Fish prove an alternative bioagent in mosquito control<\/p>\n
By Wahyuni Rizkiana Kamah<\/p>\n
JAKARTA (JP): The insecticides used in mosquito control harm
\nthe environment more than they eradicate mosquitoes. And,
\nalthough the high toxicity of insecticides on non-target species
\nhas been questioned in the past few decades, due to the evolution
\nof insecticide-resistant mosquitoes the toxicity is increasing.
\nIn the 1950s, tropical diseases were minimized through intensive
\nuse of diphenyl dichloro trichloroethan (DDT). The diseases
\nreturned after the vector insects developed increased multiple
\nresistance to most chemical insecticides.<\/p>\n
The use of environmentally friendly alternatives is therefore
\nessential to vector control. The World Health Organization (WHO)
\nintegrates various potential methods to control the disease
\nspreaders. The methods cover biological and chemical solutions,
\nsource reduction, health education and personal protection.<\/p>\n
Biological vector control is the most reasonable and
\nenvironmentally friendly alternative to the use of chemicals. The
\nbiological method of mosquito control covers the utilization of
\nnatural enemies of vectors, repetitive application of microbial
\ninsecticides, and various genetic methods such as sterile male-
\nvector release techniques. A meeting on the use of fish in
\nmosquito control was held in Geneva, Switzerland in 1981. It
\nexplored the role of larvicidal fish as a simple, cost-effective
\nand self-tool agent of vector control.<\/p>\n
Predatory fish were first used against the Aedes aegypti
\nlarvae to control yellow fever in Havana, Cuba at the turn of the
\ncentury. Since then, there have been numerous reports of the
\neffectiveness of predatory fish in reducing mosquito larval
\npopulations.<\/p>\n
Aedes aegypti<\/p>\n
The Aedes aegypti mosquito is notorious throughout urban areas
\nin Southeast Asian. This container breeding mosquito is the major
\nvector of the dengue virus which causes dengue hemorrhagic fever.
\nDengue hemorrhagic fever is a leading killer of children in
\nSoutheast Asia. According to WHO, dengue is confined to Southeast
\nAsia and the Western Pacific. At present, outbreaks occur both in
\nlarge cities and small towns, and it spreads to villages wherever
\nAedes aegypti exists.<\/p>\n
Dengue hemorrhagic fever has been a problem in Indonesia since
\n1779, but the first confirmed case was only recorded in 1969. The
\nbiggest outbreak of dengue hit Indonesia in 1988, with 47,373
\ncases being reported resulting in 1,527 deaths. The outbreak
\nappears to have a five-year cycle and tends to intensify with
\nevery occurrence.<\/p>\n
Aedes aegypti breeds in still freshwater in and around the
\nhome, and depends on human's blood to survive. It is therefore an
\nextremely efficient vector of dengue. The species is the only
\ndengue vector in most endemic countries,.<\/p>\n
The mosquito breeds in:<\/p>\n
1. Temporary water collectors, including discarded or unused
\nwater jars, old tires, broken bottles and debris holding
\nrainwater;<\/p>\n
2. Permanent water containers such as reservoirs and domestic
\ncontainers for storing water for washing, drinking and bathing on
\naccount of a lack of freshwater or unreliable supply of piped
\nwater; and<\/p>\n
3 Natural habitats such as tree or bamboo stumps near human
\nhabitation. The mosquito is also known to breed in brick lined
\nand mud lined wells.<\/p>\n
Vector control is still considered the main weapon in dengue
\nprevention. Research and training has concentrated on the use of
\ninsecticides to control adult insects or mosquito larvae. The
\nmosquito's resistance to insecticides, as well as prohibitive
\noperational costs, has shifted the control of Aedes aegypti in
\nurban areas to the reduction of man-made containers suitable for
\nlarval breeding.<\/p>\n
Hungry fish<\/p>\n
Larvicidal fish can be introduced in essential water
\ncontainers, such as large drums, cement tanks and other
\ncontainers holding large quantities of water, to decimate the
\nmosquito larvae.<\/p>\n
Fish with superior mouths are the best candidates since they
\nare usually carnivorous or omnivorous, and they swim and feed at
\nor near the water surface where many mosquito larvae and pupae
\nrest. Promising larvicidal fish are usually small (6 cm long),
\nfusiform in shape, and their terminal or superior mouths are
\nprovided with teeth. Larvicidal fish are generally omnivorous and
\nmay feed on algae, crustaceans and dipterous larvae. There are no
\nfish only eat larvae.<\/p>\n
Larvicidal fish exist in most subtropical and tropical
\ncountries. Gambusia affinis from southeastern North America is
\nwidely used for mosquito larva control. The efficacy of this
\nspecies has made it popular in many countries. Other tropical
\nfish are effective in mosquito control, such as the Indian top
\nminnow (Aplocheilus panchax ), guppies (P. reticulata), and the
\ncarp Tilapia mossambica and Cyprinus carpio.<\/p>\n
The capability of guppies to decimate mosquito larvae has been
\ninvestigated both in the laboratory and the field in Cuba, India
\nand Indonesia. The introduction of this fish in 17 wells in
\nsuburban Ram Nagar, India, reduced the mosquito population in 70
\npercent of the wells within two weeks. In rural areas in
\nVaranasi, India, 40 percent of the 17 wells were mosquito free
\ntwo weeks after the fish were introduced.<\/p>\n
Throughout tropical Asia, water is stored in different shaped
\ncontainers made from cement, ceramics, unglazed earthenware,
\nplastic and metal. The practice differs with cultural needs or
\nprevailing local conditions. In areas where freshwater is scarce,
\nsuch as coastal or rural areas, the community usually relies on
\nrainwater. They hold it for long periods in open containers.
\nThese, in turn, are breeding places for Aedes aegypti. Because it
\nis impossible to empty the containers to clean them of mosquito
\nlarvae, introducing larvicidal fish to the containers is the
\nsimplest solution.<\/p>\n
Customs and cultural sensibilities are the main constraints to
\nthis solution. Most people are not accustomed to seeing fish
\nswimming in their water containers.<\/p>\n
An answer to this obstacle is the predatory capability of
\nthree freshwater-aquarium fish. The common guppy, a familiar pet
\nwith the spectacular male and more voracious females can be found
\nin fishponds on the outskirts of Jakarta or at any pet market.
\nUnfortunately, rapid suburban development has destroyed their
\nnatural habitats. Another voracious species is Colisa lalia, a
\nbeautiful aquarium fish with the popular Indonesian name of sepat
\nralis. It is also available at pet markets.<\/p>\n
To get around the problem of fish excrement, first place the
\nfish in another container and starve them for 12 hours so they
\ncan excrete as much as possible before being introduced to a
\ndomestic container. The fish will immediately gorge themselves as
\nsoon as they are place in a larvae infested domestic container.
\nThey can be returned to the aquarium after a half hour so they
\nwill not pollute the water in the domestic container. The number
\nof fish introduced depends on the number of larvae and the size
\nof the container.<\/p>\n
The feeding frenzy is only needed once a week because Aedes
\naegypti grow from eggs to the big larvae in five to seven days.
\nThe mosquito larvae never make it to the adult stage without the
\nuse of harmful chemicals.<\/p>\n
With so many fish available for mosquito control, it is
\nlogical to ask why they are not more often utilized for this
\npurpose. One reason, of course, is the availability of a number
\nof excellent fast-acting insecticides that can be quickly and
\nuniformly applied to large and varied areas. Fish, on the other
\nhand, are seldom readily obtainable, and are difficult and
\nexpensive to maintain. They are also restricted to a distinct
\nhabitat. Fish and other forms of natural control alone will never
\nsucceed in replacing chemicals. Nevertheless, insecticides have
\nincreased in price while decreasing in effectiveness, making it
\nimperative that new biological vector controls be found to
\nreplace the environmentally harmful chemical treatments.<\/p>\n
Wahyuni Rizkiana Kamah is a graduate student of the School of
\nBiology, University of Indonesia.<\/p>",
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