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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.

Vexing rains spawn vexing mosquitoes: inland floodwater mosquito, Aedes vexans


Legions of inland floodwater mosquitoes like this egg-filled Aedes vexans will soon make their presence known as they seek human victims on warm summer nights.


Late spring and early summer of 2018 witnessed record rainfall in several parts of our region, flooding ill-fated Ellicott City and swamping low-lying lands around the mighty Potomac and Patuxent Rivers. Some hoped these rains were a “one-and-done”, but persistent rains and saturated soils have created long-standing puddles in low lying areas of lawns, parks, and woodlots and in swales along verges of roadways.  These standing water sources are the perfect breeding site for legions of floodwater mosquitoes including the nefarious inland floodwater mosquito, Aedes vexans.  Found throughout many parts of the world, Aedes vexans is an important nuisance pest in North America, Europe, and parts of Asia and Africa. It also vectors a variety of debilitating, potentially lethal pathogens of humans and pets. It has been implicated in the transmission of Eastern Equine Encephalitis, a deadly disease of humans and horses. It also transmits dog heartworm, a nasty tiny roundworm that invades the heart of a dog. Fortunately, humans do not contract this disease but this parasite extinguished the life of my childhood dachshund. 

Unlike the invasive Asian tiger mosquito, a tormentor during daylight hours, Aedes vexans extends a day of mosquito misery by seeking hosts and biting people at dusk. As with the rest of the blood-feeding mosquito clan, it is only the females that seek blood. Protein in animal blood is transformed into eggs within the ovaries of the mosquito. However, mosquitoes cannot live on protein alone. The energetic demands of searching for hosts and dodging hungry predators and murderous humans requires carbohydrates. The sources of these sugars are nectar from plants and other natural sweets such as honeydew produced by sap-feeding insects like aphids. Protein requirements of male mosquitoes are far less than those of the ladies, hence, they do not bite animals but obtain their nutrients from nectar and honeydew. Who knew that dastardly mosquitoes were also beneficial pollinators?


Watch as this female Aedes vexans tanks-up on sugar from a droplet of honey. The elongated proboscis used for sipping honey or nectar is the same weapon used to suck blood from humans and other animals.

Aedes vexans survives winter as eggs deposited on moist soil in areas prone to flooding. When spring rains arrive and low areas fill with water, eggs hatch and larvae feed on the biofilm of microbes festooning submerged vegetation. In the mid-Atlantic region, mosquito larva called wrigglers can be found from May until October and with a steady recharging of breeding sites thus far this year, we can expect continued production of these vexing vampires for the near future at least.


In this floodwater pool near the edge of a forest, mosquito larvae cruise the water column and dine on the biofilm covering submerged leaves and microbes suspended in the water.

Protecting yourself from mosquito bites

Protect yourself from aggressive biters by wearing light-weight, long-sleeved shirts and pants when working outdoors. You can purchase clothing pretreated with the mosquito repellent permethrin. I have worn this clothing in tropical rainforests where mosquitoes were ferocious and it really did help. Permethrin can also be purchased as a ready-to-use spray to treat clothing. Do not apply permethrin directly to skin and follow exactly the directions for use found on the label. Many insect repellents can be applied to exposed skin before you go outdoors. Some will provide many hours of protection, while others provide virtually none. The “gold standard” of mosquito repellents is the compound DEET. Higher percentages of DEET in a product generally result in greater levels and duration of protection but concentrations of 25 – 30 % should do the trick. Two alternatives to DEET contain the active ingredients picaridin or IR3535. These can also provide many hours for protection from mosquitoes.

In recent years many botanically-based products have come to the marketplace. Scientists discovered that wild tomato produces a compound, 2-undecanone, that prevents mosquitoes from landing on humans for many hours. Other products containing oils extracted from lemon eucalyptus, Corymbia citriodora, and products combining oils of soybean, geranium, and caster bean protect people from mosquito bites. Products based on citronella and other essential oils derived from plants vary greatly in repellency with average protection times ranging from 5 minutes to 2 hours. So, you may have to apply these products more frequently to be protected.

Questions always arise regarding the use of repellents on children. Repellents carry precautionary statements on their labels. Always read the label carefully and follow directions and precautions exactly. You should help children apply repellents and consult a pediatrician before applying any product to the very young. Some products state not to let children handle the product and even some botanically-based products warn against use on kids under the age of 3.

To learn more about insect repellents, please visit the following CDC and EPA websites:

Reducing mosquito breeding

At a residential home in the DMV, a pool of standing water breeds tens of thousands of inland floodwater mosquitoes. A single sample with this mosquito dipper holds hundreds of larvae and pupae soon to turn into adult mosquitoes ready to torment local residents. Image credit: Megan Fritz

To reduce the chances of mosquitoes breeding around your home, eliminate standing water by cleaning your gutters, dumping your birdbath twice a week, inverting your wheelbarrow and getting rid of water-filled containers. Those corrugated drainpipes that direct water from downspouts can also hold enough water to breed mosquitoes such as the Asian tiger. Installing drainpipes with slits or periodically removing pipes and draining them will help eliminate water where mosquitoes breed. If you find mosquito larvae in an aquatic water garden or a long-standing pool of water on your property, then you can use a product containing the naturally occurring soil microbe known as Bacillus thuringiensis israelensis, a.k.a. Bti, to kill larvae. Bti comes formulated in doughnut-shaped tablets that can be placed in water. If you have an uncovered swimming pool, with proper filtration and disinfection systems, it should not serve as a mosquito breeder. If your pool is covered and water collects in a non-porous tarp, then mosquitoes may breed in standing rainwater that collects in the cover. You should empty the cover regularly or treat it with a larvicide.

So, in this season of never ending rain, be on the lookout for mosquitoes as they will surely be looking for you.



Bug of the Week thanks Dr. Megan Fritz for providing inspiration and information for this episode and for mosquitoes used in its production.  The interesting reference “Aedes Vexans (Meigen): An Old Foe” by Claudia M. O'Malley was used as a reference for this episode. To learn more about this mosquito, please click on the link below: