Climate Trivia: Mosquitoes, West Nile Virus and the Weather

Mosquitoes have existed in the same basic form for at least 76 million years and have spread to every continent except Antarctica. What’s the secret to their success? Mosquitoes are one of the few insects that routinely lay eggs and mature in small, transient bodies of water, such as tree cavities and even hoof prints. Several types of mosquitoes from the genus Culex, such as the Southern House Mosquito, are known to spread West Nile Virus. First detected 13 years ago in New York City, a particularly prevalent outbreak centered in the South Central United States is occurring this year, 2012. Outbreaks generally achieve peak numbers in the late summer/early fall.

Trivia Question: Which weather related factors likely influence the prevalence of West Nile Virus in any given warm season?

a) The previous winter’s temperature
b) The current (outbreak season) temperature
c) Rainfall levels
d) All of the above

The correct answer is d. The mild winter temperatures during the 2011-2012 winter allowed many adult mosquitoes from the 2011 season to survive into 2012, boosting the total number of Culex mosquitoes. During the warm, outbreak season itself, warmer temperatures allow mosquitoes to develop faster and the viruses they carry replicate faster: West Nile begins to develop inside its hosts once the temperature reaches about 58 degrees Fahrenheit and the rate of development then doubles for each incremental 12 degree Fahrenheit increase in temperature. Lastly, and somewhat counter intuitively, drought boosts numbers of Culex mosquitoes. These mosquitoes like to breed in small pools of water rich in plant and other organic materials. Underground storm drains are generally a great breeding habitat for the mosquitoes, except when it rains and water flushes away mosquito larvae, inhibiting reproduction. The lack of rain and the particularly stagnant storm drains this year have led to a big boost in mosquito numbers. Also, the lack of rain has compelled local birds, which also carry the virus, to move from rural areas into populated city centers in search of water, leading to more transmissions of the virus to humans.

Seasons: Summer, Fall

Sources: Budiansky, Stephen. “Creatures of Our Own Making.” Science 298 (2002): 80- 86 and Kaiser, Jocelyn. “Drought Portends Mosquito Misery.” Science 301 (2003): 04 and “Outbreak Pattern Stymies Vaccine Work.” Science 337 (2012): 1030 and Reisen, WK et al. “Effects on the Transmission of West Nile Virus by Culex tarsalis (Diptera: Culicidae).” Journal of Medical Entomology 43 (2006): 309-317.

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