Fish prove an alternative bioagent in mosquito control

Fish prove an alternative bioagent in mosquito control

By Wahyuni Rizkiana Kamah

JAKARTA (JP): The insecticides used in mosquito control harm
the environment more than they eradicate mosquitoes. And,
although the high toxicity of insecticides on non-target species
has been questioned in the past few decades, due to the evolution
of insecticide-resistant mosquitoes the toxicity is increasing.
In the 1950s, tropical diseases were minimized through intensive
use of diphenyl dichloro trichloroethan (DDT). The diseases
returned after the vector insects developed increased multiple
resistance to most chemical insecticides.

The use of environmentally friendly alternatives is therefore
essential to vector control. The World Health Organization (WHO)
integrates various potential methods to control the disease
spreaders. The methods cover biological and chemical solutions,
source reduction, health education and personal protection.

Biological vector control is the most reasonable and
environmentally friendly alternative to the use of chemicals. The
biological method of mosquito control covers the utilization of
natural enemies of vectors, repetitive application of microbial
insecticides, and various genetic methods such as sterile male-
vector release techniques. A meeting on the use of fish in
mosquito control was held in Geneva, Switzerland in 1981. It
explored the role of larvicidal fish as a simple, cost-effective
and self-tool agent of vector control.

Predatory fish were first used against the Aedes aegypti
larvae to control yellow fever in Havana, Cuba at the turn of the
century. Since then, there have been numerous reports of the
effectiveness of predatory fish in reducing mosquito larval
populations.

Aedes aegypti

The Aedes aegypti mosquito is notorious throughout urban areas
in Southeast Asian. This container breeding mosquito is the major
vector of the dengue virus which causes dengue hemorrhagic fever.
Dengue hemorrhagic fever is a leading killer of children in
Southeast Asia. According to WHO, dengue is confined to Southeast
Asia and the Western Pacific. At present, outbreaks occur both in
large cities and small towns, and it spreads to villages wherever
Aedes aegypti exists.

Dengue hemorrhagic fever has been a problem in Indonesia since
1779, but the first confirmed case was only recorded in 1969. The
biggest outbreak of dengue hit Indonesia in 1988, with 47,373
cases being reported resulting in 1,527 deaths. The outbreak
appears to have a five-year cycle and tends to intensify with
every occurrence.

Aedes aegypti breeds in still freshwater in and around the
home, and depends on human's blood to survive. It is therefore an
extremely efficient vector of dengue. The species is the only
dengue vector in most endemic countries,.

The mosquito breeds in:

1. Temporary water collectors, including discarded or unused
water jars, old tires, broken bottles and debris holding
rainwater;

2. Permanent water containers such as reservoirs and domestic
containers for storing water for washing, drinking and bathing on
account of a lack of freshwater or unreliable supply of piped
water; and

3 Natural habitats such as tree or bamboo stumps near human
habitation. The mosquito is also known to breed in brick lined
and mud lined wells.

Vector control is still considered the main weapon in dengue
prevention. Research and training has concentrated on the use of
insecticides to control adult insects or mosquito larvae. The
mosquito's resistance to insecticides, as well as prohibitive
operational costs, has shifted the control of Aedes aegypti in
urban areas to the reduction of man-made containers suitable for
larval breeding.

Hungry fish

Larvicidal fish can be introduced in essential water
containers, such as large drums, cement tanks and other
containers holding large quantities of water, to decimate the
mosquito larvae.

Fish with superior mouths are the best candidates since they
are usually carnivorous or omnivorous, and they swim and feed at
or near the water surface where many mosquito larvae and pupae
rest. Promising larvicidal fish are usually small (6 cm long),
fusiform in shape, and their terminal or superior mouths are
provided with teeth. Larvicidal fish are generally omnivorous and
may feed on algae, crustaceans and dipterous larvae. There are no
fish only eat larvae.

Larvicidal fish exist in most subtropical and tropical
countries. Gambusia affinis from southeastern North America is
widely used for mosquito larva control. The efficacy of this
species has made it popular in many countries. Other tropical
fish are effective in mosquito control, such as the Indian top
minnow (Aplocheilus panchax ), guppies (P. reticulata), and the
carp Tilapia mossambica and Cyprinus carpio.

The capability of guppies to decimate mosquito larvae has been
investigated both in the laboratory and the field in Cuba, India
and Indonesia. The introduction of this fish in 17 wells in
suburban Ram Nagar, India, reduced the mosquito population in 70
percent of the wells within two weeks. In rural areas in
Varanasi, India, 40 percent of the 17 wells were mosquito free
two weeks after the fish were introduced.

Throughout tropical Asia, water is stored in different shaped
containers made from cement, ceramics, unglazed earthenware,
plastic and metal. The practice differs with cultural needs or
prevailing local conditions. In areas where freshwater is scarce,
such as coastal or rural areas, the community usually relies on
rainwater. They hold it for long periods in open containers.
These, in turn, are breeding places for Aedes aegypti. Because it
is impossible to empty the containers to clean them of mosquito
larvae, introducing larvicidal fish to the containers is the
simplest solution.

Customs and cultural sensibilities are the main constraints to
this solution. Most people are not accustomed to seeing fish
swimming in their water containers.

An answer to this obstacle is the predatory capability of
three freshwater-aquarium fish. The common guppy, a familiar pet
with the spectacular male and more voracious females can be found
in fishponds on the outskirts of Jakarta or at any pet market.
Unfortunately, rapid suburban development has destroyed their
natural habitats. Another voracious species is Colisa lalia, a
beautiful aquarium fish with the popular Indonesian name of sepat
ralis. It is also available at pet markets.

To get around the problem of fish excrement, first place the
fish in another container and starve them for 12 hours so they
can excrete as much as possible before being introduced to a
domestic container. The fish will immediately gorge themselves as
soon as they are place in a larvae infested domestic container.
They can be returned to the aquarium after a half hour so they
will not pollute the water in the domestic container. The number
of fish introduced depends on the number of larvae and the size
of the container.

The feeding frenzy is only needed once a week because Aedes
aegypti grow from eggs to the big larvae in five to seven days.
The mosquito larvae never make it to the adult stage without the
use of harmful chemicals.

With so many fish available for mosquito control, it is
logical to ask why they are not more often utilized for this
purpose. One reason, of course, is the availability of a number
of excellent fast-acting insecticides that can be quickly and
uniformly applied to large and varied areas. Fish, on the other
hand, are seldom readily obtainable, and are difficult and
expensive to maintain. They are also restricted to a distinct
habitat. Fish and other forms of natural control alone will never
succeed in replacing chemicals. Nevertheless, insecticides have
increased in price while decreasing in effectiveness, making it
imperative that new biological vector controls be found to
replace the environmentally harmful chemical treatments.

Wahyuni Rizkiana Kamah is a graduate student of the School of
Biology, University of Indonesia.