Please Don't Bite Me!
by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station
Last week, island voters passed a $100,000 override at the ballot box to develop a management plan for mosquitoes. People are passionate about mosquitoes, afraid of their bites, leery of catching mosquito borne diseases, and extremely irritated by their ability to ruin a nice day outside. People are often unaware that mosquitoes are an important part of the food web. There are ways to reduce our contact with mosquitoes without spraying toxic pesticides or polluting our groundwater and further endangering ourselves. In fact, the proponents of the warrant article debated and passed at town meeting and funded for the first year this past week wanted to have a comprehensive mosquito plan for the county to reduce the amount of spraying and lawn treatment individuals might take upon themselves. As we’ll find out, many common mosquito products can be toxic to pets, aquatic insects, and shellfish larvae; so using them sparingly, if at all; depending on more selective and less dangerous solutions benefits everyone.
The word "mosquito" is Spanish for "little fly," and its use dates back to about 1583 in North America. Mosquitoes belong to the order Diptera, true flies. Mosquitoes are like flies in that they have two wings, but unlike flies, their wings have scales, their legs are long and the females have a long mouth part called a proboscis for piercing the skin of their victims. Only females are to blame for bites because they require our protein-rich blood to acquire enough energy to reproduce. The males of all mosquito species are perfectly happy feasting on fruit and flowers. Mosquitoes have been around for more than 30 million years, which is more than ample time to develop stealth-bomber like skills for finding warm blooded mammals to bite. Mosquitoes have a battery of sensors designed to track their prey, including:
Chemical sensors — mosquitoes can sense carbon dioxide and lactic acid up to 100 feet away. Mammals and birds gives off these gases as part of their normal breathing. Yet another reason to talk less and listen more at your next backyard party, as each word you utter releases more carbon dioxide. Certain chemicals in sweat also seem to attract them (people who don't sweat much don't get nearly as many mosquito bites).
Visual sensors — if you are wearing clothing that contrasts with the background, and especially if you move while wearing that clothing, mosquitoes can see you and zero in on you. It's a good bet that anything moving is "alive," and therefore full of blood, so this is a good strategy. Dark clothing seems to be especially attractive to mosquitoes.
Heat sensors — Mosquitoes can detect heat, so they can find warm-blooded mammals and birds very easily once they get close enough. Some scientists have estimated that mosquitoes can distinguish a 0.5 degree difference in temperature from 20 yards away. When kids visit the lab at the Field Station, we take turns checking each person’s natural skin temperature to determine if they would be tasty and obvious targets. Exercising increases CO2, sweat, and your skin temperature, all of which make you a mosquito magnet. Dark clothing also absorbs more of the sun’s rays, providing a warmer and more obvious target. We use both heat and chemical bait to trap mosquitoes on island in order to find out what species are prevalent in different habitats and to determine if they carry any diseases that can pose a human health risk.
There are more than 2,700 species of mosquitoes in the world, and there are 13 mosquito genera (plural for "genus") that live in the United States. Of these, most mosquitoes found in North America belong to three groups:
Aedes — These are sometimes called "floodwater" mosquitoes because flooding is important for their eggs to hatch. Aedes mosquitoes have abdomens with pointed tips. They are strong fliers, capable of traveling great distances (up to 75 miles/121 km) from their breeding sites. They persistently bite mammals (especially humans), mainly at dawn and in the early evening. Their bites are painful. Many of the Aedes mosquitoes have been reclassified in 2000 as Ochlerotatus species. Aedes vexans are often found in the wetlands near Hulbert Avenue.
Anopheles — These tend to breed in bodies of permanent fresh water. Anopheles mosquitoes also have abdomens with pointed tips. They include several species, such as the common malaria mosquito (Anopheles quadrimaculatus), that can spread malaria to humans. These are not very common on Nantucket (one or two found each year out of thousands).
Culex — These tend to breed in quiet, standing water. Culex mosquitoes have abdomens with blunt tips. They include several species such as the northern house mosquito (Culex pipiens). They are weak fliers and tend to live for only a few weeks during the summer months. They persistently bite (preferring birds over humans) and attack at dawn or after dusk. Their bite is painful. They have been collected near Pocomo and Brant Point.
All mosquitoes lay their eggs in water or in the case of Aedes, in areas which are likely to flood. The life cycle of a typical mosquito starts with the eggs laid either singly or attached together in rafts on the water surface. The mosquito eggs hatch into larvae or "wigglers," which live at the surface of the water and breathe through an air tube or siphon. We’ll discuss below methods of reducing mosquito populations which take advantage of the fact that the larvae breathe through these “straws.” The larvae stage can last from a few days to a couple of weeks depending on the species.
Mosquitoes molt like many other insect species to progress through their life stages. After the fourth molt, mosquito larvae change into pupae or "tumblers," which live in the water anywhere from one to four days depending on the water temperature and species. The pupae float and roll at the surface and breathe through two small tubes (trumpets). Fish and aquatic insects eat mosquitoes in the larval and pupal stages.
At the end of the pupal stage, the pupae encase themselves in pupal cases and transform into adult mosquitoes. The adult uses air pressure to break the pupal case open, crawls to a protected area and rests while its external skeleton hardens, spreading its wings out to dry. Once this is complete, it can fly away and live on the land. One of the first things that adult mosquitoes do is seek a mate, mate, and then feed. Male mosquitoes are attracted to female mosquitoes by the whine of the females' wings; that whine is especially annoying to me, not being a male mosquito. After they feed, females lay their eggs (they need a blood meal each time they lay eggs). Females continue this cycle and live anywhere from many days to weeks (longer over the winter); males usually live only a few days after mating. Some mosquito species can over-winter in cold climates in either an adult or egg stage if they can find a relatively warm moist area.
So what is actually happening to a victim of a mosquito “bite”? The female mosquito lands on your skin and probes around for an easy entry for her proboscis; she then inserts six specialized mouth parts called stylets. Four of the stylets serrate capillaries, 1 stylet injects an anticoagulant and the last stylet acts like a trough for the blood to be drawn into the mosquito abdomen by a pump located in the head of the mosquito. I am pretty sure this whole process has been featured a few times on HBO’s “True Blood” series. Her saliva contains proteins (anticoagulants) that prevent your blood from clotting. After she has bitten you, some saliva remains in the wound. The proteins from the saliva evoke an immune response from your body. The area swells (the bump around the bite area is called a wheal), and you itch, a response provoked by the saliva. Eventually, the swelling goes away, but the itch remains until your immune cells break down the saliva proteins. To treat mosquito bites, you should wash them with mild soap and water. Try to avoid scratching the bite area, even though it itches. Some anti-itch medicines such as Calamine lotion or cortisone creams may relieve the itching.
Fortunately, on the health safety front, the majority of mosquitoes collected in traps placed around the island were identified by experts as salt marsh species who although aggressive and “daytime biters” usually do not carry mosquito borne diseases such as the West Nile virus (WNV), malaria, or Eastern Equine Encephalitis virus (EEEv). Mosquitoes belong to a section of the animal kingdom called arthropods. Viruses that are spread by arthropods (including mosquitoes) are referred to as arthropod-borne viruses or arboviruses. For several years, I have placed a variety of mosquito traps provided by Mass. Dept. of Public Health (http://www.mass.gov/dph/wnv) around the island to catch mosquitoes in order for them to be identified and tested for West Nile and EEE. I collect the mosquitoes captured, pack them up quickly in dry ice in biohazard containers and ship them off for testing by the state. I was assisted by the late (and greatly missed) Nantucket Department of Public Works (DPW) mosquito control employee, Peter Brady.
The two types of traps we most often deployed were the Center of Disease Control carbon dioxide (CDC/CO2) traps and “stink water traps.” The CO2 traps are suspended from trees and supplied with a light and a fan which are run off of a portable battery. The trap is “baited” with frozen carbon dioxide (better known as dry ice) that gases out of a black insulated container, which attracts female insects and sucks up the unsuspecting visitors using the attached fan into a clear container. The dry ice traps seem to work well for the Culiseta species although they are non-selective and also trap moths and other insects. The stink water is made of fresh hay, brewer’s yeast, and malt agar, all of which smell pretty good separately, but when mixed and allowed to ferment in water make up a substance that can quickly clear out a room. The female mosquitoes find this stagnant, stinky water an irresistible site to lay their eggs. The stink water traps are known as “gravid female” (mated and capable of egg laying) traps and are favored by the Culex species. Areas tested include the Coskata Red Cedar swamp, areas around the Point Breeze and the wetlands off of Easton Street (Brant Point area), the Windswept cranberry bog, the Hidden Forest, neighborhoods near wetlands in the mid-island section of Nantucket, Madaket, and Wauwinet and many others. The Culex species of mosquitoes favor vernal ponds and wetlands in acidic soil, while species such as salt marsh loving mosquitoes Ochlerotatus sollicitans and Ochlerotatus taeniorhynchus are some of the most common mosquitoes on Nantucket.
Many private individuals allow us to sample their “mosquito magnets” which can be effective if used properly. This website, http://www.consumersearch.com/mosquito-traps/important-features, provides useful information on selecting and placing a mosquito trap and includes a discussion of their carbon footprint which is relatively small. Mosquitoes collected on island are sent to Matt Osborne, the Field Coordinator for the Arbovirus Program in the Division of Infectious Disease Laboratories at the Massachusetts Department of Health State Laboratory Institute. After the mosquitoes are separated into species, Osborne and his staff will blend the mosquitoes of each species into a soup and test the entire batch. The two mosquitoes he's most interested in grinding up are the Culex pipiens — the primary carrier of West Nile virus — and Culiseta melanura, the primary carrier of EEE, just two of the 40 or so species in Massachusetts that will bite humans but two of the most dangerous. Species found in 2006 include 12-17% Ochlerotatus sollicitans (Golden Saltmarsh Mosquito), 35% Coquillettidia perturbans, and 40% Culex salinarius. The remaining were primarily Ochlerotatus cantator and Aedes vexans. If you are thinking ahead, and I know you are, you might be thinking of yet another reason why sea level rise is NOT a good thing, it creates more habitats for salt water mosquitoes!
EEE is one of the rarer mosquito borne diseases and is typically cycled between birds and the Culex melanura. According to the CDC, from 1964 through 2002 there have been 153 confirmed human cases in the US of EEE, for an average of less than four cases annually. Like West Nile virus (WNV), most human cases normally occur in late summer and fall. EEE has been isolated from, or antibodies have been found in a multitude of wild and domestic birds, particularly passerines (song birds) but also including owls and shore birds. People and horses become infected by the bite of infectious "bridge vector" mosquitoes such as the Ochlerotatus sollicitans that have become infected by feeding on infected birds. If you go to one of the yearly state reports such as the “Arbovirus Surveillance Summary, 2009” report (www.mass.gov/Eeohhs2/docs/dph/cdc/arbovirus/2009_summary.pdf) you’ll see that Nantucket ranks as “remote” for EEE danger and “low” for West Nile occurrences. Of the over 400 tests done on several thousand mosquitoes collected on Nantucket from 2005 to 2010 no positive hits have occurred for West Nile, malaria, or Eastern Equine Encephalitis virus.
So how do we control these creatures and reduce our exposure to them? Formulas containing 7.5-100% DEET (N, N-diethyl-meta-toluamide) are the most effective mosquito repellents, although parents should use formulas with lower levels of DEET (no more than 15%) for their children. Clothing is available coated with insect repellents that last for 20-25 washes. Wearing long sleeves and long pants also can reduce your likelihood of being bitten.
Elimination of standing water in tires, plant containers, birdbaths, or kiddie pools is the most important tool for reducing the stagnant habitat most freshwater mosquitoes need. Another method to reduce mosquito bites is to provide a different source of attractant for mosquitoes, which is why some people use candles (heat and carbon dioxide producers) placed away from the BBQ. Citronella oil, which is a product of several types of plants that can be made into candles or burned directly, is an effective mosquito repellent in high concentrations, but individual citronella-producing plants do not make enough oil to effectively repel mosquitoes. Other natural plant oil extracts have been proven to be mosquito repellents although they need to be applied frequently. These include: citronella, lemon eucalyptus, cinnamon castor, rosemary, lemongrass, cedar, peppermint, clove, and geranium oils. Each year at the field station we try out a variety of organic products for mosquito control. The Chrysanthemum cinerariifolium and C. coccineum plants produces pyrethrum, an insecticide used in many products (see warning below on man-made permethrin below), although researchers have discovered that some chrysanthemums attract and some repel mosquitoes (they can work well for repelling garden pests like aphids). Ultraviolet lights (as used in bug zappers) and ultrasonic devices are not effective and they kill as many or more of the predators that eat mosquitoes such as dragonflies and damselflies. Although bats and Purple Martins can be prodigious consumers of insects, many of which are pests, less than 1% of their diet typically consists of mosquitoes. Neither bats nor Purple Martins are known to control or even significantly reduce mosquito populations.
Part of the new mosquito plan will most likely advocate the most common surveillance and treatment methods for mosquitoes which is to collect larval samples around the island and then treat the water containing larvae with non-toxic larvicides designed to capitalize on the larvae’s “straw breathing” technique by coating the water surface with BTI (Bacillus thuringiensis israelensis), a naturally occurring soil bacteria. BTI is an endotoxin that affects the gastrointestinal tract of mosquito larvae, killing them at that stage and then breaking down right away into harmless organic compounds. The Environmental Protection Agency sanctions its use as safe around humans and drinking water supplies. The DPW used this process up until 2010. What I (and mosquito officials who work in similarly fragile areas on the Cape) do NOT recommend is to spray or fog with Sumithrin or similar products which are permethrin based insecticides that should only be used in a officially declared health emergency based on positive test results for EEE or WNV. Sumithrin is a neurotoxin that can pose a very serious risk to other beneficial aquatic insects, bees, the food web upon which our harbor health depends, and even humans and pets. Some products also contain organophosphates such as malathion which are a very potent neurotoxin both to mosquitoes and many other species including humans. Permethrin is extremely toxic to fish, bees, and wildlife and may be linked to liver damage and minor carcinogenic risks in humans. Details and specific toxicity information regarding permethrin can be found at http://pmep.cce.cornell.edu/profiles/extoxnet/metiram-propoxur/permethrin-ext.html.
Massachusetts’ DPH’s mosquito plans emphasize: monitoring of mosquitoes and mosquito-borne disease; public education; personal protection from mosquito bites; elimination of artificial mosquito breeding sites around developed areas, and; targeted applications of pesticides when necessary to protect the public from mosquito-borne diseases based on monitoring and risk thresholds. The Mass Audubon’s advice and policies can be found at www.massaudubon.org/Nature_Connection/wildlife/index.php?subject=Insects&id=82
More than any other nuisance species, people often ask me what ecological function mosquitoes serve in our environment. The mosquito larvae and pupae are important food sources for fish in aquatic ecosystems and adults are fodder for birds and bats, and their most efficient predator, dragonflies. In salt marshes that have not been altered by humans, mosquito eating fish keep mosquito populations under control. Although, literally, a pain, mosquitoes do form part of our natural environment and are essential food, whether in egg, larval, pupal, or adult stages, for our local flora and fauna.
This article is dedicated to Peter Brady, a retired Department of Public Works employee and the mosquito control official for Nantucket for several years. More facts and some great drawing of mosquitoes can be found at http://www.mosquitoes.org/downloads/MosqFacts.pdf.
