Mosquito control has been a point of focus in scientific communities and health-providing units. Various methods have been tried and are still underway, to eliminate the mosquito population.  However, certain environmental issues related to the chemical control of mosquitoes have convinced experts to opt for eco-friendly methods.

By Prof Dr Abdullah Arijo

Mosquitoes are a family of small, midge-like flies: the Culicid. Although a few species are harmless or even useful to humanity, most are considered a nuisance because they consume blood from living vertebrates, including humans. The females of many species of mosquitoes are blood-sucking pests. While feeding on blood, some of them transmit extremely harmful human and livestock diseases, such as malariayellow fever, and filariasis (Anonymous, 2013).


Mosquitoes are the worst enemies of mankind resulting in disturbing sound sleep followed by sucking of blood that may serve as means of pathogen transmission. Different methods have been tried to repel or kill mosquitoes using various materials such as mosquito mats, coils, mosquito repellents, spraying chemicals, fumigation, netted windows and doors, mosquito nets etc. But the results are not promising, because, instead of reduction, the mosquito population increasing and spreading day by day. One reason may be the fault of approaches in controlling the mosquito population. Attempts are made to kill adult mosquitoes that are coming into our houses or sleeping places but destroying their breeding sites may be a more reliable approach for permanent mosquito control (Rajput et al., 2009).


Surprisingly, many species of mosquitoes are not bloodsucking. However, in the bloodsucking species, only the females suck blood. Furthermore, even among mosquitoes that do carry important diseases, neither all species of mosquitoes, nor all strains of a given species transmit the same kinds of diseases, nor do they all transmit the diseases under the same circumstances; their habits differ. For example, some species attack people in houses, and others prefer to attack people walking in forests (Anonymous, 2013).


There are about 3,500 species of mosquitoes identified from various parts of the world. Some mosquitoes that bite humans routinely act as vectors for several infectious diseases affecting millions of people per year (Molavi & Afshin 2003). Others that do not routinely bite humans, but are the vectors for animal diseases, may become disastrous agents for zoonosis of new diseases when their habitats are disturbed, for instance by sudden deforestation (Wilcox & Ellis, 2006).


For effective control of mosquitoes, it is imperative to know about the biology and behaviour of targeted mosquitoes. Mosquitoes are generally nocturnal hexapods that feed on host blood for Oogenesis (formation of eggs).  However, for breeding purposes, they find their way to swamps where they lay eggs.  The feeding habits of mosquitoes are unique in that it is only adult females that bite humans and other animals. Some female mosquitoes prefer to feed on only one type of animal or they can feed on a variety of domesticated animals, such as cattle, horses, goats, etc.; all types of birds including chickens; all types of wild animals including deer, rabbits; and they also feed on snakes, lizards, frogs, and toads. Most female mosquitoes must feed on an animal and get a sufficient blood meal before they can develop eggs. If they do not get this blood meal, they die without laying viable eggs. However, some species of mosquitoes have developed the means to lay viable eggs without getting blood (Wikipedia, 2013).


Mosquitoes are the principal vector responsible for to spread of many diseases, like malaria, yellow fever, dengue fevers, chikungunya, filariasis, Japanese encephalitis, and leishmaniasis, which cause thousands of deaths per year (WHO, 2007).

Various species of mosquitoes cause multiple types of direct and indirect damages Culex mosquitoes for example are mostly house-resting mosquitoes in many tropical countries and act as an important vector of filariasis and cause a nuisance to their victims. Lymphatic filariasis is probably the fastest spreading insect-borne disease of man in the tropics, affecting about 146 million people (WHO, 1992).

Anopheles mosquitoes are another important vector of malaria in Asian countries affecting more than 500 million humans each year, killing approximately 1.2 to 2.7 million per year (Mittal et al., 2005).


Since mosquitoes are quite dangerous and are responsible for direct and indirect damages, therefore, their control has always been focused on. The scientific community has tried many methods of mosquito control including chemical, genetic, and management approaches, however, there are certain limitations associated with environmental issues. Many scientists have suggested that the complete eradication of mosquitoes would not have serious ecological consequences (Fang, 2010). Therefore, there has always been room for developing the safest and environmentally friendly control methods.


The mosquito may be controlled by adapting attempts to hit on the weakest life-cycle links. For example, larval control measures are intended to reduce malaria transmission indirectly by reducing the vector population density near human habitations. Since different species of mosquitoes have various habitat associations, it is, therefore, necessary to know the aquatic behaviour of larvae. Generally, larvae are exclusively aquatic; therefore, their distribution is determined by the locations of suitable water bodies. It is a known fact that immature larval stages prefer slow-moving or stagnant water in which they can stay close to the surface with their breathing syphon to breathe air from the atmosphere. However, Anopheles larvae require relatively clean water for development. Whatever the case may be, before larval control methods are implemented, the majority of the vector larvae’s productive breeding sites must be identified. (Keiser et al., 2004).


 There are several methods of mosquito control, but biological control is the deliberate use of natural enemies to reduce the number of pest organisms. It comprises methods that have gained acceptance for controlling nuisance. In the case of arthropod-borne disease vectors, biological control is a potentially effective strategy for regulating and preventing transmission of diseases such as dengue, malaria and lymphatic filariasis, amongst others.


A biological control refers to the introduction or manipulation of organisms to suppress vector populations. A wide range of organisms helps to regulate mosquito populations naturally through predation, parasitism and competition. As biological mosquito control agents, carnivorous fish (i.e., those that feed on immature stages of mosquitoes) are being used extensively all over the world since the early 1900s (Raghavendra, 2002).


Biological control, particularly using carnivorous fish, has been important to malaria control programs in the 20th century, particularly in urban and peri-urban areas in developed and developing Countries (Gratz, 1988). It has a very positive role to play in the integrated control methodologies in which both pesticides and fish or other biotic agents have their roles (Mulla, 1961).


The selection of biological control agents should be based on their potential for unintended impacts, self-replicating capacity, climatic compatibility, and their capability to maintain very close interactions with target prey populations (Waage, 1988). They eliminate certain prey and sustain in such environments (i.e., they eat the prey, when introduced) for long periods thereafter (Marten, 1994).


The biological control of mosquitoes and other pests involves introducing into the environment their natural enemies, such as predatory animals. Larvivorus fish are more efficient to control mosquitoes at their larval stages. Gambusia affinis and Poecilia reticulata (Larvivorus fish) have been used worldwide for controlling mosquito larvae ( Phukon & Biswas, 2013 ).


Larvivorus fish have been used for over 100 years in mosquito control. Gambusia affinis has been widely used to control the immature stages of various vector mosquitoes. Other fish species include Tilapia spp., Poecilia reticulata, and Cyprinidae (Lacey et al., 1990). The benefits of larvivorous fish are that the mosquito larvae cannot build up physiological resistance, and the fish populations are generally self-sustaining and do not depend on the presence of larvae (Wright et al., 1972). Even if some Anopheles larvae survive despite the presence of fish, these emerge as smaller adults (Bond et al., 2005). The fish are relatively inexpensive, and 6 months after stocking the larger fish can be harvested, providing a sustainable source of income and protein for rural farmers (Howard et al., 2007).


It is generally believed that biological control is relatively safe as compared to chemical control. All other methods have certain limitations and environmental issues. In chemical control, for example, larvicides are applied to the water where mosquito larvae develop or where it may provide a habitat for mosquitoes. Currently, light mineral oils and insect-growth regulators and some organophosphates such as temephos and malathion are used as mosquito larvicides in many countries. Larvicides have very little residual effect. As the life cycles for mosquitoes are about 7 days, therefore larvicides have to be applied to larval habitats continuously for 7 days (Durden & Gray, 2002).

The harmful effects caused by chemicals, for instance, DDT, in mosquitoes as well as on non-target populations and the development of resistance in mosquitoes have prompted an alternative use of simple and sustainable methods of mosquito control. The eradication of mosquitoes using adulticides is not a good strategy, as the adult stage occurs alongside human habitation, and they can easily escape remedial measures (Milam et al., 2000).


In genetic control, a gene is introduced into the mosquitoes, which stops their cells from functioning normally. This gene can act as a switch to control the activity of other genes. In the modified insects, the expression of this gene ties up some of the cell’s essential machinery and stops other mosquito genes from being expressed correctly.  As a result, the mosquitoes can’t develop properly and die before becoming adults. If the larvae are given tetracycline, this stops the effect and so tetracycline acts as an antidote. But the insects can’t access the antidote in the right quantities in the environment, so when they are released the lethality factor is activated and the offspring will not reach adulthood (Anonymous, 2004).


Since other mosquito control methods have their consequences, hence emphasis may be given to developing environmentally friendly methods of mosquito control. With this object, the present study was designed to evaluate fish as larvicidal agents.