Orchids exploit skeeters in an unusual ecological niche
Most people think of mosquitoes only as insects that suck blood, but they have another niche in the ecosystem–they pollinate flowers. Male mosquitoes never bite, and the females need the protein in blood only to produce eggs, so the normal food of adult mosquitoes is actually nectar from plants.
Photo from Phys.org
Though they don’t gather pollen like bees, they fly from flower to flower to feed, and along the way, they carry pollen from one blossom to the other.
In most cases, mosquitoes are just one of many insects that plants use for pollination, so even if mosquitoes were all eradicated by humans, plants would still survive. Some species are more vulnerable than others. One common plant, that isn’t at any risk, though is…
Goldenrod
Goldenrod uses mosquitoes, as well as other insects, for pollination. You might think it appropriate that an annoying insect would help pollinate a flower notorious for causing hay-fever, but scientists believe that goldenrod really isn’t responsible for fall allergies, compared to other less noticeable plants that bloom at the same time, but which have smaller grains of pollen that blow in the wind. [1]
The sticky grains of goldenrod pollen adhere to mosquitoes and other insects as they forage over the blooms.
But there’s no need to worry about goldenrod going extinct. Enough other insects feed on goldenrod nectar that getting rid of mosquitoes wouldn’t have any effect.
Mosquitoes also act as pollinators for grasses and a few other flowering plants. Their role in spreading pollen hasn’t been studied nearly as much as their role in drinking blood and spreading diseases. But the niche they’re best known for is pollination orchids, including the blunt-leaved bog orchid, (Habenaria obtusata, also called Platanthera obtusata), and other rare Arctic bog orchids.
Orchids
John Smith Dexter was the first to report that mosquitoes pollinated orchids, in 1913. [2] A researcher, Ada K. Dietz, noticed a mosquito in Michigan that had two yellow masses of something stuck to its head. Dexter went to investigate and found other similar mosquitoes. “In a few minutes I had caught a half dozen or more, all of them females, bearing the yellow masses,” he reported. He discovered the pollen was from the blunt-leaved bog orchid that was blooming nearby. The mosquitoes who had been feeding on the orchid had one, two or three bits of pollen stuck to their eyes. One had four grains.
Dexter put some of the orchids and some mosquitoes in a glass aquarium and within a few days, most of the mosquitoes also had pollen stuck to their eyes in the same way. They picked them up from feeding on the orchid blooms, and eventually would transfer them to orchids when they fed on them and the pollen fell off.
In the years since Dexter reported his discovery, scientists have found mosquitoes pollinating other varieties of orchids too, particularly in the northern U.S., Canada and Alaska. [3]
The fact that orchids use mosquitoes for pollination is just one more surprise in the wild and crazy history of these rare plants.
Mosquitoes aren’t the only insects which can pollinate the orchids, however. Moths also pollinate the bog orchid that Dexter studied. [4] A few rare Arctic orchids, growing in ecological niches where insects are scarce, may be more dependent on mosquitoes for pollination, though they may also be partially self-pollinating.
Other plants and animals use mosquitoes for food or pollination, but if mosquitoes were entirely eliminating by humans, rare arctic orchids would probably suffer the most, since their ecological niche is more dependent on mosquitoes, and many of them grow where there are few other insect options.
They provide food and pollination, but spread diseases
Mosquitoes can have both positive and negative impacts on the ecosystem. As part of their useful role, the larvae of mosquitoes live in water and provide food for fish and other wildlife, including larger larvae of other species such as dragonflies. The larvae themselves eat microscopic organic matter in the water, helping to recycle it. Adult mosquitoes make up part of the diet of some insect-eating animals, such as birds, bats, adult dragonflies and spiders. They also help pollinate some flowers, when they consume nectar.
But mosquitoes also can have a damaging role, harming other animals by being a vector for diseases, such as malaria, yellow fever, encephalitis and dengue. The mosquitoes don’t cause the diseases themselves, but only act as carriers. They need to feed on a person or animal who is already infected, then when they bite a healthy person or animal afterwards, they pass on the disease.
In places where a particular disease is not already present, there’s no risk of catching it from mosquitoes, but ecologists worry, because if infected humans or animals do come into the area, the mosquitoes that already live there will spread the disease among the rest of the healthy population.
Usually the bites themselves are just an annoyance if there’s no chance of illness, but sometimes mosquitoes are so overwhelming, their sheer numbers have a major impact. In some places around the Arctic where summers are short, mosquito season is brief, but so many are competing for a bite of mammal blood that they cluster in dense swarms, causing caribou herds to flee.
As annoying and dangerous as mosquitoes are, scientists don’t want to recommend exerminating all of them, or any other species, unless they’re sure there won’t be any unintended bad consequences. Thirty years ago, people understood that eradicating one species might affect others, but they looked more for large, obvious changes. Today, scientists examine the ecosystem more closely, since even small changes can produce large alterations over time. Bugs, in general, have a long history of both helping and hurting human life, and interacting with people and the ecosystem in ways one might not expect. A humorous, scientifically accurate book about them, by a professor of entomology is Bugs in the System: Insects And Their Impact On Human Affairs.
The problem with getting rid of all mosquitoes is that we don’t know everything about them yet. They may be useful in ways we can’t imagine. John Carlson, studying at Tulane University, wrote, about Costa Rica, “If all of the mosquitoes were killed, the ecosystem would probably not suffer, unless the poisons used to kill them also killed organisms that are required for the balance of the rainforests.”
But he cautioned that we also can’t be sure that some useful chemical might one day be found in mosquitoes, so they may have some value in the future that we’re not even aware of now.
Still, many scientists think the world would survive the loss of mosquitoes without too much damage. The problems may be caused more by how we get rid of them.
Even if mosquitoes themselves don’t have a vital role in a particular ecosystem, insecticides used to kill them may harm other creatures that do. A Florida mosquito control white paper points out that some small organisms, such as arthropods the same size as mosquito larvae, may be more vulnerable to pesticides than the mosquitoes themselves. Trying to kill off the mosquitoes may kill off all similar creatures. Fish and other animals which ate mosquito larvae wouldn’t be able to switch to another diet, because not enough similar creatures would survive. So scientists not only need to learn the role of mosquitoes in the ecosystem and judge how important they are, they also need to study the role of any other creatures that might be affected by our attempts to get rid of them.
A possible way of eradicating them without killing any other living things, is a new plan scientists are working on, to release males with a lethal gene which prevents their offspring from surviving.
One company has made a mosquito which won’t live unless it receives the antibiotic tetracycline, according to an article in the Oct. 30, 2011 New York Times. The company can breed the mosquitoes in a lab by giving them the antibiotic to keep them alive, then introduce them in the wild, where they will live long enough to mate with normal females, but their offspring will die. The company released 19,000 of the special mosquitoes in 2009 on a Grand Cayman island, and the males with the lethal gene successfully reduced the mosquito population, though they still weren’t as successful at breeding as normal males, so many of the next generation were normal and survived.
Unlike other insecticides, this way of controlling them can only affect mosquitoes, but there are still ethical concerns.
Though scientists are fairly sure that mosquitoes’ useful role in the ecosystem is small enough that other insects could take over, if we ever have the ability to eliminate them completely we’ll want to be sure they’re not serving some purpose we’re currently unaware of.
Tag : mosquitoes role in the environment, what is the role of mosquitoes in nature
Mosquito season starts slowly in the spring, when warm weather brings out the first of the bugs, peaks in summer, and tapers off into fall, when humans and animals finally get a break from the annoying and sometimes dangerous bites of these little critters. That’s if they live where winters get cold.
Photo from Mosquito Magnet
Mosquitoes don’t go away for good until the first freeze, followed by temperatures consistently below 50 degrees. In the southern continental U.S. and Hawaii, there may be at least a little mosquito activity year round.
In locations where a warm spring follows a cold winter, some species of mosquitoes emerge from hibernation, while others are born from eggs laid last year. Different species have different life cycles, habitats and tolerance to cold, but almost all the females need to start looking for a meal of blood after they’ve mated, in order to produce eggs. That’s when humans begin to notice them.
The exact start of the season depends on both temperature and rainfall. Mosquitoes that hibernate need warm weather to become active, while mosquitoes that spend the winter as eggs need rainfall to flood the eggs and make them hatch.
Since weather makes such a difference, mosquito activity can begin at different times each year, and even in the same year, some species may become active before others. Still, it’s possible to estimate roughly when mosquito season will begin and end, depending on what part of the country you live in.
As the hot weather of summer arrives, mosquito season reaches its peak. The warm temperatures make them pass through their life cycle faster, so more are laying eggs and more eggs are hatching.
By the end of the summer, you may notice a decline in bites, since there are fewer mosquitoes around. Those which were born earlier in the summer are gradually disappearing from accidents and predators, and fewer new eggs are hatching.
At the end of the season, mosquitoes species which die off for the winter won’t disappear completely until frost, though they become less active as temperatures drop below 50 degrees. Those that hibernate as adults will begin their dormancy when winter weather arrives, but some may come out on warm winter days, so you might see a few any time of year that the temperature is warm enough. Still, you’re less apt to be bitten as fall and winter come.
Scientists have tried to predict exactly what makes some mosquito seasons worse, but there are several factors. One study found that Culexmosquitoes were most apt to be infected by West Nile virus when the temperature was warmer, though wetter conditions sometimes also made a difference. Still, it came down to weather more than any other factor. “Overall, 80% of the weekly variation in mosquito infection was explained by prior weather conditions,” according to the study. [1]
Mosquito season varies from year to year depending on temperature, moisture and other conditions. The map shows approximate periods when mosquitoes may be most active, with activity tapering off for a month or more before and after. Alaska’s short but strong mosquito season peaks in June and July. In Hawaii, mosquitoes are least noticeable in fall, but are active most of the rest of the year.
Time to Prepare
Though no one can change the temperature or rainfall to control when mosquitoes begin their season, humans can discourage them from breeding by eliminating standing water in backyards, around houses, in gutters and ditches. If you want to keep some shallow standing water, such as in a lily-pond or bird-bath, you can use mosquito control methods such as dunks to prevent mosquito larvae from surviving.
The start of mosquito season is also the time to begin control methods such as traps or sprays, to make sure window and door screens are secure, to put up screened gazebos for outside use or to begin wearing long clothing or using repellents for camping or other outdoor activities, especially at dawn and dusk.
Health Departments and Extension Agents around the country offer advice on how to deal with the local mosquito population. For example, the City of Nashville’s health department holds a Backyard Inspection Day at the beginning of May each year, to usher in the start of mosquito season. The goal is to educate people about safe ways to prevent mosquito bites and to encourage them to eliminate breeding sites in backyards, such as anything that will hold stagnant water.
Alaska’s infamous mosquito season is short, but not sweet. It peaks from the middle of June to the end of July, when swarms can be severe, but the mosquitoes aren’t as bad in cities, above the tree line, or when there’s a breeze, according to Alaska.org, which offers other tips for vacationers who don’t want to get bit.
Whether you’re dealing with mosquitoes at home or planning a vacation, it’s useful to be aware of when mosquito season is likely to be at its peak. The website Weather.com has a handy customized mosquito activity forecast, where you can type in your zip code and see a prediction of mosquito activity for the next two days. It’s based on local weather conditions as well as the time of year, so it’s updated regularly for specific local conditions, though of course can only be an estimate.
Mosquitoes go through their complex life cycle quickly, typically maturing to adults in a week or two. Adults then live a few weeks to several months more, while they breed and lay eggs to start the next generation.
All mosquitoes begin their life cycle as eggs, laid either on or near water. The eggs are tiny, less than a millimeter long. They begin white, but darken to black, brown or reddish brown within the first day.
Some species lay their eggs connected together in rafts which rest on the surface of the water, while others lay the eggs so they float individually. These eggs hatch within a few days after they’re laid, but other mosquitoes, typically of the Aedes genus, lay eggs on the soil near water, rather than on the water itself. Their eggs can survive long periods of drought or cold, before rain or snow-melt comes and washes them into water or raises water levels to where they are. Aedes eggs may require certain conditions before they’ll hatch, such as a certain day length, a period of freezing, or being wetted and dried several times. This prevents them from hatching in midwinter or too soon in spring, before conditions are right. These kinds of eggs can be transported hundreds of miles before they hatch, and in fact, that’s how the Asian tiger mosquito came to the United States from the other side of the globe.
Going Snorkeling
Once any kind of mosquito eggs hatch, they producing the next stage in the life cycle, the larva (plural “larvae”). They’re commonly called wrigglers or wigglers, because they move in the water by wriggling. You can see them swimming or hanging close to the surface in rain barrels, flooded ditches, the edges of swamps or other stagnant water. They live in water but breath air, so like snorkelers, they have a breathing tube and must come close to the surface often to get oxygen. When resting, they usually hang down from the surface or, in some species, lie horizontally just below it.
The larvae eat tiny organic particles in the water. A few of the larger species, like those of the Toxorhynchitesgenus, eat smaller creatures, including other mosquito larvae. While the larvae feed, they grow, and as they get larger, they shed their skin. Each stage in between shedding is called an “instar,” and by the fourth instar, the larvae are ready to go on to the next stage in their life cycle, the pupa (plural “pupae”). The larvae typically go from egg to pupa in less than a week, though some take a week or more.
Almost Done: Pupae
Just as caterpillars emerge from coccoons to fly away as butterflies, the little wrigglers go into a pupal stage so they can eventually fly away as mosquitoes. The pupae still live in the water and move around, though they look different. The larvae are generally long and straight, while the pupae are comma-shaped, with a larger section that contains the mosquito’s head and front part of its body. The thinner curved section has paddles at the end which allow it to swim, though it doesn’t eat during this stage. It still needs to come to the surface to breathe through tubes on its head.
The pupal stage may last a few days, after which the adult mosquito emerges from the top of the pupa up into the air, for the final stage in the life cycle. A few species, though, spend the winter in their underwater stages and don’t emerge to fly away as adults until the next spring.
Adults
Both male and female adults look similar at a distance. Both buzz, though only the female bites. With a magnifying glass, you can see that the mouth parts of the female are uniquely adapted to be able to bite animals, as well as to feed on nectar and similar plant products, which are the usual food of both males and females.
After maturing for three to five days, the adults mate. The male dies within a week or so, but the female may live several weeks or even months, if she belongs to one of the species which hibernate over the winter as adults. After she mates, she needs to find an animal to get a meal of blood, to make the eggs grow within her. Though each female mates only once, she may lay several sets of eggs during her lifetime.
Different species prefer different hosts. Some will bite either humans, animals or birds, while others prefer one type, and a few choose cold-blooded animals. A small number of species don’t need blood at all, though most do.
After drinking her fill of blood, either in one bite or several, the female finds a sheltered place to rest for several days until her eggs are fully grown inside her. At that point, she will look for a location to lay the eggs, either on water or near it, depending on her species.
After laying, she’ll look for another creature to bite, so she can grow another batch of eggs, but she won’t need to mate again. Females may not survive long enough to lay one batch of eggs, or they may lay only one, or they may survive to lay several during one summer. At the end of summer, the female will either die or, if she belongs to a species that hibernates, will spend the winter hibernating with fertile eggs inside her, waiting to lay them in the spring.
Larvae live in stagnant water & adult females need blood to drink.
Mosquitoes multiply fastest in tropical and sub-tropical habitats, but they have managed to adapt to a wide variety of habitats. The eggs or larvae of some kinds can survive frozen in ice, while others hibernate as adults so they can live where snow covers the ground for months and the temperature falls below zero. Thanks to their adaptability, mosquitoes thrive on every continent except Antarctica.
Mosquitoes need a habitat with stagnant or slow-moving water, since that’s where their eggs hatch and where their larvae live until they’re old enough to transform into adults. They pass through the larval stage quickly, though, sometimes going from eggs to adults in a week or less, so they don’t necessarily need permanent water sources. They can survive even in places where temporary puddles appear after a hard rain and dry up soon after. Warm summer weather encourages mosquitoes to go from egg to adult the fastest. “The shortest life cycle on record for a mosquito is about 4.5 days,” according to “Mosquito Breeding Habitats,”by Dr. David N. Gaines, Public Health Entomologist.
Larvae have difficulty living in fast-moving water, though some kinds can survive in woodland creeks. “Stream breeders will find vegetation along banks with which to anchor themselves or attempt to remain away from the main flower of the stream by seeking isolated eddies,” according to Larval Habitats of Mosquitoes, posted by the Rutgers Center for Vector Biology.
Besides water, the other thing that almost all female mosquitoes need in their habitat is a source of blood. The males survive on nectar and other non-living food resources, but the females of most species use the nutrition in other creatures’ blood to develop their eggs, so they need a breeding site close enough to humans, livestock, wild animals or birds so they can get nourishment.
Though each species of mosquitoes may prefer a slightly different habitat, almost all kinds of still or slow-moving water will be attractive to at least some kind of mosquito, unless humans take action to discourage them.
The species Aedes sollicitans prefers stagnant salt marshes along the ocean for its eggs and larvae, but will fly 10 miles or more inland to find animals to bite. Culex salinarius will also breed in salt water marshes, or even roadside ditches filled with cattails.
Anopheles quadrimaculatus, a common mosquito in the United States which used to spread malaria, is more particular about where it lays its eggs. It prefers clear, shallow water, that’s not too acid or stagnant, but it has found plenty of suitable habitat, wherever there are ponds, slow-flowing streams or the kind of clean, attractive bodies of water with shallow edges, that humans like to live near.
The CDC says that Water Hyacinth can be a breeding habitat for mosquitoes, whose larvae use the plants to anchor themselves against waves.
Aedes aegypti, the mosquito which used to spread yellow fever in the United States and is still common here, will breed in cities, needing only the standing water in old tires, barrels or gutters. Aedes albopictus, the invasive Asian tiger mosquito, can live in a similar habitat. Introduced to the United States in the 1980s, it has spread over most of the southeast and is taking over some of the yellow fever mosquito’s habitat. Both can find blood nearby, from humans, birds and pets.
Some mosquitoes prefer flooded rice fields, Florida swamps, irrigation ditches out west or northern forests, but all their habitats contain some kind of water and some kind of food–usually the kind of food that doesn’t want to be eaten, like us and our animals.
Changing Habitat to Get Rid of Mosquitoes
One way to have fewer mosquitoes is to get rid of standing water, so their larvae can’t survive. Some forms of standing water are unattractive to humans, so removing discarded tires, old cans and other trash that holds rainwater beautifies the landscape as well as discouraging the little pests.
Other forms of standing water are a necessary part of human activity, such as drainage ditches and culverts for roads and fields, watering tanks for domestic livestock, or the puddles caused by farm irrigation. Natural wetlands are important habitats for useful and necessary species, and even dry woodland contains holes in trees or low spots that collect water or melting snow. Humans even create some kinds of standing water to beautify the landscape, by building ornamental ponds or installing birdbaths. Mosquitoes consider these perfect breeding grounds. In those cases, other ways of getting rid of mosquitoes may be necessary.
Sometimes slightly altering the habitat will discourage them. They don’t like deep ponds or ponds that also have a flourishing population of predators that like to eat mosquito larvae, such as frogs, fish and insects and spiders. Other options are trapping and killing the adults, or using dunks or mosquito bits that are easy to add to water.
They spread a bacillus which is a natural enemy of the larvae but won’t harm fish, birds, people or pets. When mosquito larvae swallow Bacillus thuringiensis israelensis, it damages their mid-gut cells, but is harmless to almost all other species except for a few closely related flies, according to the article “Mosquitoes” by Bruce Eldridge, former director of the UC Mosquito Research Program. The article also contains other useful information on controlling mosquitoes by altering their habitat.
Fortunately, we have lots of practice in controlling them
Researchers agree that global warming will increase the number of mosquitoes, but they disagree whether we can do anything about them and, more importantly, about an outbreak of mosquito-borne diseases.
Global warming is controversial in general, not just on the topic of mosquitoes. Even readers of children’s books divide radically into both one-star and five-star reviews for the same book.
Adult mosquitoes like temperatures above 50 degrees, so there’s no doubt more mosquitoes will be active if the average temperature rises. They also need wet conditions to breed, and global warming can create more of that in some areas, even if it makes other areas drier.
Scary scenarios
Some of the scarier scenarios were summaried by Sheila Dichoso of Northwestern University in a post reporting on the 2009 conference of the American Association for the Advancement of Science. There, experts on the environment and public health warned that diseases have increased since 1970, including West Nile virus, malaria, yellow fever and the plague, all spread by mosquitoes.
But not all experts agree. More recent research indicates that it takes more than warmer, wetter conditions to spread disease, even if there are more mosquitoes.
Human intervention
Although malaria and similar diseases are more common in the tropics today, entomologist Paul Reiter argues that what really matters, in stopping malaria, is human intervention. [1] People have learned to screen out mosquitoes, get rid of standing water, drain marshes and swamps, and spray mosquito habitats, as well as give better medical care to those suffering from the disease.
Peter Gething published information about the historical incidence of malaria in the journal Nature in 2010, showing that between 1900 and 2007, there had been a decrease in the disease, in both its range and intensity, even though global temperatures had increased.
The southeastern U.S., which has always been warm and humid, had period outbreaks of malaria in 1900, but that no longer happens today. The hardest place to fight the disease is central and western Africa, where there’s widespread infection of malaria in the population and mosquitoes bite year round. But Gething and his co-authors argued that climate change is not the main factor in the spread or control of malaria.
Encephalitis carried by mosquitoes also increases when temperatures and rainfall increase, according to Michael Gale Moore’s book, Climate of Fear. But diseases like yellow fever and dengue, as well as malaria, used to occur regularly in the United States, and now they’re rare, despite warmer temperatures, so we have learned to control them.
He cites an example of an outbreak of dengue in 1995 in the Caribbean, Mexico and Central America, with 74,000 cases of the disease reported. Some areas, however, were barely affected, while people living nearby suffered from the epidemic. “The contrast between the twin cities of Reynosa, Mexico, which suffered 2,361 cases, and Hidalgo, Texas, just across the border, is striking,” he writes. “Including the border towns, Texas reported only 8 nonimported cases for the whole state.” Better sanitation, education, elimination of standing water, window screens, and other control methods made the difference. [2]
The U.S. Environmental Protection Agency, citing information from the World Health Organization, is also cautiously optimistic, noting that climate change can provide new opportunities for mosquito-borne diseases, but the actual increase “will depend not only on climatic but also on non-climatic factors, primarily the effectiveness of the public health system.” [3]
Mosquitoes are Adapting
But even if global warming doesn’t cause an upswing in diseases, it will probably cause an increase in mosquitoes which will need controlled. Mosquitoes have survived millions of years because they’re adaptable. Scientists have already noticed how they’re responding to climate change, to increase their survival.
Pitcher plant mosquitoes, Wyeomyia smithii, spend their winters hibernating as larvae in water trapped inside the pitcher plant. They use day length, rather than temperature, to signal when to go into hibernation.
As the climate changes and winters get shorter, the larvae who take advantage of a few extra days of warm weather before hibernating are more “fit” for the new environment. Scientists have noticed those larvae do survive better, and therefore the hibernation period of pitcher plant mosquitoes has changed because temperatures have become warmer. “Between 1972 and 1996 mosquito populations at 50 N latitude evolved to wait 9 days later to go dormant,” according to an article on microevolution at the Understanding Evolution website. [4]
So there is no doubt that mosquitoes will be able to adapt and thrive, as global warming creates temperatures which favor them. The good news is that humans have had decades of success in learning how to control them, as well as the diseases they spread. If we apply what we’ve learned, we should be able to prevent disaster in the long run.
Asian Tiger Mosquito – Mosquito with black & white stripes
If you saw a mosquito that has black and white stripes on its body and legs, you may have seen one of the Asian tiger mosquitoes that are invading the United States. Originally from Asia, they first came to this country in the 1980s as eggs which stowed away in used tires and other cargo. The eggs hatched and found the new habitat to their liking. Asian tiger mosquitoes have now spread over most of the southeastern United states. Their Latin name is Aedes albopictus, though some scientists also identify them as Stegomyia albopicta.
The Asian tiger mosquito, an invasive species, was first found in the continental U.S. in August, 1985, in Harris County, Texas, in significant numbers. Two researchers published an article about the discovery in the Journal of the American Mosquito Control Association. [1] The next year, more tiger mosquitoes turned up around a pile of used tires near Jacksonville, Florida. [2] Since then, they have been spreading rapidly, from both new arrivals and breeding.
Like other invasive species, the Asian tiger, Aedes albopictus, has out-competed mosquitoes who already lived here, such as Aedes aegypti, the yellow fever mosquito, but that’s not necessarily good. The tiger mosquito can spread diseases too, such as dengue fever or Eastern equine encephalitis. In fact, its habit of staying close to where people live and biting multiple hosts in the daytime, make it an even more efficient vector for some diseases. We’re lucky that it came to the United States as uninfected eggs, rather than as infected adult mosquitoes, and that not enough people or animals infected with the diseases it spreads are available here, so no epidemics have started. Yet…
By now, tiger mosquitoes live in most of the southeastern United States, as shown by the orange areas in the map below. Other places, such as California, have also reported invasions which officials are trying to eradicate before they become permanent.
Orange areas show where Asian tiger mosquitoes are naturalized. For a more detailed county-by-county map, see the CDC’s map.
To see close-up photos of the larva, pupa, and adult stages, with identifying points, visit this website, produced by the University of Florida. You’d need a powerful magnifying glass or microscope to see some of the details, but the bold black and white stripes are easier to notice.
The adults are average in size for mosquitoes, ranging from 2 to 10 millimeters long, but only a few reach the upper end of that range. The males are about 20 percent smaller than the females.
The invading Asian Tiger Mosquito has black and white stripes on its body and legs.
You can identify them not only by their stripes, but by the way they behave. Most mosquitoes bite between dusk and dawn, but fierce tiger mosquitoes live up to their name, biting during the day, especially in the afternoon. For this reason, they’re sometimes also called “forest day mosquitoes.” Both males and females have white stripes along the thorax, or body, though you’re more apt to notice the females, because they’re the ones who bite. Like other mosquitoes, they sense carbon dioxide and typical chemicals and smells which humans give off, so fortunately they can be caught in the same traps that give off similar vapors to attract and kill other kinds of mosquitoes.
Tiger mosquitoes bite birds and mammals as well as people, but they’re easily scared away while biting, so they may draw a little blood from several different humans or animals before they get one full meal. This characteristic makes them more than just an annoyance, because they can spread dangerous diseases from one host to another, such as dengue fever, encephalitis and yellow fever, as well as heartworm in cats and dogs. In 2005-2006, they caused an epidemic of Chikungunya fever on La Reunion island, off the southeast coast of Africa, that sickened a quarter million people and killed 248.
Asian tiger mosquitoes can transmit diseases to humans, but have not yet become a significant vector in the United States. They can carry several kinds of encephalitis viruses, as well as dengue and Cache Valley virus, though other moquitoes do too, and no significant outbreaks have occurred in the United States because of the new invasion. Scientists are watching the situation, monitoring the spread of the mosquitoes as well as testing them to see if they’re carrying any disease.
According to the Centers for Disease Control, Eastern equine encephalomyelitis and Cache Valley viruses have actually been found in Asian tiger mosquitoes in the United States, though they’re potential carriers of additional diseases, if people or animals who have yellow fever or other diseases come to the United States and get bitten. Right now, that’s not a danger yet.
In North America, some other mosquitoes have a similar pattern of black and white stripes on their legs, but the Asian tiger is unique in having the same striking stripes on its body as well. You’re most apt to encounter the Asian tiger in the areas in orange on the map below. Based on the CDC’s county-by-county map, it shows where this pest is naturalized, but new populations are being discovered and, hopefully, eradicated, so people are occasionally finding them outside this area.
Though they first permanently settled here in the 1980s, a few eggs arrived in shipments even before then. Some were found in California in 1971, but officials worked quickly to kill them off. This article describes California’s efforts to keep them from naturalizing in that state. Because their eggs are so small and can survive long periods of drying, keeping them out of the United States is almost impossible, as long as shipments of cargo continued to arrive from areas of Asia where they live. Many eggs rode in used tires that had contained standing water, and California also received some eggs that hitched a ride with “Lucky Bamboo” from south China, sold as an ornamental plant in the United States.
For more information and resources about this invasive species, what diseases it can carry and how it’s being controlled, see the USDA species profile.
Controversial insecticide is still used in the third world.
Dichloro Diphenyl Trichlorothane ( DDT ) was once a common insecticide in the United States, but it was banned in 1972 due to health concerns and danger to other wildlife. It’s still used in other countries to kill mosquitoes that spread malaria and other diseases, and that’s why there’s a problem when mosquitoes become resistant to DDT.
Malaria sickened 225 million people and caused 781,000 deaths in 2009, according to the World Health Organization (WHO). Most of the deaths occurred among African children, where 20 percent of childhood deaths are caused by malaria. The only way to get malaria is by being bitten by an Anopheles mosquito which is carrying the disease, so reducing the number of mosquitoes, preventing mosquito bites, or shortening the lifespan of mosquitoes so the disease can’t develop inside them, will eliminate malaria.
The WHO currently recommends pyrethrum-impregnated mosquito nets in countries with malaria problems, but also recommends spraying: “Indoor residual spraying (IRS) with insecticides is the most powerful way to rapidly reduce malaria transmission,” according to a WHO fact sheet on malaria. “DDT can be effective for 9-12 months in some cases.” [1] But if mosquitoes develop resistance to DDT, it becomes less effective.
Even though DDT causes serious health problems in humans, some people, like this author, defend its use, and blame its poor reputation on distortions by the mainstream media.
DDT use peaked in the United States in 1959, when 79 million pounds were spread and manufacturers exported another 76 million pounds to other countries, but soon afterwards, increasing insect resistance and tighter safety regulation caused a decline. By the time the Environmental Protection Agency banned it for crops in the United States in 1972, most of it was being used only for pests on cotton crops. [2]
Health Dangers
Several serious human health problems, including diabetes and hormonal disruptions, have been linked to DDT, and it is suspected to be carcinogenic. It also can harm other unintended species, such as when it endangered Bald Eagles in the United States before it was banned. For these reasons, its use is only justifiable as a last resort where greater health problems are being spread by mosquitoes or insects, where the mosquitoes aren’t resistant, and no other effective control is practical. Though the WHO still recommends DDT, it hopes to phase out its use as better and safer control methods are developed. [3]
Alternatives to DDT include pyrethroids, organophosphate insecticides such as malathion, and carbamate insecticides, but they also come with problems. They are more expensive, though pyrethroids last longer, so the actual expense is about the same, but they have their own health side effects and mosquitoes can develop resistance to them as well.
Other alternatives include solutions such as eliminating mosquito larvae habitats by draining ditches and standing water, though this may be difficult or impossible in some areas. Netting over beds to prevent mosquitoes from biting is a safer alternative, though not effective for people who need to be outdoors or working in the evening and night.
Failed Worldwide Campaign
Paul Muller of Switzerland first recognized DDT’s value as an insecticide. During World War II, the United States and its allies sprayed it in the South Pacific to control mosquitoes that spread malaria and dengue fever. Though malaria was already in decline in the U.S. and Europe, the use of DDT helped hasten its eradication there also.
The World Health Organization attempted a major world-wide malaria eradication program, starting in 1955, that emphasized the use of DDT as well as medical treatment and inspection. Though there were some successes, lack of funding, wars, supply shortages, and increased mosquito resistance to insecticides forced the WHO to abandon the plan for eradication. Instead, they now hope to reduce the amount of disease and death. [4]
Resistance was recognized as a potential problem within a few years after widespread spraying began. Mosquitoes which survived the toxins in DDT lived to breed and pass on whatever small amount of resistant they naturally had. As more generations passed with only the survivors breeding, their resistance increased, and fewer succumbed to the toxins.
In 2008, scientists discovered exactly how some Anopheles gambiae mosquitoes were able to survive the poison. They found that resistant ones produced more of a specific protein, which helped them metabolize DDT. Scientists hope that better understanding why some individual insects are able to withstand the pesticide will help them develop a way to overcome the resistance. [5]
Still Repellant
Even though mosquitoes are becoming resistant to the fatal effects of DDT, one study in 2007 showed that more than half of resistant Aedes aegypti mosquitoes were still repelled by it when it was sprayed in huts. Aedes mosquitoes carry yellow fever and dengue, but scientists predicted that Anopheles mosquitoes, which carry malaria, would be at least as likely to be repelled, since they are normally more sensitive to DDT. This may explain why DDT still helps reduce diseases in areas like India, where most mosquitoes have become resistant. [6]
The video at right is a 1944 educational film about DDT, made by the U.S. War Department during World War II. It is presented as a historical artifact and is not necessarily scientifically correct today, nor politically correct either!
Mosquitoes come in hundreds of different kinds, divided by species
There are hundreds of different species of mosquitoes, so many that even scientists don’t agree on how they should be classified. Researchers have spent thousands of hours exploring, catching and staring at mosquitoes under magnifying glasses and microscopes, grouping them into genera (the plural of genus) and species.
Can you imagine paying this much just for one professor’s book about mosquitoes? Amazing. But people who dedicate their lives to learning about these little insects are actually doing important work, because each kind of mosquito has a slightly different secret for surviving cold, drought, predators and other dangers, and different types of mosquitoes carry different diseases. By classifying them into species and studying their habits, scientists can figure out the best way to get rid of the ones that carry the worst diseases.
Listed below are the major genera of mosquitoes found in the United States and some of the important species within each genus. For illustrations of the main differences between the eggs, larvae, pupae and adults of three common genera, Anopheles, Aedes and Culex, see this pdf chart.
ANOPHELES
There’s at least one species of the Anopheles genus in every state of the U.S. except Hawaii. They’re the easiest genus to recognize because they rest with their body slanted, unlike other genera who keep their body level. They used to carry malaria in the United States, though they rarely do now, but they still spread the disease in other countries.
The adult females lay their eggs individually in pools of water, with little floats attached to keep the eggs on the surface until they hatch. They can lay over a hundred eggs after each breeding. After a few days, the eggs hatch, producing larvae which become adults in a week or several weeks. The larvae rest parallel to the water surface, rather than hanging down from it like most mosquito larvae.
Anopheles mosquitoes usually bite at night, just after dusk and just before dawn. Fertilized females spend the winter hibernating.
Anopheles quadrimaculatus, the malaria mosquito, enters houses and is the mosquito of this genus most apt to bite people. It lives throughout the eastern and central U.S. and lays its eggs in the shallow, clear water of swamps and ponds which are not too stagnant or acid. They bite at night, choosing livestock as well as people. In winter, the fertilized females hibernate in protected areas, coming out on warm days.
Anopheles walkeri lives in places similar to quadrimaculatus, though it overwinters in egg form, producing a special “winter egg” to survive the cold.
Anopheles freeborni lives in the western half of the United States, and bites aggressively at twilight and dawn, choosing both humans and animals and entering houses or barns. It can spread malaria and used to be a significant vector of that disease in the west. Its larvae develop in the standing water of irrigation canals, stream edges and rice fields. The adults hibernate, coming out on warm days, then beginning to breed in early spring.
AEDES
About half of the mosquitoes in North America are in the Aedes genus. They thrive in cooler temperatures, not requiring tropical climates. Their eggs can withstand long periods of being dry or cold, so in cold climates, the adults die off while the eggs survive until spring. They lay eggs individually at or above the waterline or on dry ground. The eggs wait until wet weather or flooding dampens them, before they hatch. The larvae live in puddles, pools, marshes or wherever there’s temporary standing water. The adults usually bite in the evening, though some species bite in the day or night. Aedes mosquitoes carry diseases, and can be vicious biters, found in large numbers.
Aedes aegypti was infamous for spreading yellow fever. Originally from Africa, it’s now found throughout most of the southeast U.S. and the larvae will thrive in any stagnant water around cities or homes, from old tires to gutters. Sensitive to cold, it overwinters by leaving eggs to hatch when warm weather returns, but can go from egg to adult in 10 days when temperatures are hot. It prefers to bite in the morning or late afternoon rather than at night and will choose people over animals.
Aedes albopictus, the Asian tiger mosquito, arrived in the U.S. as eggs or larvae in cargo and now is established throughout the east and midwest. It is a possible vector for dengue, dog heartworm and encephalitis, and can breed in small water containers like Aedes aegypti, which it is out-competing in some areas.
Aedes canadensis is common in the northern U.S., though it breeds in woodland habitats and stays close there, rather than venturing to cities. In the spring, it appears early in the year, at the first sign of warm weather in March. It can carry equine encephalitis and dog heartworm.
Aedes sollicitans breeds in salt marshes along the mid and north Atlantic coast. The adults can swarm and migrate in the evenings, congregating in towns 10 miles or more from the marshes, where they bite aggressively at night and even in the day if stirred up from their hiding places. They carry Eastern equine encephalitis to horses and people.
Aedes nigromaculis breeds in floodwaters and irrigated areas of the western plains, as far south as Mexico, and California. It bites aggressively and painfully, and can spread encephalitis.
CULEX
Culex mosquitoes prefer the tropics, but there are about a dozen species somewhat common in the United States. They lay their eggs connected together in groups called “rafts,” which float on quiet pools of water as big as lakes or as small as buckets, or as stinky as sewage cesspools. One raft may contain a hundred or more eggs, which hatch in two or three days. The adults usually bite in the evening or at night and have been blamed on disease outbreaks in several places.
Culex erraticus is a small, dark species of the southeastern U.S. The larvae live in ponds and are also found in Arkansas rice fields. The adults bite hard, but prefer birds to people.
Culex nigripalpus prefers the warm temperatures of Florida and has been linked to outbreaks of St. Louis encephalitis there.
Culex stigmatosoma is common in California and the Pacific coast in general, but prefers biting animals to humans.
Culex pipiens, also called Culex fatigans, the house mosquito, is divided into several subspecies, but at least one kind is found in every state. The adults don’t fly far, but their larvae thrive in any stagnant water, so they’re common around houses wherever water is allowed to accumulate in puddles, old tires, barrels or gutters. They can carry St. Louis encephalitis.
Culex salinarius doesn’t mind salt water, so it lives near the Atlantic and Gulf coasts, biting at night. The larvae live in either fresh or salt marshes, roadside ditches or cattail bogs.
Culex tarsalis spreads encephalitis to people and horses west of the Mississippi, and occasionally is found in the east too. The larvae can live in irrigation puddles, ditches, ponds, drinking troughs or open cesspools. The adults bite at night, preferring birds but also biting livestock and humans. They hide in sheltered areas during the day and, like most Culex mosquitoes, survive the winter by hibernating.
CULISETA
There are eight species of the Culiseta genus in the United States. They are unusual in that they don’t mind cold weather. In the south, some will come out of hibernation in the middle of winter if there are a few warm days. The adults usually prefer to bite animals rather than people and they have been blamed for outbreaks of equine encephalitis.
Culiseta incidens has spotted wings. It’s generally found from Texas west and prefers biting livestock rather than people.
Culiseta inornata lives everywhere in the United States outside of northern New England. It prefers biting animals to people and doesn’t mind cold weather, sometimes coming out of hibernation in the winter while snow is still on the ground, earning the nickname “snow mosquito.”
Culiseta melanura lives throughout the eastern United States. Scientists believe it’s the main vector for spreading Eastern equine encephalitis between birds, but it rarely bites humans.
MANSONIA
The Mansonia genus has only one species of importance in the United States, since it prefers tropical weather.
Mansonia titillans lay their eggs on the underside of water lettuce leaves in Florida. The larvae live among the roots of the plant and grow slowly, emerging the next year as adults that bite aggressively.
COQUILLETTIDIA
Mosquitoes of this genus lay their eggs in rafts, like the Culex genus. Unlike Culex, when the eggs hatch, the larvae live underwater, breathing air through tubes they insert into the roots of aquatic plants, and don’t turn into adults until the following year.
The most significant species in the U.S., Coquillettidia pertubans is found in most of the eastern states and some of the midwest. They’re vicious biters, mostly in the evening and night, indoors and outdoors, but they’ll also come out to bite during the day in damp, shady places.
PSOROPHORA
There are fifteen species of this genus in the United States, in the south and east. Their eggs, which they lay on the ground, can withstand months of drying, then hatch quickly when floodwaters come.
Psorophora ciliata is a big yellow-brown mosquito found throughout the eastern U.S., called the “gallinipper.” It’s the bully of the mosquito world. The larvae are two or three times bigger than most, and they eat insects including other mosquito larvae. The adults bite aggressively day and night.
Psorophora columbiae (also called P. confinnis) typically lives in rice fields, though is found elsewhere. It’s aggressive enough to swarm and kill livestock and drive people indoors.
Psorophora columbiae also likes rice fields or other places that flood occasionally. The eggs can grow to adults within a week in warm wet weather and the adults may live for a few weeks or a couple of months.
Psorophora cyanescens is a shiny blue color and is found over most of the south.
Psorophora ferox is bright bluish purple and prefers swampy woods in the south, west to Texas and north to Nebraska.
Psorophora signipennis lives in the midwest from Texas to North Dakota and has a painful bite.
TOXORHYNCHITES
A few other minor genera live in the U.S. One of them is unusual: a mosquito that doesn’t bite! Toxorhynchites doesn’t suck blood. Its big larvae actually eat other mosquito larvae, while the adults live on nectar and fruit. We could use some more of them!
This pdf article from the Centers for Disease Control tells more about common mosquito species in the United States.
Mosquitoes lay eggs which hatch into larvae or “wrigglers” that live for a while in water, then emerge as new adult mosquitoes. Though mosquitoes live on land, they’ve evolved three different ways to get their eggs in the water, so the larvae can hatch where they need to be. The video shows a Culex mosquito laying her eggs to create an egg “raft” and, later, the larvae hatching from the eggs.
Rafts
Mosquitoes of the genus Culex lay hundreds of eggs connected together to form a raft which floats on the surface of still water in a place like a pond, swimming pool, ditch, discarded tire or even a sewage cesspool. When the eggs hatch about a day later, the larvae wriggle free from the bottom of the eggs, swimming directly down into the water. They look like strange alien creatures, as you can see in the video.
To lay the eggs, the mosquito first lands on the surface of the water and takes a drink. Scientists think she’s testing it, to make sure it’s suitable for her larvae, and she may also be filling her abdomen so she can more easily push the eggs out through her oviduct. Though scientists aren’t sure exactly what the mosquito is looking for, they know she likes water where other mosquitoes have already laid rafts and where other larvae are successfully living. When she is satisfied that she has found a good place, she deposits eggs one at a time, close together, so they stick into a raft of a hundred or more.
The more blood the female mosquito drinks, the more eggs she can lay. Scientists have found that for every milligram of human blood, she can lay an average of about 40 eggs. A milligram of blood is tiny, smaller than a grain of sand, so even one good bite can give a mosquito enough nourishment to lay hundreds of eggs at once. The eggs are so small, though, that a single raft of 200 to 300 eggs is only 1/4 inch long. Mosquitoes usually lay the eggs at night and the rafts are so inconspicuous, you may never notice them.
Floats
Mosquitoes of the genus Anopheles also lay a hundred or more eggs at once, but they lay them separately, rather than connecting together, though the eggs will sometimes drift together in a loose mat. Each of the scattered eggs rests on the surface of the water like a tiny boat, because it has a “float” attached on either side. This page from the CDC, which describes the life cycle of the malaria mosquito, has a good drawing of the eggs and their floats.
The floats are “air-filled chambers formed from the outer layer of the egg, the exochorion,” according to Mosquitoes and Their Control by Norbert Becker. The eggs are so small you can barely see them without a magnifying glass. The photo on this page of an Anopheles mosquito laying her eggs shows the mosquito larger than life. You can imagine how small the eggs are compared to a real-life mosquito.
Anopheles mosquito eggs
After two or three days, the floating eggs hatch and the wriggling larvae drop into the water. Both Anopheles and Culex mosquitoes have solved the problem of how to get their larvae into the water the same way, by finding a good pool or puddle of water and floating the eggs directly on top of it, but Aedes mosquitoes do it differently.
Waiting for Water
Mosquitoes of the genus Aedes lay their eggs on soil where the eggs wait like seeds, until spring rain washes them into shallow puddles or pools, or wet weather causes ponds to flood and rise to reach them. When conditions are too dry and cold, the eggs can lie dormant for months, hatching within hours or days when warm wet weather returns.
Aedes mosquitoes need to choose where to lay their eggs carefully. Scientists have determined that the moisture of the soil, the type of soil and the plants nearby all help them decide where to deposit their eggs, but they also choose locations near where other mosquitoes have laid their eggs, resulting in more eggs near the edges of pools which regularly rise and fall in rain and drought. This knowledge can help scientists develop better ways to control them, since they can encourage most mosquitoes in an area to lay in the same place, then use an insecticide that targets their larvae.
Since larvae may not survive if they hatch on an unseasonably warm winter day, too early in the spring, they may enter a condition called diapause.After a long cold period, they may require two weeks or more of warm temperatures before they’ll hatch, or they may wait for longer spring days, to be sure warm weather has finally arrived for good.
Aedes mosquitoes don’t need to survive the winter as adults, since their eggs can wait out the cold months and hatch only when conditions are right in spring. Other kinds, whose eggs float on the water and hatch quickly, must hibernate as adults.
Winter Eggs and Summer Eggs
There are a few kinds of mosquitoes, like Anopheles walkeri, which lay two kinds of eggs, ones that hatch quickly in the summer, and special “winter eggs” that they lay in the fall.
Scientists discovered that the winter eggs, which are larger than the summer ones, won’t hatch unless they have been exposed to freezing temperatures, then warmed up again, to signal that winter has come and gone. The winter eggs appear larger than the summer ones, because they’re protected by “enlarged floats that nearly cover the dorsal surface of the exochorion” or outer shell, according to an article by Wayne J. Crans of Rutgers University.
All these different ways help mosquitoes continue for another generation, but studying them helps people figure out new and better ways to prevent mosquitoes from multiplying.
Tag : What Do Mosquito Eggs Look Like, mosquito eggs in water, where do mosquitoes lay eggs
Photo from Phys.org
