Each year, more than 400 million people in the world become infected with malaria transmitted from Anopheles mosquitoes. Over two million people, mostly in Africa, die. Although South Africa has been vigilant about malaria control, this disease continues to kill. Frontiers takes viewers through the laborious task of trying to control malaria, made even more challenging by mosquitoes that have developed a resistance to insecticides. One new technique involves the high-tech Global Positioning System (GPS).
Activity 1: Floating Nurseries
Activity 2: Real World Connections
For Further Thought
Mosquitoes are an amazingly prolific relative of flies and gnats that need nothing more than a sip of blood and a bit of brackish (stagnant) water to set up a nursery teeming with squirming larvae (wrigglers). Discarded tires, bird baths, clogged gutters and even cupped leaves serve as a haven for baby mosquitoes after a spring or summer rain.
Roughly 2,700 species of mosquitoes are classified into 35 groups; each group is called a genus. Mosquitoes in three main genera attack people.
A minor player in the food chain, mosquitoes serve no useful purpose except as food for bats and birds. They range in length from 6.4 mm to 12.7 mm - almost half an inch long. No wonder bumper stickers from Minnesota to Louisiana proclaim the insect their "state bird."
As you see in this episode of Frontiers, malaria is often fatal. The female Anopheles mosquito transmits the parasites that cause malaria into the bloodstream. The parasite infects and destroys red blood cells. Only 76 new cases of locally acquired malaria in the U.S. were reported to the Centers for Disease Control and Prevention (CDC) from 1957 through 1994; but outside the U.S. malaria is still a killer - especially in Africa, where 90 percent of cases occur.
The mouth of a mosquito contains a full complement of serious tools. The proboscis, or lower lip, is a thin sheath hiding half a dozen razor-sharp implements. The mosquito does not really "bite" its victim, but stabs it with needle-like stylets at the center of its proboscis.
After the mosquito pierces the skin of its victim, it injects a mixture of saliva and anticoagulant into the opening. (The malaria parasite is transmitted at this stage.) When the victim's blood has thinned to a feeding consistency, the mosquito plunges in a straw-like digestive tube and begins to feed on the blood. Most people are allergic to the mosquito's saliva, which causes the familiar itching and swelling.
Male mosquitoes feed off plant sap, as do females, but only adult females are blood-suckers; a meal of blood is essential to producing a healthy brood of eggs. Depending on the species, eggs are laid singly or in "rafts" on standing or stagnant water containing organic material on which the larvae will feed. Air trapped under the eggs keeps them afloat.
ACTIVITY 1: FLOATING NURSERIES
Surface tension allows the surface of water to behave like "skin." Insects like mosquitoes are able to walk on it. In some species, the larva's breathing tube has water-repellent hairs that break through the surface tension.
You can investigate surface tension to better understand how the egg rafts stay afloat and how mosquitoes are able to stand on the water's surface. Carefully float a sewing needle on the surface of a clean container of water and look at it with a magnifying glass. What do you see? Try floating other objects on the surface. How does shape affect the object's ability to successfully rest on the surface? Using aluminum foil, create a "raft" with air trapped under a convex floor. Use paper clips, staples or pennies to see how much weight the raft can hold before it sinks.
ACTIVITY 2: REAL-WORLD CONNECTIONS
As more landfills refuse to accept scrap tires, illegal tire dumps have proliferated across the U.S. and resulted in major sites of mosquito breeding. In 1992, authorities located a pile of tires only seven miles from Walt Disney World in Florida, where billions of mosquitoes had taken up residence. Many were infected with the deadly Eastern equine encephalitis.
In late summer of 1996, residents of the Northeast became concerned when mosquitoes infected with Eastern equine encephalitis were captured and identified. Officials ordered the spraying of pesticides to prevent a possible epidemic.
CONDUCT A PUBLIC HEALTH CAMPAIGN
As the head of public relations for your city, you are asked to design a campaign to persuade residents to dispose of tires properly and to identify mosquito breeding grounds. You realize the urgency of the job because a neighboring state has identified a potentially fatal virus transmitted by mosquitoes.
Work in teams to plan your strategies. Some questions to consider:
- How can you make people aware of the potential dangers of mosquitoes without causing panic?
- Pesticides, vaccines and biological controls (spraying spores of a bacterium to kill the larvae) have been used effectively. What methods would you recommend to control the mosquitoes?
- What are you going to do with the tires and how will you get citizens to cooperate?
Mosquitoes develop into adults and leave the water about seven days after the female lays her eggs. Depending on the species, a female can lay between 100 and 400 eggs in one brood and produce a new brood every two to three days (or about 3,000 eggs in a lifetime). Most eggs are lost to predators who find them an easy snack.
Assuming that one adult female produces a new brood every three days, but only 10 percent of the eggs survive to become adults, how many offspring might one female mosquito produce in one week (providing the female remains alive that long)? Now assume that half of the offspring are female; how many mosquitoes could be born in the next generation? Why are controls in the early spring the most important?
In his poem "The Mosquito," D.H. Lawrence describes the sound of the mosquito as "a small, high, hateful bugle in my ear." He calls the creature a "ghoul on a wing." Look up the poem and the West African folk tale, "Why Do Mosquitoes Buzz in People's Ears?" (it can be found in the children's book of the same title by Verna Aardema). Then write your own poem or story about this unpopular insect. Don't forget illustrations!
FOR FURTHER THOUGHT
- What other diseases are transmitted by mosquitoes? Map the areas in the world where malaria, dengue and other mosquito-borne diseases are still a problem. Identify which species carries the disease.
- Find out what mosquito control methods are used in your community. What is the cost to maintain this program?
- If mosquitoes are a "minor player in the food chain," why not just eradicate them as a species? Explain your answer.
- Explain and demonstrate why running water in a stream or pond would be a poor choice for where to lay mosquito eggs.