Payback time for mosquitoes

Payback time

Mosquitoes bite us, drink our blood and give us disease. With the help of bacteria, microbiologists suggest we can retaliate against the insects by killing their offspring.
Picture by SMITH SUTIBUT

Research into the use of bacteria may soon give us the upper hand in the long battle against mosquitoes

Story By Pongpet Mekloy

Mosquitoes have always been one of humans’ greatest nemeses. Over the millennia, people have used a variety of strategies-from barehanded combat to chemical warfare-to battle against these blood-sucking, disease-carrying insects. But the fight never really ends and often our defences backfire.

Now, microbiologists suggest, it’s time we get help from a third hand-bacteria.

Scientists have long known that several species of bacteria-Bacillus sphaericus and Bacillus thuringiensis, for example-can act effectively as bug busters. Several strains of Bacillus thuringiensis, better known as BT, have been widely used to tackle problem insects in farming. Their genes have been engineered into certain plants to create pest-free crops.

However, in contrast with agriculture, the use of microbes for public health-in fighting against mosquitoes, to be specific-has been relatively limited. But with the mosquitoes’ increasing resistance to insecticides and the chemicals’ high prices and dangerous consequences, the need for safer solutions has become more and more prominent, both in Thailand and elsewhere. And careful uses of bacteria as biological controls look very much like a good answer.

In this magnified cutaway photo of a Bacillus, you can see the bacterium’s round spore, which is covered with tissues laden with a toxic protein.
Picture courtesy of THE THAILAND INSTITUTE OF SCIENTIFIC AND TECHNOLOGICAL RESEARCH

So how do they work, you may wonder.

Of course, the bacteria will never be able to chase after the winged adult mosquitoes. Instead, in sort of a suicide mission-they must be eaten first-they take on the insects’ larvae.

The weaponry of the microbes is their toxicity. They develop protoxins in parts of their single-celled bodies, especially around the spores. When a hungry wriggler feeds on them, the alkaline condition in the larva’s mid-gut turns the protoxins into toxins which then wreak havoc in the larva’s digestive system and finally kill it.

However, these selected microbes are considered harmless to people and many other organisms whose acidic stomachs would not change the bacteria’s protein-based protoxin into hazardous substances. And this is an edge that biocontrol has over chemical pesticides-it doesn’t kill everything in its path, just the targeted troublemakers.

Besides, as long as you eat well-cooked food you never have to worry about having these microscopic strangers running around inside your body, since they and their toxic quality are completely destroyed within 10 minutes when exposed to heat higher than 80 degrees Celsius.

Another advantage of biocontrol is that it is harder for the targeted insects to develop resistance against it, because the victims are attacked by several toxins at the same time, not by a single active ingredient as they are when sprayed with chemical insecticides.

Over the past years, Mahidol University, the Thailand Institute of Scientific and Technological Research and a number of agencies related to the Public Health Ministry have conducted studies on the cultivation of useful bacteria as well as on the technologies needed to incorporate them in biocontrol products.

Lab tests and field trials so far have been successful, but it will be a while before private investors are convinced of the products’ potential in the market and produce them on a commercial scale.

Anyway, the business aspect is one thing, but the twist in this never-ending war between man and mosquitoes is that after infecting countless generations of people with a variety of diseases, this time it’s the mosquitoes who are infected.

Bacterial products for biocontrol come in a variety of forms _ from powder and concentrated liquid to floating briquettes like these.

Truths about bacterial products

- Each year, Thailand imports more than 10-million-baht worth of chemicals for the formulation of Arbate sand, which is used for killing mosquito larvae, and more than 100-million-baht worth of bacterial products for pest control in vegetable farms.

- For several years already, Thai researchers have been able to develop the formulae and technologies for the production of several bacterial biocontrol products. However, the locally produced products-which are expected to be cheaper and more effective because of their freshness-will not be commercially available unless private investors step in.

- Unlike chemical pesticides, biological control agents do not kill the target insects instantly-one reason why they are not so popular with mainstream farmers. But that doesn’t mean they do not work. As soon as the pests get infected, they stop feeding and no further damage is done to the crop, although the insects are not yet dead.

- For more details about bacterial mosquito larvicides and their production, contact the Technology Transfer Group of Thailand, Institute of Scientific and Technological Research, on 579-5515 or 579-1121-30, or Mahidol University’s Faculty of Science, on 246-0063 ext 1111 (Dr Amaret Bhumiratana) or ext 6413 (Tawatchai Monkholwai). The microbiologists at Mahidol University can also provide you with information on bacterial products for agricultural use.

Mosquito facts

- City folks tend to be complacent about mosquitoes during the daytime, but the truth is, it’s the day biters-Aedes mosquitoes, or yung lai in Thai-that transmit the feared haemorrhagic fever. Night-time nuisances are those of the Culex species or yung ramkhan, which sometimes carry viruses that cause encephalitis, while Anopheles mosquitoes, or yung kon-plong, the bearers of malaria, do not exist in urban areas.

- Contrary to popular belief, mosquitoes do not lay eggs in heavily polluted water.

- To prevent mosquito larvae from developing in your flower vases, change the water every three or four days. Also, to make sure the larvae do not survive, you should empty the vases onto dry ground-not into a drainage system.

- If you keep water lily basins, put some small fish in there, too. The fish will add life to the lily basins and at the same time prevent mosquitoes from using them as breeding sites.

Outlook Nature wishes to thank:

- Dr Amaret Bhumiratana, dean of Mahidol University’s Faculty of Science

- Puangpen Suyanandana of the Thailand Institute of Scientific and Technological Research

- Laojana Chowanadisai of the National Institute of Health’s Biological Control Section

Copyright The Post Publishing Public Co., Ltd. 2000