May to July is mosquito season. The long dry spell, uncomfortable though it was, gave us some respite from the vicious little bloodsuckers. Now the conditions are ripe for their regeneration and they are breeding in droves, making their presence known in the house day and night. Sri Lanka is home to 140 mosquito species [...]

The Sunday Times Sri Lanka

Watch out, the mossies are back!

Following the rains, beware of the sting, says Daleena Samara in our series ‘De-bug’

May to July is mosquito season. The long dry spell, uncomfortable though it was, gave us some respite from the vicious little bloodsuckers. Now the conditions are ripe for their regeneration and they are breeding in droves, making their presence known in the house day and night.

Sri Lanka is home to 140 mosquito species of the world total of 3,500. Although concern is currently concentrated around dengue, there are five main mosquito-borne diseases in Sri Lanka: Malaria spread by the Anopheles group, Dengue and Chikungunya by the Aedes group; Filariasis and Japanese encephalitis by the Culex group. The female of the mosquito species does the most damage because all the males need to get on in life are sips of plant nectar. The females need blood to nourish their eggs and keep the line going.

Upturned dry leaves, banana groves which harbour water in their crevices, even habarala leaves are potential nurseries for wriggling mosquito larvae. Our own carelessness or callousness in storing containers, dumping rubbish by the roadside, ignoring clogged drains and allowing water to stagnate in unused structures aggravate the problem.

“Mosquitoes are hard to eliminate. Serious attempts to control mosquitoes begin by controlling their larvae in stagnant water,” says Raja Mahendran, expert in urban and agro pest control and consultant to leading pest control organisations in Sri Lanka and overseas. “One needs to begin by eliminating stagnant water sources because mosquito eggs are laid in water. They hatch and the larvae live on water and pupate before becoming the adult pest mosquito. The larvae are 80 percent of the problem; the adults are only the tip of the iceberg. Real control begins with eliminating stagnant water not only on one’s own property but also on neighbouring properties. It is as much a community effort as it is an individual effort.”

The buzz about mosquitoes

There are different species within the three groups, and both groups and species may have varying habits and preferences. For example, the Aëdes tends to feed through the day, from dawn to dusk, whereas the Culex feeds from dusk to dawn. Aedes eggs take a day to hatch, while Culex eggs take two. Aedes larva pupate after four days, whiles Culux larva take over a week. In general, all groups follow a uniform life cycle: the egg hatches when exposed to water giving birth to larvae that live in the water, molting several times and surfacing to the top to breath air. The larvae then molt into purpae, a non-feeding stage just before emerging as adults. The newborn adult is soft, and after a short period of hardening, flies away.

The diseases mosquitoes carry are also very complex. For example there are four different types of the dengue caused by four related viruses (serotypes) all belonging to the Flavivirdae family, and transmitted through the bite of infected female Aëdes aegypti and Aëdes albopictus mosquitoes. Contracting one serotype will not provide immunity against the other three. In fact, it could complicate future infections of the other three serotypes. It’s also possible to contract multiple forms of dengue, intensifying the severity of the illness. That’s why some people require intensive medical care after contracting dengue, while others are mildly affected and recover quickly.

Here are more odd mosquito facts:

-   A single malaria mosquito can infect over 100 people.

-   A child dies of malaria every 45 seconds in Africa, according to WHO.

-   They are the slowest flying insect of all, moving at 1.5mph at top speed.

-   Alexander the Great is said to have died of a mosquito bite.

-   60 Culex mosquito species carry the West Nile virus.

-   Mosquitoes spit and drink because their saliva contains an anticoagulant that stops blood from clotting, so ensuring they drink their fill.

-   Their spit irritates the skin.

-   They are attracted to sweat while chocolate confuses them.

-   Mosquitoes infected with viruses tend to drink more blood than other mosquitoes.

-   They are an important food source to a number of organisms

Prevention is better than cure

Source reduction is crucial, says Mahendran. Eliminate breeding sites: check old tyres, bottles, cans, flower pots, roof gutters with leaves, even some leafy plants with cavities that hold water.

A serious infestation may call for professional larviciding. “It’s important to treat the larvae with larvicides,” says Mahendran. “For example, Temephos, an organophospate, is used in Sri Lanka. However, organophosphates have fallen out of favour because they were suspected to affect humans in the long term. Alternatives like IGRs (insect growth regulators) and microbials like bacillus thuringiensis for biological control are now being used in developed countries.”

The US Environmental Protection Organisation has declared Temephos safe when used in the highly diluted form described in the instructions on the label. However, it has been found to damage the nervous system when used indiscriminately in high concentrations. It was also found to affect bees, and some bird species.

Microbials on the other hand are natural bacteria introduced into breeding sites to disrupt the growth cycle of larvae by affecting the gut, thus effectively disrupting the new generation. These are used in combination with other methods in an integrated pest management approach.

The introduction of certain mosquito species whose larvae feed on the larvae of other mosquito species is another natural solution.

Nature’s answers

What can one do about the swarms of adult mosquitoes that knock on our windows at dusk? We could lock ourselves in, shut windows at dusk and dawn when mosquitoes become very active. We could also slather on insect repellent purchased from the local supermarket.

The internet is full of natural mosquito-repellent solutions such as cultivating citronella or garlic in the home. “These plants do repel but unless you have a large number of them all round your house, on balconies and inside house, you are not going to observe any benefits,” says Mahendran.

He suggests the use essential oils or sprays containing concentrations of the repellent properties of the plants. “A recent study showed that Victoria Secret’s Bombshell perfume does a good job in repelling insects and so does Lemon Eucalyptus oil. I would place my bets on those than planting a few repellent plants in the garden,” he says.

To protect himself and his family from mosquitoes, Mahendran prefers to have fly screens on his doors and windows and to use the fan or the airconditioner. “I don’t like to apply chemicals on my body. The less contact with pesticides the better – they should be avoided, especially in daily use,” he says.

What would you do if you found a large mosquito breeding ground in your neighbourhood? This writer did and brought it to the attention of the National Dengue Control Unit. That was three days ago; they have not yet responded.

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