Colorado is gloriously sunny, high and dry. That ought to mean living in mosquito-free bliss. So why are we slapping so many skeeters off our legs lately? And why are some neighborhoods up in arms about the recent mosquito boom and efforts to quell it?
Perhaps it’s because they make us itch like crazy and bring disease to us, our pets, and to livestock. Or… like other bloodsuckers such as fleas, ticks, bedbugs, and lice—maybe it’s because they creep us out.
Luckily, our mosquitoes are as predictable as the Creature from the Black Lagoon—because their life cycle is tied to stagnant water. Our most common local mosquito lays its eggs atop standing water and its brethren lay eggs on wet soil that later floods. Adult males live about a week and exclusively feed on flower nectar and pollen. Females often live up to 6 weeks—but suck blood as well as nectar and pollen. Proteins in blood are needed for most of the eggs that they lay.
Like the amber-encased mosquito from Jurassic Park, today’s mosquitoes get most of their blood from living dinosaurs—a.k.a. birds. As they suck on finches, sparrows and quail, they acquire diseases that they transmit to humans, horses, pets and other animals.
What are the primary types of mosquitoes here?
Among the dozens of species of mosquitoes inhabiting Colorado, two types stand out. The first includes members of the genus Aedes, commonly called floodwater or “nuisance” mosquitoes. These are the fast-breeding ones that carry heartworm larvae that infect your pets. They also bite the heck out of you shortly after your neighborhood is deluged, irrigated, or experiences a spring snowmelt. Aedes eggs, which comprise the vast majority of skeeters near my house, need as little as a half-inch of standing water and 5 days to mature into biting adults. The second group is dominated by members of the genus Culex, which whine in your ear and feed at night. These are the mosquitoes that carry viruses like West Nile, St. Louis encephalitis and Western equine encephalomyelitis. Typically, a tenth of a percent to 10 percent of them are infected.
What are the chances of getting sick from a mosquito bite?
There are a half-dozen mosquito monitoring sites in Stapleton. Among the mosquitoes trapped at these sites this year, about 6 percent are Culex mosquitos, which can carry West Nile virus (WNV).
Among Culex, infection rates vary considerably—but in Denver the average historical rate of mosquitoes that carry WNV is about 7 percent. If a mosquito successfully transmits WNV to a person, it will remain in their body on the order of two days to two weeks after being bitten. Only 20–30 percent of people infected by WNV actually develop symptoms—the majority don’t even know they have it.
West Nile virus symptoms can include a fever, headache, joint or muscle aches, rash and/or vomiting. Most people who develop such symptoms recover completely within days to months with no recurrent symptoms. Concern is merited, though, because in some cases WNV can be transmitted to a fetus during pregnancy or by breastfeeding. And, because less than a percent of WNV sufferers have their brains invaded by the virus to the extent that they have swelling of the brain and spinal cord syndromes that cause fever, headache, seizures, tremors, respiratory issues, paralysis and the like—including lasting neurological issues. Unfortunately, about a tenth of these people die.
Mosquito abundances aside, these data might equate to less than a one-in-a-thousand chance that you’ll get WNV and develop symptoms, and less than one in a million that such an experience will be fatal.
To put these infection rates into perspective, last year Colorado had an epidemic year, with 322 cases of WNV reported, 12 of which were in Denver. This year there have been two cases of WNV reported in the whole state.
Why so many mosquitoes?
Both of these mosquito types owe their abundance to the ponded water around us. Some of this water comes from rainwater that accumulates in natural depressions, but much of it comes from our infrastructure and development.
For example, graded land, construction sites, and well sites are littered with puddles. Agricultural areas have abundant lingering water in furrowed fields, irrigation ditches and perforated canals. And let’s not forget golf courses, where man-made “water hazards” provide a veritable mosquito bonanza.
To put these in perspective—one flower pot saucer of stagnant water can produce thousands of mosquitoes per summer. Thus all these sites combine to have profound effects.
Even our neighborhoods contribute to the problem. Manicured developments like mine are the perfect haven for mosquitoes during the day by providing cool, moist, shady plants for mosquitoes to hang out in. At night they provide refuges in humid, well-watered lawns.
So what should we do?
First, eliminate standing water to prevent mosquitoes from reproducing. Second, consider screening your front door, wearing long-sleeved, loose-fitting clothing and pants, and minimizing time outdoors at dawn and dusk when mosquitoes are gorging. Third, consider your repellent options. Moderate concentrations of DEET, when used on skin and according to instructions, are safe for the vast majority of people—including kids and most other mammals. That said, until we know the outcomes of studies of how DEET impacts the endocrine and neurobehavioral systems of babies, toddlers and pregnant women, minimization of DEET among such populations is warranted. Other chemical repellents like IR3535 or Picaridin are less well tested, but are widely used in Europe and appear to be safe for people. Citronella and similar botanical repellents work, but are less effective. Their side effects in humans are also poorly known and many contain concentrated allergens. Achoo!
As a society, we can best solve our mosquito woes by using integrated pest management programs. These programs are employed by many cities and are successful at reducing both nuisance and hazardous mosquito populations. Such programs involve monitoring where and how mosquito populations change so problems are addressed before they get out of hand. Usually this approach involves eliminating, draining, or introducing bug-eating fish to sites where skeeters reproduce, and bombarding larval populations with mosquito-toxic substances derived from soil bacteria. Where necessary, pyrethroid insecticides can be sprayed at night to kill adult populations. The latter can sometimes damage desirable insect communities, though.
Despite this stinging news, our state’s overall mosquito situation is much improved. In historical times, malaria and equine encephalitis were rampant in Colorado. Today’s mosquitoes are mostly a nuisance and don’t typically cause public health emergencies. Yet they can significantly impact our quality of life and our economy. In Colorado their abundance is mostly catalyzed by man-made activities and structures. Mother Nature just provides some of the water and the entomological potential.
Like immunization, dealing with mosquitoes benefits most from systemic prevention. This weekend I’d better check those cobwebbed flowerpot saucers behind our grill.
James Hagadorn, Ph.D., is a scientist at the Denver Museum of Nature & Science. Suggestions and comments are welcome at firstname.lastname@example.org.
Editor’s Note: The Stapleton Master Community Association (MCA) is continuing to apply a non-toxic larvicide at 24 sites in Stapleton, monitoring areas where they’re getting complaints and adjusting their mosquito management accordingly. The City of Denver is applying larvicide to about 60 sites throughout the city and also has a half-dozen mosquito traps in addition to the ones in Stapleton. Culex mosquitoes from all the traps are sent to a state laboratory to analyze the rate of WNV.
The July mosquito trap counts posted by Stapleton MCA can be viewed here: 7-9-14 count and 7-6-14 count. Additional information from the Stapleton MCA can be found at www.StapletonCommunity.com > What We Do > Community Maintenance.
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