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Bug of the Week is written by "The Bug Guy," Michael J. Raupp, Professor of Entomology at the University of Maryland.

Bloodsuckers in the basement: Subterranean mosquito, Culex pipiens form molestus

 

As possible vectors of human disease, Culex pipiens form molestus is a femme fatale, both beautiful and potentially deadly.

 

Last week we visited bloodsucking bed bugs. This week we continue on the ectoparasites theme and visit another group of urban adapters, city dwelling mosquitoes. About this time last year under the cloud of Zika virus we met a population of yellow fever mosquitoes living beneath the streets and buildings of Capitol Hill. We discussed the role they might play in the local transmission of Zika virus. This week we travel 200 miles north to New York City to meet another urban adapter that brings untold hours of itching and scratching to residents of the Big Apple. These tiny bloodsuckers are close kin to the Northern House Mosquito, Culex pipiens form pipiens. Northern House mosquitoes are important vectors of deadly arboviruses including West Nile virus and Japanese encephalitis.  Culex pipiens form molestus, a subterranean-dwelling relation to the Northern House mosquito, was once called the London Underground Mosquito. During the Blitzkrieg, Londoners seeking shelter from German bombers were assaulted by hungry mosquitoes in the shelters beneath the city. Fast forward to 2016 where on the Upper West Side of Manhattan apartment dwellers have been bitten throughout the winter by tiny vampires breeding in a still unknown redoubt. Residents in the seemingly expanding mosquito zone have shared creepy stories of their nightly search and destroy missions to get a good night’s sleep in their apartments.

 

Somewhere beneath the sidewalks of New York mosquito larvae filter food from the water and breathe air through a siphon on their rear end.

While its appearance is identical to the Northern House mosquito, form molestus mosquitoes have different behaviors that make them particularly bothersome to humans.  Instead of mating and laying eggs above-ground like their cousins, female molestus mate and deposit their eggs in water sources below ground.  Human-induced changes to the landscape, particularly urbanization, produce plenty of underground habitat for molestus to infiltrate.  Examples of molestus habitats include underground subway tunnels, sewer systems, and buried water holding tanks. This ability of molestus to exploit man-made habitats bring them into close contact with human populations – often times too close for comfort.

While most female mosquitoes must blood feed to obtain the protein required for egg production, molestus females acquire all the protein needed for oviposition from their larval diet.  When larvae metamorphose into adults, they can lay one batch of tens to hundreds of eggs before they need to search for a warm blooded victim. In the lingo of mosquito biology, this first so-called “autogenous” egg raft ensures a self-sustaining underground population.  But adult females require blood feeding to produce further offspring, and guess who is on the menu? Female molestus in search of warm-blooded hosts can move above ground, where humans, dogs, and rats are abundant. These little vampires are capable of blood-feeding and egg production every 3-5 days for up to several weeks. That’s a whole lot of itching and scratching! Even worse, this pesky behavior isn’t limited to summer time. Most mosquitoes in Northern latitudes have to find some way of surviving the cold winter months. For example, their Northern House mosquito relatives use fall temperature and daylight cues experienced in their larval stages to, as adults, move into resting sites known as hibernacula. To see an example of an NYC hibernaculum click on the following link:

https://www1.nyc.gov/assets/doh/downloads/pdf/wnv/parity-rates-isolation-of-wnv-publication.pdf). 

These structures, combined with seasonally dependent physiological changes, help the above-ground Northern House mosquito survive cold wintry months. But molestus females do not have these same behaviors. Their urban underground breeding sites are buffered from seasonal temperature changes and they get very few daylight cues underground. Therefore, they never enter into the state of hibernation experienced by their above-ground relatives. Adults are active all year round, and a warmer than average February day makes for an exciting foray above ground in search of a tasty blood meal. So much for a winter reprieve from those itchy bites.

 

Watch as a female Culex pipiens form molestus extracts all the blood she needs to produce the next batch of eggs and scurries away to the shadow of a knuckle to hide. Filmed at twice life speed.

It is unclear exactly how these Manhattan molestus are making their way into apartment buildings from their underground breeding sites. To avoid being bitten, residents have resorted to sleeping beneath mosquito nets and squashing mosquitoes with pillows. The body counts of smashed mosquitoes are impressive. Other US cities, including Chicago, have faced similar infestations by Culex pipiens form molestus. Hopefully, similar recon will reveal the source of Manhattan’s misery and put an end to these tiny bloodthirsty troglodytes.

References

Bug of the Week thanks Dr. Megan Fritz for sharing her research and mosquitoes and for coauthoring this episode. To learn more about the Culex pipiens form molestus in New York, please listen to the following broadcast by WNYC News.

http://www.wnyc.org/story/seven-years-uws-mosquito-infestation-remains-mystery/