winter meaning some mosquitoes could survive, according to Fredericks. Even if the population significantly decreases, the disease can still be transmitted sexually if a person is infected.

To date there are more than 800 Zika infections in Florida, with 71 of them non-travel related, according to the Florida Department of Health. There are nearly 3,000 total cases across the U.S. with 43 of them directly resulting from mosquitoes, CDC statistics show.

Mosquito season has been increasing across the lower 48 states, according to a report published in July by Climate Central. The study found that rising temperatures and humidity since the 1980s have caused an increase in the annual number of days that provide ideal weather for mosquitoes, which is also increasing the risk of disease being spread.

Cities in the Midwest and East Coast such as Baltimore (37 days), Durham, North Carolina (37 days), and Minneapolis (34 days) saw the biggest increase. The report also states that from 2006-2015, Miami was the top city with an average of 337 days a year of ideal mosquito weather.


Have questions, comments, or a story to share? Email Kevin Byrne at Kevin.Byrne@accuweather.com, follow him on Twitter at @Accu_Kevin. Follow us @breakingweather, or on Facebook