Mosquito bites are annoying, and some mosquitoes spread diseases such as malaria and West Nile virus. But we know so little about their role in the ecosystem outside their impact on humans.
Mosquitoes have been around in their present form for over 100 million years thanks to their ability to survive in a wide range of different environments. The adults can be found from seashore and river bank to mountain tree lines, while the immature larvae can be found in a wide variety of environments, from small cavities in trees to large marshy areas in the Arctic.
There are at least 74 different species of mosquitoes in Canada and approximately 3,500 world-wide. The females of most Canadian species require a blood meal before they lay eggs, but both males and females feed regularly on nectar and assist in pollinating plants. “In the Arctic, plants make use of vast hordes of nectar-hungry mosquitoes for pollination during the short growing season,” says Dan Peach, a University of British Columbia researcher specializing in mosquitoes, emphasizing their importance in a world with declining pollinator populations. In addition, not all blood-eating mosquitoes target humans. Some feed on birds and reptiles. One Yukon species only feeds on amphibians.
Larval Mosquito Are Worth Saving
Fernand Saurette, Winnipeg biologist and educator, is opposed to the City of Winnipeg’s practice of killing mosquitoes at the larval stage. “I’m like anyone else. I don’t like mosquitoes buzzing around me when I go to bed at night,” Fernand says, “but mosquito larvae are innocent. They don’t transmit disease or draw blood. We shouldn’t annihilate them at the larval stage just because some of the adults are a nuisance.”
Mosquitoes spend most of their life in water, only emerging into the air for a short period of time as adults. The eggs, four progressively larger larvae, and pupae spend all their time in water and are a significant element in the aquatic food chain. The larvae eat plant debris, fungi, and bacteria in the water and the larvae in turn are eaten by larger creatures.
In addition, aquatic invertebrates such as mosquitoes and mayflies provide a valuable link between the aquatic and terrestrial environments, transferring aquatic nutrients to the terrestrial systems once the adult insects emerge. Some of the mosquitoes and mayflies will be eaten by other creatures such as birds, bats, and frogs. Those that survive die very shortly after laying eggs, decomposing rapidly and contributing nutrients for terrestrial plants.
By killing mosquitoes at the larval stage, as the City of Winnipeg does, we’re also killing other aquatic invertebrates. And yet, we know so little about small aquatic invertebrates and the role they play in the ecosystem. “Let’s be careful,” Fernand says. “They may be sentinel species or indicators of water quality. Eradicating them at this stage is too radical.”
Fernand points out how little we often know about the role played by small creatures in the larger ecosystem. “Zebra mussels are an invasive species and pose a risk for Canada’s waterways, but they’re also very efficient filter feeders,” Fernand says. “Lake Erie had an algae problem; it was a huge surprise when the zebra mussels cleaned the water.”
A Look at the Alternatives
Killing mosquito larvae has for many years been seen as the most effective way to rid communities of a nuisance and potential disease vector. But the focus has been very narrow, not taking into account the implications to a wider ecosystem.
The City of Winnipeg uses both biological and biorational larvicides. Biological larvicides, such as Bacillus thuringiensis, were believed to be harmless to non-target species, but recent research indicates they could harm frogs. They recommend “that the application of Bti products in amphibian-rich ecosystems be targeted and minimized, taking into account sensitive periods during a frog’s life cycle, including reproduction and development of eggs into young frogs.”
According to the Sierra Club of Canada, methoprene, the biorational larvicide that is being used in Winnipeg, has toxic effects on fish and crustaceans, while its effect on amphibians such as frogs, toads, and salamanders are alarming as they cause severe malformations in species that are already in decline.
Other approaches are worth investigating in order to maintain mosquitoes as pollinators and important elements in the food chain. Fernand suggests a campaign to get rid of stagnant water as this is where mosquitoes thrive. Dan Peach suggests “targeting specific [mosquito] species or making the mosquitoes themselves immune to pathogens and thus unable to spread them would protect humans while keeping the ecosystem function of mosquitoes intact”.
An American researcher is taking a similarly targeted approach. Laurence Zwiebel has found that “mosquito larvae are surprisingly complex, with a sophisticated sense of smell that enables them to find food, avoid predators and thus become healthy adult mosquitoes with greater ability to transmit disease to humans.” They can also lift their antennae above the surface of the water to sniff odours in the air. “Understanding how larvae sense the world gives us ideas about how to keep them where we want them and away from where we don’t,” Zwiebel said. “By understanding a larvae’s adult-like sophistication, we can envision ways to make them weaker adults and reduce their ability to transmit disease.”
The City of Edmonton recently decided to eliminate aerial spraying of insecticides. Instead, they plan to rely on biological pest control, including dragonflies and bats. They will use a biological larvicide but only on temporary bodies of water. They are also planning an educational program and encouraging residents to remove stagnant pools of water such as a paddling pool.
Surprising Facts About Mosquitoes
Mosquito larvae are known as wigglers or wrigglers. They have no legs but can swim by moving their abdomen from side to side. Although they spend much of their time at the surface of the water breathing air through a tube on their abdomen, they will wiggle their way to the bottom of the body of water if they sense movement. The pupae are known as tumblers as they move by scooping up water with paddle-like projections at the base of the abdomen (similar to lobsters). They breathe through a trumpet-like tube on their thorax (chest).
The larvae have very elaborate mouthparts. Small brushes open and close rapidly creating a current that brings food particles closer so they can be trapped in the brushes. Hairs along the jaw comb out the food particles and move them backwards into the throat. Large particles can be broken up by the brushes.
Toxorhynchites rutilus is a big mosquito (1/2 in. wingspan) with a big appetite – for other mosquitoes! They are found in the southeastern United States north to the Great Lakes and west into Kansas, Oklahoma, and Texas. Tx. Rutilus lay their eggs in tree cavities, and their larvae can eat up to 5,000 prey larvae by the time they mature. Scientists are exploring ways of exploiting this capacity to get rid of disease-spreading mosquitoes.
Some female mosquitoes (Anopheles, Culex, and Culiseta) hibernate over the winter, seeking a blood meal and laying their eggs in the spring. Aedes mosquitoes, on the other hand, have developed eggs that survive winter temperatures, even in the Arctic. Mansonia perturbans are unique. They over-winter as larvae by piercing the air-filled roots of cattails and other aquatic plants in order to breathe.
Wyeomyia smithii mosquitoes spend their entire lives in or near a purple pitcher plant. The eggs are laid in the small pocket of water at the base of the pitcher-shaped leaves. Most insects are trapped, digested, and absorbed by the pitcher plant, but the digestive fluids are too weak to dissolve mosquito larvae, which then feed on the smaller insects and insect detritus found within the pitcher plant.
Photo credit: Gerald Yuvallos, https://www.flickr.com/photos/gyuvallos/71410563
Further Information
Mosquito [Canadian Encyclopedia]
The Ecological Importance of Mosquitoes
Mosquito Wrigglers and Tumblers [short video]
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