The War Against Mosquitoes

In the ice-cold lakes of the Alaskan tundra as well as in the swampy and stagnant waters of Louisiana in the United States, the one common perennial enemy of man is the female mosquito which sucks blood through her tiny tube-like proboscis from an infected individual and injects her saliva into another. This act causes a red swollen itching blotch and sometimes leads to serious diseases such as malaria, encephalitis, dengue fever and yellow fever. After completing her dinner in this rather gruesome fashion, she deposits 50 or more eggs in different watery places such as tree holes, wet basements, marshes, swamps and even muddy boot prints. While the male mosquito prefers to dine on nectar, the female more than compensates for his vegetarianism, by gorging herself in blood like a vampire.

Mosquitoes are found in almost every part of our planet. In order to tackle the mosquito menace, experts all over the world rely on a wide range of methods—old as well as new. Gradually, the emphasis is shifting to newer and safer ways of destroying mosquitoes without endangering the environment. Some of the most ingenious new methods involve turning nature itself against the insect marauders. The simplest form of natural mosquito control is centuries old— draining or filling in the watery dwelling places of the mosquitoes.

Another common method developed at the end of the nineteenth century and which is now still in use in the rice fields in California is introducing larvae-eating fish into the waters where mosquitoes are known to breed. But these methods, though effective, are clearly not adequate.

In some quarters doubts have been expressed about the safety and long-term effectiveness of insecticides. In many areas mosquito control has relied heavily on chemicals, particularly on larvicides sprayed on stagnant pools and other watery areas where mosquito larvae abound.

It has been observed that adulticides sprayed on habitats of adult mosquitoes are far less effective. Both these types of chemicals have common drawbacks even though they are not as dangerous as DDT. Adulticides and some larvicides can still harm the environment and what is more disturbing is the fact that mosquitoes gradually develop a resistance to the chemicals. This development has therefore necessitated an urgent need to reduce dependency on insecticides. (California which used to rely on pesticides for mosquito control for a few years has brought down its demand drastically.)

Since mosquitoes differ widely in their breeding places and habits, scientists have begun to think seriously in terms of devising biological methods to deal with them. They are now in a position to destroy whole colonies of mosquitoes by infecting larvae with bacteria, fungi and parasites. They can introduce into the larvae a hormone that prevents them from growing into adults capable of biting.

In one place in the United States they have tried out hormone- saturated briquettes that gradually release the substance after each summer rain. Scientists can also sterilise male mosquitoes with radiation and permit them to mate with females who will then produce sterile eggs. They can even turn one bug against another by inducing one breed of mosquito larva to swallow up another.

One of the most promising weapons in the new biological arsenal exploits the fact that mosquitoes, like all other organisms, are vulnerable to disease. Since many bacteria, viruses and fungi that attack mosquitoes are harmless to other forms of life, researchers are seeking methods to harness and direct them in force against their natural enemies. In Israel a bacterium known as Bacillus thuringiensis israelensis (BTI) has been discovered that lives in the soil and which produces a toxin that is effective against many species of mosquito larvae. When BTI is sprayed it is eaten by larvae which mistake it for natural food. Once inside the mosquito’s body, in its intestines, BTI explodes in the cells and kills the immature insects. Unlike some of the standard insecticides, it kills 90 per cent of the larvae.

The main disadvantages of BTI are: (a) the exorbitant cost (BTI is nearly twice as expensive as standard insecticides), (b) The toxin must be laboriously extracted from the sack-like crystals on the outside of the bacteria. As it is very hard to purify, mosquito controllers must use more of the substance to obtain the same results they get from the pesticides. Nevertheless, entomologists are of the opinion that BTI can be rated high as a useful weapon against mosquitoes.

Mosquito controllers are also investigating the effects of another soil bacterium, known as Bacillus sphaericus, which can prove fatal to larvae that feed on them. However, these bacteria are not prolific; they do not make enough of the poison for commercial use and they also tend to lose their ability to produce the toxin.

In order to make the bacteria more powerful and effective, micro­biologists in the University of Arizona began to use genetic engineering. They found that the bacterial gene that controls the toxin production of Bacillus sphaericus is located on ‘superplasmids’, i.e. large packets of genes that lie outside the gene in the nucleus. Using the latest recombinant DNA techniques, they have successfully cut a gene out of the plasmid with enzymes that act as chemical ‘scalpels’. The scientists were planning to insert the gene into the normal plasmid of another species, Bacillus subtilis, a soil organism which has the ability to reproduce rapidly.

The progeny of such re-engineered Bacillus subtilis would carry the gene for the toxin and soon sufficient new strain of bacteria would be available to ensure an ample supply. Scientists believe not only that the bug is harmless to people, animal and plants but also that the mosquitoes would have very little chance to develop resistance to it. (Resistance to a chemical comes about from a single gene change, e.g. one that blocks the entry of the chemical.) With this toxin a number of events would have to take place; for instance, it has to be digested by several systems in the mosquito’s gut, each of which would have to be genetically altered for resistance to develop—a process unlikely to occur. The microbiologists of the university of Arizona tested the toxins for a while in the pools of stagnant water in California, confirming their theories.

All mosquitoes however do not breed in pools of stagnant water. Some of the most troublesome mosquitoes breed in relatively inaccessible places like tree holes. Scientists in California have successfully tested a lethal fungus whose infectious spores invade the mosquito larvae, eventually killing them and then produce millions of infectious spores. They were planning to develop a strategy by which mosquitoes can be attracted to special traps where they get contaminated with fungus spores. The females then carry the spores back to the tree holes when they return to lay eggs; they not only die from infection but also poison the tree holes with deadly spores. Researchers are now testing the potency of the spores against a particularly bothersome mosquito which breeds in water-filled containers and which transmits dengue fever. (The same species carries yellow fever in some other parts of the world, like Africa).

Since it is difficult to manipulate fungus, scientists are willing to try even a new method such as ‘cannibalism’ to wipe out tree hole mosquitoes. One particular variety of large mosquito (Toxorhynchites rntilus), nicknamed BigTox, feeds on the larvae of smaller mosquitoes. However, as it does not appear in large enough numbers to take care of the entire mosquito population, plans are underway to breed this mosquito in the laboratory. If large numbers of the mosquito’s eggs could be collected, they can be placed in tree holes or, alternatively, the females can be raised and allowed to enter the tree holes where they could deposit their eggs. It is fortunate that the larva of BigTox does not produce the biting mosquito. Its proboscis cannot penetrate the human skin and the Big Tox dines only on nectar and plant detritus.

It may thus be seen that scientists are constantly devising new methods to vanquish an enemy whose threat to human life though totally out of proportion to a deceptively tiny size, cannot be underestimated. But they are justifiably optimistic that the latest weapons in the biological arsenal have given humankind, for the first time, an advantage over a foe in a battle that has been going on for decades.