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Slapping the mosquito - scientifically.

Slapping the Mosquito--Scientifically

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It's too bad the mosquito is such a pain (not to mention itch)--its life cycle is really quite fascinating.

Just listen to Gary A. Mount, Donald L. Barnard, and Jack A. Seawright, of ARS' Medical and Veterinary Entomology Research Laboratory in Gainesville, Florida, and you may even be tempted to admire the delicate little horrors.

For all larvae, life begins in still water. For some species, this means water trapped in discarded tires, buckets, planters, ditches, tree holes, and other containers. Other species hatch in lakes or even sewage ponds.

The larvae (one-tenth to one-fourth inch long) usually float at the top of the water, breathing through a snorkel-like device. They stay afloat because they need air to survive. "If something upsets them, they go down quickly, stay down a minute or so, and then come back up," Barnard says.

A brushlike apparatus circulates water containing food particles through the larva's mouth. Mosquito larvae are filter feeders, feeding on anything in the water that they can filter through their mouths, such as organic debris and small invertebrates.

Barnard says organic matter is plentiful wherever mosquitoes are because the water they live in is stable. Decomposing vegetation usually sinks down into standing water, and because food for algae, bacteria, fungi, and invertebrates thrives. "It's like a community of organisms within the container."

Adults are less alike, in behavioral terms, than larvae. Some adults breed and live near fresh water, while others prefer brackish. Some choose foul water; others, clear. Some breed in anything that will hold water while others stick to just tree holes. Although most species become active and bite at dusk, some also attack during the day.

Many species prefer to bite birds and mammals, but others are known to attack reptiles, amphibians, fish, and even other insects for a blood meal.

Actually, blood isn't a source of sustenance; both male and female mosquitoes seek plant nectar and juices for that. Females use blood protein to complete egg development before they lay their eggs.

Entomologists Albert Undeen and James J. Becnel at the Gainesville lab study natural mosquito control. One of the most promising organisms right now, Undeen says, is a tiny parasite called Edhazardia aedis. Its spore looks like a tiny egg and works like a hypodermic syringe to infect mosquito larvae, attacking container-breeding mosquitoes like the yellow and dengue fever transmitter Aedes aegypti.

As they are filter feeding, explains Becnel, mosquito larvae ingest dormant spores of the parasite. When the spores germinate, they put out a tubelike projection that penetrates the larva's gut and infects it.

Some larvae die, but most emerge as infected adults. Inside an infected adult female, the parasite produces yet another kind of spore that infects the eggs.

Many larvae that emerge from infected eggs die. Their corpses release the original type of spore that, in turn, infects other larvae, completing the parasite's life cycle.

Females that emerge from infected eggs and somehow survive also lay infected eggs.

"This transmission from females to offspring is what makes this such a good candidate for biological control of container-breeding mosquitoes," Undeen says.

Becnel explains, "With container breeders, you must have a pathogen that the mosquito itself carries around because you can't find all the other little containers." With E. aedis, female mosquitoes take care of spreading the disease.

In laboratory tests, the parasite infects and kills 100 percent of larvae in a container--depending on the dosage of spores used. "But that doesn't mean we're going to get that kind of control in the field, generation after generation," Undeen warns. Small-scale field tests may begin soon, if the Environmental Protection Agency approves them.

As Becnel prepares for field tests on E. aedis, Undeen is steering his own research in new directions. He plans to search for spots where "you'd expect to find mosquitoes in large numbers but you don't." Previously, scientists brought mosquitoes into the lab and looked for diseases in them. This could have been biasing the search toward chronic pathogens that would debilitate mosquitoes but perhaps not actually kill them, Undeen says.

He will begin looking in waste water habitats this year, following populations closely and trying, ironically, to not find mosquitoes. If an area that should have mosquitoes proves to have none, Undeen will look for organisms that could be keeping the region mosquito-free. That organism could be, Undeen reasons, a powerful biocontrol agent. "I may be going out on a limb with this approach, but it's worth a try."

Protecting the Victim

Gainesville scientists have invested three decades of research in protecting people from biting insects--outdoor workers and recreationists here and military personnel abroad.

"The research emphasis has been on malaria transmitters," Barnard says, pointing out that the Department of Defense funded much of this research.

The work has led to development and wide use of deet, a strong repellant of biting flies, mosquitoes, ticks, and chigger mites. deet was discovered by ARS chemists in Beltsville, Maryland, and entomologists of the Gainesville lab nearly 40 years ago. It has been used both by civilians and troops ever since.

But deet is coming under fire. Its oiliness makes it messy to use. It can occasionally irritate skin and will burn mucous membranes and sensitive areas, for example around the eyes and mouth. And it dissolves some paints and plastics such as vinyl.

So a few years ago, 3M Company made new deet formulations and entomologist Carl E. Schreck tested them at Gainesville, recommending which one might be best suited for military use. The result: a new cream with special polymers that help it stick to the skin better. "The new formulation has half of the active ingredient, with just 35 percent deet but performs just as well as the military formulation that had 75 percent."

Deet strongly repels species of the Aedes and Culex genera but isn't as effective with other important genera, like Anopheles.

Schreck says that the program to develop new repellants never quits, because "we're always trying to find the ideal product that both repels all mosquitoes and doesn't irritate even sensitive skin."

Over the last three decades, chemists Albert B. DeMilo and the late Terrance McGovern of the Insect Chemical Ecology Laboratory at Beltsville, Maryland, developed 600 to 800 new materials for Schreck to screen for repellancy. That work has led to a few good candidates; all are, unfortunately, more expensive to develop and use than simply staying with deet.

"Industry is reluctant to spend the development moneys it takes to get EPA registration, because it's not a very economical investment for them," Schreck says. A company could invest several million dollars and "find they only sell the product in May, June, and July, when mosquitoes are out in full force. Then they'd have to wait until next season."

"Basically, the synthesis programs that industry used to run are at about zero today. It's up to us to develop these things," Schreck says.

Schreck has also extensively tested a mosquito killer developed by chemists in Britain. It is a synthetic pyrethroid called permethrin.

In recent studies, Schreck found that permethrin lasts 6 to 9 months on tents--in terms of both knocking down the total number of mosquitoes entering a tent and reducing the number of bites. "For up to 9 months, people got either zero bites or very few," he says.

The tents were kept outside for a year straight--a situation that probably wouldn't occur for the average camper or even military personnel. "That should give you some idea of how effective the treatment is. In intermittent use, one treatment would probably last the tent's life," he says.

The compound is approved for spray application to civilian clothing. It also has EPA registration for U.S. armed forces use in several forms for application to clothing. In fact, permethrin, along with the new formulation of deet, was widely used by troops in the Persian Gulf during Operation Desert Storm.

Go Bite Somebody Else

Schreck has also begun a project for personal protection--one with an interesting twist. Instead of focusing on victims' clothing or tents, he's looking at the victims themselves. He's trying to answer that age-old question: Why do mosquitoes absolutely adore you, while someone standing next to you gets relatively few bites?

Schreck tested volunteers in the lab to see who most and least attracted hungry mosquitoes.

Volunteers put a hand into a chamber. A fan blew air over the hand and into a chamber next door, which housed 100 female mosquitoes. Schreck counted how many responded.

The most attractive: technician Daniel Smith.

To find out what makes the difference, Schreck had Smith roll a few glass marbles in his hands and drop them into ethanol. The chemical removed the residue so that chemist David C. Carlson could analyze it in a gas chromatograph.

Results are still being analyzed, and studies continue. The trail could lead scientists to what mosquitoes are responding to when they search for a blood meal and then allow the scientists to find a chemical that will mask that component.

Schreck says they will even consider whether dietary factors affect the insect's attraction to different people.

Cow Breath Comes in Handy

Octenol, a component of the breath of ruminants, may offer a unique new mosquito control method. Originally studied in Africa by scientists trying to control tsetse flies, octenol has proven to be very attractive to mosquitoes, says entomologist Daniel Kline.

In outdoor cage tests, octenol attracted the same number of mosquitoes as carbon dioxide, another proven attractant. But combining the two produced a synergistic reaction: 3 to 10 times more mosquitoes were attracted than with either compound alone. Scientists believe the octenol attracts mosquitoes from far away, while carbon dioxide attracts closer ones.

The best way to increase a mosquito catch is to combine the two, Kline says. And so the researchers are "taking a monitoring tool and adapting it to make it a control device."

He uses a standard mosquito trap with a wick that emits octenol. "We're trying to figure out how much octenol needs to be released for ideal results. With a wick, the release rate is subject to wind and temperature, and we'd like to be a little more precise than that."

He also says he has to improve the trap so it can be produced more cheaply if it is to be used as a control method.

Finally, the method needs to be proven in large-scale field tests. Within the next 5 years, Kline hopes to go into the Florida Everglades, mark mosquitoes with a fluorescent dye, and test the system with natural populations.

Until then, he'll continue to work with two Florida mosquito districts in small-scale tests to find out how many traps are needed, how far apart to put them, what release rate for carbon dioxide and octenol is best, and whether mosquitoes respond throughout their lives or only during certain periods.

But if Kline could prove the method effective, mosquito abatement officials and homeowners alike would have a nature-friendly mosquito control method.

This project has so far centered on the salt marsh mosquito, Aedes taeniorhynchus, a terrible biting pest that occurs as far north as New Jersey. This is a wetland breeder, and "wetlands are very sensitive as far as using chemical pesticides," Kline says. So a removal trapping method would be welcomed, because it would offer an alternative to the chemical pesticides that are currently used.

Though he developed the system with the salt marsh mosquito, Kline says that octenol attracts 4 other species of the 69 he tested, including 3 that cause disease.

Psorophora columbiae is a freshwater flood mosquito associated with rice fields and suspected of spreading Venezuelan equine encephalitis in the United States every few decades.

Mansonia titillans and Coquillettidia perturbans, two biting pests implicated in transmitting eastern equine encephalitis, are also attracted.

Kline says octenol could replace carbon dioxide in traps used to monitor any of these species. Abatement officials often have trouble getting or using carbon dioxide, which comes in the form of dry ice or in compressed gas cylinders. A small vial or wick of octenol would be a lot more efficient and would provide a "good index of the mosquito numbers that are out there," he says.

In fact, the Collier County Mosquito Control District in Florida is already using octenol to monitor for salt marsh mosquitoes and has reported very good results.

Mosquito Population Prediction

Engineer Danel G. Haile and entomologist Dana A. Focks have developed three computer simulation models for mosquitoes.

MALSIM predicts how many cases of malaria military officials can expect with certain weather patterns and control measures. The scientists furnished the program with basic information about the mosquito's life cycle, including how temperature, moisture, and other climatic factors affect its survival. They told the program that malaria must incubate inside a mosquito 12 to 14 days before it can be transmitted to another victim.

The model predicts the benefits of different mosquito control programs, including aerial spraying or personal protection measures for troops.

Though the model hasn't been corroborated fully in a field situation, the military is already using it as a training tool for new recruits. The program is first run to demonstrate how many of them will get malaria if they don't use their personal protection gear.

The program is run a second time, with personal protection gear in place. The simulations serve as good motivation to use permethrin on clothes and deet on skin.

PCSIM simulates population dynamics of Psophora columbiae, the riceland mosquito, while CIMSIM models container-inhabiting mosquitoes. Like MALSIM, these programs predict insect numbers based on weather conditions and control measures.

Once validated, computer simulations could be used to develop integrated management strategies for mosquito abatement programs.

To contact ARS scientists mentioned in this article, write or call the Editor, Agricultural Research Magazine, Bldg. 005, 10300 Baltimore Ave., Beltsville, MD 20705-2350. Phone (301) 344-3280.

PHOTO : Donald Barnard examines yellowfever mosquitoes to see if they have had their blood "meal," a prerequisite for complete egg development. (K-4159-5)

PHOTO : Mosquito larvae infected with the protozoan Edhazardia aedis. The males will die; females will lay protozoan-spiked eggs.

PHOTO : Entomologist Carl Schreck examines a collection tube containing yellowfever mosquitoes (Aedes aegypti). (K-4161-1)

PHOTO : Yellowfever mosquitoes (Aedes aegypti) attack the untreated arm of researcher Ken Posey. Below, an arm that is treated with the repellant deet suffers far fewer bites. (K-4158-7,18, 19)

PHOTO : Biological technician Ken Posey clocks length of time mosquitos are repelled by various treatments. (K-4157-16)

PHOTO : These white marbles could help explain why mosquitoes seem to prefer certain individuals. Subjects who are particularly "tasty" roll the marbles in their hands. Residues are then washed from the marbles and analyzed. (K-4160-5)
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Title Annotation:research on mosquitos control
Author:Silva, Jessica Morrison
Publication:Agricultural Research
Date:Aug 1, 1991
Words:2467
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