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Volume 38 Issue 8 • June 19 - 25, 2008
now in our 38th season

Zzzzz! Island Mosquitoes

by Dr. Sarah D. Oktay
Managing Director UMass Boston Nantucket Field Station

One of the few unfortunate realities of any summer spent outdoors is an occasional encounter, or ten, with mosquitoes, and Nantucket idyllic shores are no different. Fortunately, sea breezes sometimes can keep these bloodsuckers away and the majority of our mosquitoes are salt marsh species who although aggressive and “daytime biters” often do not carry mosquito borne diseases such as West Nile virus (WNV), malaria, or Eastern Equine Encephalitis virus (EEEv).

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 noshing on fruit and flowers.

Mosquitoes have been around for more than 30 million years. And it seems that, during those millions of years, mosquitoes have been honing their skills so that they are now experts at finding people 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. 

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). 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. 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 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 a great way to remove the standing freshwater habitat some 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. The Chrysanthemum cinerariifolium plant produces pyrethrum, an insecticide used in many products, although researchers have discovered that some chrysanthemums attract and some repel mosquitoes. Ultraviolet lights (as used in bug zappers) and ultrasonic devices are not effective.

On a local level, the Department of Public Works’ mosquito control employee, Peter Brady, collects larval samples around the island and then treats the water 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.

What Brady does not do is spray or fog with Sumithrin or similar products which are permethrin based insecticides that the DPW can only use when Health Inspector Richard Ray and the Board of Health declare a health emergency based on positive test results for any of the aforementioned diseases. 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.

In addition, Brady and I set Center of Disease Control carbon dioxide (CDC/CO2) traps and “stink water traps” around the island to capture mosquitoes in areas of the island where they are common. The CO2 traps are hung from trees and supplied with a light and a fan which are run off of a portable battery. Frozen carbon dioxide, better known as dry ice, 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 (mated and capable of egg laying) female traps and are favored by the Culex species. It is crucial to only leave the traps out for 24 hours and to recover the mosquitoes alive, quickly stun them in a freezer, and get them shipped off as quickly as possible so that any blood they are carrying can be easily analyzed.

Many private individuals allow us to sample their “mosquito magnets” which can be effective if used properly. All the mosquitoes trapped or collected are packed up in sterile plastic centrifuge tubes and shipped overnight on ice to Matt Osborne, the field coordinator for the Arbovirus (arthropods such as mosquitoes and tick borne viruses) Program in 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. Of the over 200 tests done on several thousand mosquitoes collected on Nantucket from 2005 to 2007 no positive hits have occurred for West Nile, malaria, or Eastern Equine Encephalitis virus. Species found in 2006 include 12-17% Ochlerotatus (formerly a subgenus of Aedes) sollicitans (Golden Saltmarsh Mosquito), 35% Coquillettidia perturbans, and 40% Culex salinarius. The remaining were primarily Ochlerotatus cantator and Aedes vexans.

Different traps are designed to trap different species, for instance, most mosquito control experts use the CDC/CO2 traps to catch Culiseta melanura, Culex salinarius and salt marsh mosquitoes such as the Ochlerotatus sollicitans. The gravid trap and associated “stink water” are targeting Culex pipiens, Culex restuans, Ochlerotatus japonicus and Ochlerotatus triseriatus. Where mosquito control can be confusing is in tracing the vectors in which the blood borne disease travels. For instance, 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, for an average of less than four cases annually. Like West Nile virus (WNv), most human cases normally occur in late summer and fall. EEEv 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.

Areas tested include the Coskata Red Cedar swamp, areas around the Point Breeze and the wetlands off of Easton Street, 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. I keep samples of the mosquitoes that don’t get mailed off for students to see what they look like under a microscope. Most of the species can be identified by stripes on their legs or by the length of the proboscis.

As mentioned before, only female mosquitoes bite. The female lands on your skin and sticks her proboscis into you (the proboscis is very sharp and thin, so you may not feel it going in). Her saliva contains proteins (anticoagulants) that prevent your blood from clotting. She sucks your blood into her abdomen. 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.

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.

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