Friday, May 24, 2013

Mosquitoes

It’s “Fly Day Friday,” and while not everyone thinks of mosquitoes as flies, that is exactly what they are. Mosquitoes make up the family Culicidae. The majority are aquatic in the egg, larva, and pupa stage of their life cycle. They do not require a great deal of water, or time, to complete their development, and this makes them very difficult to control.


Female Culiseta incidens, the "Cool Weather Mosquito"

The adult female insect, in most species, needs a meal of blood in order to nourish her eggs. Human beings are not usually the host of choice, but we will do in a pinch. Mosquitoes are often cited as the most dangerous of all animals because of their efficiency in delivering disease pathogens to human populations. West Nile Virus may make headlines, but other dangerous mosquito-borne illnesses fly under the radar. Our pets and livestock are also at risk.

The Mosquito Life Cycle


Many mosquitoes breed in treeholes

Female mosquitoes deposit eggs in “rafts,” or singly, on the surface of still water. Larvae, called “wrigglers,” emerge from the eggs and begin their aquatic lives. They breathe through a spiracle at the end of an elongated tube called a “siphon,” at the tip of the abdomen. Most species are filter-feeders that use their mouthparts to strain food particles and/or microorganisms from the water. A few species are predators of other mosquito larvae.

Larvae grow by molting, shedding their flexible yet constraining exoskeleton periodically. Eventually, they molt into the pupa stage, known as a “tumbler.” The pupa is not inert, but quite active, able to dive when danger threatens by thrashing its abdomen. Tumblers breathe through paired siphons (“trumpets”) on its “shoulders.”


Mosquito Life Cycle, © Eric R. Eaton

The adult mosquito pops out of the top of the pupa, and sits on the water surface while its new, winged body hardens and pigments become evident. Adult mosquitoes are covered in scales that may serve to attract mates, but also allow the insects to skip off spider webs, shedding scales instead of becoming entangled.

Mosquito Diversity
There are 176 species of mosquitoes currently recognized in North America. This includes species introduced from other parts of the world through commerce. A new species, Anopheles grabhamii, was described from the Florida Keys in 2002, so there is certainly potential for future additions to our mosquito fauna through several avenues. Only female mosquitoes bite, and not all species bite mammals. Many feed only on birds, a few on amphibians. Some species don’t bite at all.


Male Culex mosquito

Male Mosquitoes
Male mosquitoes are often easily identified by their plumose (feathery) antennae, which they use in part to find females of the same species. They also frequently have enlarged, brush-like palps, paired appendages that are part of their mouthparts. The palps might be mistaken for antennae themselves. Both male and female mosquitoes fuel their flight muscles with flower nectar which they sip through that needle-like proboscis.

Asian Tiger Mosquito


Asian Tiger Mosquito

One prominent example of an exotic species is the Asian Tiger Mosquito, Aedes albopictus. It was first documented in Texas in 1985. One year later it turned up in Florida, among tires imported from overseas for re-treading. This is probably how it spread, as this is one of the “container-breeding” mosquitoes that needs little water to complete its life cycle. Rainwater collects in discarded tires exposed to the elements, and mosquitoes in general are adept at finding such resources. The Asian Tiger Mosquito is now found over much of the eastern U.S. An outbreak in Los Angeles, California in 2001 was traced to a shipment of “lucky bamboo” from China. The insects were eradicated, but subsequent introductions may not have been so successfully suppressed.

Mosquito vs. Mosquito
Aedes albopictus has largely replaced the Yellow Fever Mosquito, Aedes aegypti here in the U.S. Larvae of the Asian Tiger Mosquito compete better for food, and parasites brought with the Asian Tiger Mosquito have had an adverse impact on A. aegypti. Sterility of offspring from interspecific matings has also affected the Yellow Fever mosquito disproportionally. Today, Aedes aegypti is limited to the southeast U.S., a few isolated areas in New York state, and Arizona.


Yellow Fever Mosquito

Disease Transmission
The success of the Asian Tiger Mosquito is ironically somewhat helpful to us. While A. albopictus is known to be able to transmit over thirty viruses, it is not a very efficient vector. Western and Eastern Equine Encephalitis, St. Louis Encephalitis, LaCrosse Encephalitis, and dengue fever are all serious diseases potentially vectored by the Asian Tiger Mosquito.

Malaria is, thankfully, not currently a problem in North America. This was not always the case. Anopheles quadrimaculatus was the vector of this disease in the U.S. and Canada. Malaria affected most of the United States by 1850. One century later, thanks to improvements in sanitation that reduced breeding spots for mosquitoes, and the widespread use of DDT and other potent pesticides, the disease was largely eradicated.


Female Anopheles mosquito

West Nile Virus is transmitted almost exclusively by mosquitoes in the genus Culex. Birds and horses suffer much more frequently from this disease than people; and there is a vaccination available for equines. The elderly, and those people with compromised immune systems, are most at risk.

Dog Heartworm is also a mosquito-borne disease that occasionally afflicts cats as well. The illness itself is caused by a roundworm that is transmitted by at least sixteen species of mosquito. Risk for Dog Heartworm is greatest along the southern Atlantic and Gulf coasts, and the Mississippi River Valley (Nayar & Rutledge-Connelly, 2012).


The "Gallinipper," Psorophora ciliata, a real giant whose larvae eat other mosquito larvae

Mosquito Control and Prevention
You can do a great deal to reduce your risk of exposure to mosquitoes and the diseases they may carry. Consider taking the following measures and precautions:

  • Eliminate standing water on your property by cleaning gutters regularly, storing toys, flowerpots, and other potential rainwater collectors indoors, and draining water wherever else it accumulates.
  • Apply insect repellents with DEET as the active ingredient, paying careful attention to the directions on the product.
  • Comply with local city and county vector control regulations.
  • Maintain swimming pools properly
  • Change the water in the birdbath frequently, remembering mosquitoes can complete their life cycle in about a week.
  • Sleep under mosquito netting when traveling overseas to locations where malaria, dengue, and yellow fever are still problematic.
  • Get your pet checked for, and immunized against, Dog Heartworm.

Mosquitoes have their own pests: this one has mites (red spots)

What good are mosquitoes?
When asked this question, I am sometimes tempted to answer “Ask a Plasmodium (the malaria parasite).” We are naturally anthropocentric in our view of other organisms, especially when we see no direct benefit to us. We do know that mosquito-borne diseases have driven our own evolution. Sickle-cell Disease was an evolutionary response to malaria, the misshapen blood cells being inhospitable to the malaria parasite. What else do we owe to mosquitoes, positive or negative? We have much yet to learn, no doubt. Meanwhile, mosquitoes are a fundamental building block in the food chain, responsible for supporting the enormous diversity of fish, birds, bats, and predatory insects found around the globe. We also don’t know what we might lose with the microbes dependent on mosquitoes for transportation from host to host. The next medical breakthrough might come from studying one of those organisms. Lastly, mosquitoes can be important pollinators of flowers. Both male and female mosquitoes visit blossoms for nectar. Still want to wipe them off the face of the Earth?


Yellow Fever Mosquito sipping nectar

Sources: “Asian Tiger Mosquitoes,” Greater Los Angeles County Vector Control District, 2012.
Rios, Leslie, and James E. Maruniak. 2004. “Asian Tiger Mosquito (EENY-319),” Featured Creatures. University of Florida.
Nayar, Jai K., and C. Roxanne Rutledge-Connelly. 2012. “Mosquito-borne Dog Heartworm Disease,” EDIS. University of Florida.
”Mosquito-borne Diseases,” American Mosquito Control Association, 2011.

1 comment:

  1. No, it is not my goal to be bitten by every mosquito species possible, but I am an opportunist when it comes to getting images :-)

    ReplyDelete