7.5 Conserving natural enemies of mosquitoes

Mosquitoes are some of nature's most unloved creatures. Their bites itch, and they transmit dreaded diseases like malaria, filaria, dengue and Japanese encephalitis. Mosquitoes have enormous reproductive potential: if left unchecked, a female mosquito could produce 8 billion other females in only two months.

Fortunately, nature has provided a variety of biological control agents-pathogens, parasites and predators-that affect every stage of the mosquito's life cycle. They vary from micro-organisms to mammals such as bats, and even pitcher plants. Besides these natural enemies, abiotic factors such as temperature, humidity and rainfall also limit mosquito populations. The net result is to inflict considerable mortality on each generation of mosquitoes.

Threats to big-control agents

In the quest to eliminate insect pests of crops and insects that carry diseases, humans have resorted to the indiscriminate use of insecticides to support both agriculture and health.

Initial successes were tremendous. But through repeated use, the target insects (including mosquitoes) have gradually acquired physiological resistance to these insecticides. Many insecticides have a very broad-spectrum action, so they kill the natural enemies as well as the pests. And some pesticides enter the food chain, accumulate in the bodies of certain organisms, and disrupt the food web. These effects have caused incalculable harm to the natural enemies of mosquitoes and other insect pests.

Conserving, propagating and using biological control agents have gained tremendous importance. Chemical insecticides should be used only when absolutely essential, and then with extreme caution.

Promoting natural enemies

Mosquitoes have many natural enemies, but of this long list, only a few can be used on a large scale. These include toxins of bacteria (Bacillus sphaericus and Bacillus thuringiensis israeliensis) and certain types of fish.

Mosquito life cycle

Mosquitoes galore

A female mosquito lays on an average of 150 eggs every alternate day. Imagine a female lays eggs on 4 occasions in her life span (i.e., a total of 600 eggs) and half of the eggs hatch into females and the other half into males. After the 4th generation (i.e., in about two months), each female would theoretically produce 300 × 300 × 300 × 300 females, or about 8 billion females!

Natural enemies of mosquitoes Plants (Utricularia) Viruses Bacteria Fungi Protozoan parasites Nematodes (Romanomermis) Dragon fly adults Spiders Insects (Nepa sp., Notonecta sp.) Larvivorous fishes Frogs and toads Turtles Geckos House lizards Swallows Bats

Fish

Several indigenous species of fish have been identified and tested against mosquito larvae in Goal A small fish, Aplocheilus block) or "kankatre", is very effective at controlling mosquitoes and malaria. This fish is found throughout Goa in estuaries, fishponds, backwaters, lakes and streams. It is a much better predator of mosquito larvae than exotic species of fish such as Gambusia affinis (mosquito fish) and Lebistes reticulates (guppy). Aplocheilus block) is easily identified by the white spot between the eyes. Four to five fish per square metre surface area of water effectively control mosquitoes.

Another very agile, hardy and good larvivorous fish is Rasbora daniconius, or "dadiyo". These fish occur naturally in groups in ponds and streams. They are easily identified by a prominent dark line running from the operculum to the tail fin. People in Goa keep this fish in wells to keep the water clear of debris and the well walls free of algae and ferns. That they also predate upon mosquito larvae is not so well known. Two or three fishes per square metre of surface area can control mosquitoes in ponds, wells and tanks.

Aplocheilus blocki

Rasbora daniconius

Puntius ticto

Aplocheilus lineatus

Number of mosquito larvae consumed per day

The bigger the fish, the more mosquito larvae it eats. But it is better to keep many small fish than a few large ones. The table above shows that in relation to their size, smaller fish eat more larvae than do bigger fish.

Frogs

Frogs are well-known big-control agents of insects. Adult frogs prey upon flying insects by shooting their sticky tongues out at the prey. The tadpoles devour aquatic insects, including mosquito larvae. Unfortunately, the number of frogs is dwindling because of insecticide use. Frogs are also hunted for their hind legs, which are considered a delicacy. Frogs can control mosquitoes in ponds. rice fields, lakes and water tanks. They are effective against mosquito vectors of Japanese encephalitis, particularly during the monsoon.

Frogs

Turtles

Many villagers in Goa keep freshwater turtles in their wells. The people believe they bring good luck and help keep the well water clean. Scientists in the Malaria Research Centre in Goa have found that turtles also control mosquitoes. In laboratory studies, fist-sized juvenile turtles can consume about 5000 mosquito immatures per day. Unfortunately, turtles are hunted for their flesh and carapace; their numbers have been dwindling very rapidly.

Turtles

Lizards

Lizards are often hated in the household as they are thought to be scary. A careful watch would show that they prey on various insects, including adult mosquitoes.

Lizards

Spiders

Spiders spin webs on walls, in corners and in dark, secluded places to trap their prey. Adult mosquitoes can often be seen trapped in the webs.

Spiders

Need to conserve natural enemies of mosquitoes

Research has shown the usefulness of larvivorous fishes and bacterial toxins in controlling mosquitoes and the diseases they spread. But we need to conserve and use natural control agents of mosquitoes at the household level. These friendly creatures face a constant dual threat from chemical insecticides and humans. It is common in Goa to see people hunting frogs for their hind legs immediately after the first pre-monsoon showers. The hunters fail to realise that killing frogs during this breeding season prevents them from producing offspring. Public awareness needs to be created to stop the slaughter of these friendly animals. Similarly, turtles also need to be protected. People should also use insecticides in a very selective and judicious manner to avoid harming big-control agents.

Some indigenous big-control agents of mosquitoes

Prepared by Dr. Ashwani Kumar