Mosquitoes are a common pest in the upper Midwest. Globally, they cause three kinds of damage: nuisance from itchy bites, direct harm from disease transmission, and all the secondary costs that come about from the disease burden (lost productivity, healthcare costs, etc.). Fortunately, a combination of effective pest management, effective health care, and a temperate climate inhospitable to some of the world’s worst disease-vectoring mosquito species have made disease transmission by mosquitoes in the United States a relatively minor concern, but mosquitoes are certainly still a big nuisance.
There are over 3000 species of mosquitoes on earth, with 176 documented in the United States. Most of the states in our area are home to 50-60 species of mosquitoes, with roughly 10-15 per state that sometimes achieve pest status. Not all mosquitoes drink blood, and of those that do, not all feed on humans, so a great many mosquitoes are not harmful to humans at all. While it may be tempting to say “a mosquito is a mosquito,” they actually have great differences in flight ranges, behavior, and preferred habitats, so knowing which type of mosquito is plaguing you can go a long way towards correcting the issue.
Despite the diversity of mosquitoes, there are some unifying features. All mosquitoes have aquatic larvae, though the volume of water they need to develop can range from expansive marshes to the water that collects in bottle caps. The exact larval diets of mosquitoes vary a bit from species to species, but the majority have filter brushes around the mouth that they use to strain bacteria, algae, and particles of organic matter from the water in which they live. The adults are not particularly strong fliers, and are most active during calm, cool to warm humid weather. When the adults are not active, they typically roost in areas that stay cool and humid and protect the mosquitoes from sunlight, extreme temperatures, desiccation, and wind. Adult mosquitoes feed extensively on nectar from flowers, and only females take blood meals in order to acquire the protein needed to produce eggs.
Three groups of mosquitoes are responsible for the greatest amount of human biting and disease transmission across the United States: Aedes, Anopheles, and Culex. Aedes has been a battlefront of systematists and taxonomists for roughly twenty years, with conflicting editorial opinions from the American Mosquito Control Association and the Entomological Society of America’s Journal of
Medical Entomology and the debate raging in the realm of peer-reviewed papers as well (see Savage & Strickman, Wilkerson et al., Reinert, and Reinert et al.). My posts about scientific names and some reading through the International Code of Zoological Nomenclature may help to add context to the issue. For the purposes of this post, I am aligning with the Entomological Society of America’s position that we do not yet have a clear enough picture of how the systematics should shake out to break up Aedes.
In any case, Aedes mosquitoes include floodwater, saltmarsh, and container-breeding species. Adult Aedes lay eggs on hard surfaces just above the water line, and the eggs dry out and go dormant – in some cases for up to five years. When the eggs are flooded, they hatch – making Aedes mosquitoes particularly responsive to rainfall events. While several Aedes species can be nuisances or minor disease vectors, the most problematic in our area seem to be Ae. vexans, the floodwater mosquito, Ae. triseriatus, the treehole mosquito, Ae. japonicus, the rock pool mosquito, and Ae. albopictus, the Asian tiger mosquito. Treehole, rock pool, and Asian tiger mosquitoes typically breed in small containers of water, which means that their breeding sites can be very close to human occupation but also means that their breeding sites can potentially be eliminated. Floodwater mosquitoes breed in any area that is prone to flooding during periods of heavy rains, and can form tremendous swarms of aggressive biters when conditions allow. Most of these mosquitoes fly at dawn and dusk, but Asian tiger mosquitoes are active during the day. Flight ranges of these mosquitoes can range from around a hundred meters for Asian tiger mosquitoes to several miles for floodwater mosquitoes, though they will typically stay near their breeding site if they can. Most Aedes prefer to feed on mammals (including humans), and this group includes several important disease vectors: treehole mosquitoes vector La Crosse encephalitis, rock pool mosquitoes vector West Nile Virus, and Asian tiger mosquitoes vector yellow fever, Chikungunya, Dengue fever, and Zika virus.
Anopheles mosquitoes are somewhat odd-looking, with long palps held along the proboscis, a headstand posture, and speckled or patterned wings. They lay single eggs on the water’s surface in vegetated wetlands: swamps, marshes, etc. That means source reduction is often impossible for Anopheles, and other methods must be used to manage them. Fortunately, Anopheles seem to be less common than are Aedes and Culex. Anopheles mosquitoes are the only mosquitoes that can vector malaria, though there has not been a case of malaria acquired within the United States for several decades.
Culex mosquitoes include container breeders and wetland breeders, though they prefer breeding in scummier water with more organic matter. Prominent species in our area include C. pipiens, the northern house mosquito, and C. tarsalis, the western encephalitis mosquito. Culex lay their eggs in clusters, or rafts, on the water’s surface, and often breed in storm sewers, ditches, and other areas where decomposing leaf litter or grass clippings may be submerged and provide rich food for the larvae. Culex mosquitoes tend to prefer feeding on birds over mammals, but will readily feed on both. In line with their preferred avian hosts, adult Culex often rest in sheltered parts of tree canopies, where they may be out of reach of typically mosquito barrier sprays. Northern house mosquitoes are considered the primary vector of West Nile virus, while the western encephalitis mosquito primarily vectors St. Louis encephalitis, western equine encephalitis, and West Nile virus.
Some other mosquitoes that periodically become important include Coquillettidia perturbans, the cattail mosquito, Psorophora ciliata, the shaggy-legged gallinipper, and Culiseta melanura, the black-tailed mosquito. Cattail mosquitoes are distinct because their larvae breath air through cattail roots, which allows them to stay submerged, buried, and at least partially protected against many mosquito larvicides. Gallinippers look like enormous Aedes mosquitoes and can easily bite through single layers of clothing, though they are not significant disease vectors and offer the service of having predatory larvae that consume other mosquito larvae. Black-tailed mosquitoes are the primary vector of eastern equine encephalitis among birds, but the virus requires a bridge vector such as a cattail mosquito or Aedes mosquito to transfer the virus from birds to mammals.
When it comes to living with these blood-suckers, it is important to recognize that no single treatment is 100% effective all the time. Source reduction by eliminating adult harborage and breeding sites should be the first action taken. That means draining any outdoor container that may hold water for at least 4-5 days at a time, even if it only holds a fl oz or two, and thinning vegetation so that sunlight and wind can penetrate the branches (good news – this practice also reduces the likelihood of plants developing fungal infections!). Limit outdoor activities at the times and places where mosquitoes are most active; if you must be out at dusk, try to stick to open areas that are more prone to windy conditions or areas that are still exposed to sunlight. Wearing long sleeves and long pants, especially when mosquitoes are most active, will physically block bites over much of your body, and an effective repellent recommended by the EPA and/or CDC will deter bites from exposed skin.
Professional treatment options for mosquitoes include larviciding, trapping, baiting, barrier treatments, and fogging, and should always also include thorough inspection and a communication/education component. Larviciding is the treatment of breeding sites that cannot be eliminated with a pesticide that is toxic to mosquito larvae. Many larvicides are remarkably specific, so larviciding may even be done in animal water troughs, farm ponds or ornamental fish ponds that house fish. Because the larvae are the most concentrated life stage of mosquitoes and the larvicides are target-specific, larviciding is typically the most effective and most environmentally-friendly of all the chemical mosquito treatments. Traps come in different styles and work in different ways; two trap models that both use the mosquitoes’ egg-laying behavior against them include water with decomposing organic matter as the attractant. In one style, a mesh barrier prevents the mosquitoes from reaching the water to lay eggs and a glue board intercepts the mosquitoes so they never escape from the trap and can be identified and tallied later. In the other style, concentrated larvicides ensure that any larvae deposited in the trap will not develop to adulthood, while the adult mosquitoes leaving the trap may have so much larvicide on their exoskeleton that they can inadvertently treat other breeding sites that they visit. Only one bait is currently on the market for mosquitoes, and its efficacy is dependent upon mosquitoes’ nectar-feeding behavior. Baiting is likely to be the second safest chemical treatment for mosquitoes after larvicides, with properly placed bait being predominantly consumed by true flies, including mosquitoes, and predators that eat mosquitoes engorged with bait are not detectably harmed by the bait. Barrier treatments are applications of a residual, usually pyrethroid insecticide to resting areas used by adult mosquitoes. They can be highly effective, but variables such as mosquito species, rainfall, and application equipment have large effects on efficacy, and the broad application of broad spectrum insecticides means barrier treatments kill tremendous numbers of non-target animals. Fogging disperses miniscule droplets of broad-spectrum insecticides – typically pyrethroids or organophosphates – to kill mosquitoes on the wing. The pesticides used in fogging do not persist in the environment, so fogging must be done while the mosquitoes are active, and also when weather conditions are conducive to the treatment. Like barrier treatments, fogging kills large numbers of non-target insects, and one of my professors at North Dakota State University collected the data to show that community fogging programs can decimate whole communities of insects.
Guardian’s mosquito services are currently limited to inspections, larvicides, and barrier treatments, but we are working on incorporating each of the other treatment types except for fogging. A continuous mosquito management program will allow for preventative work and will be far more effective and environmentally friendly than responsive treatments to mosquito outbreaks.