Dengue mosquitoes: These are a few of their favourite things

Dengue mosquitoes can breed in as little as 5 ml of stagnant water. Bengaluru is full of potential breeding sites for these mosquitoes.

Bengaluru grapples with a dengue crisis yet again and it is worth asking why. What about the city makes it ripe for a dengue epidemic? Dengue is a tropical disease caused by the dengue virus, which is transmitted to humans by mosquito bites.

Most dengue transmissions are caused by one mosquito species, Aedes aegypti. This species also transmits Chikungunya, Zika, and Yellow fever.

Aedes breeding behaviour

Dengue mosquitoes are large, slow fliers and have distinctive white markings on their legs, making them easy to spot. As with most mosquito species, only the females bite and drink blood, usually just before they lay eggs.

Dengue mosquito species, Aedes aegypti
Dengue mosquito species Aedes aegypti is large and has distinctive white bands that make it easy to identify. Pic: Muhammad Mahdi Karim (Wikimedia)

While some mosquitoes lay their eggs directly in stagnant water, Aedes species often lay their eggs on the edges of water, such as the bank of a puddle or the walls of a bucket. The eggs can lay dormant for several months. The eggs hatch when these containers or crevices fill up with water, usually after rains.

Multiple studies have found that dengue mosquitoes thrive in artificial or man-made containers with stagnant water. These include drums, unused grinding stones, flower pots, discarded paper cups and other waste.

I took a walk in my neighbourhood and found a few sites where dengue mosquitoes can thrive.

Read more: Dengue in Bengaluru: The situation so far

Discarded pots

Discarded pots are perfect containers to hold stagnant water over a long period of time.

A discarded pot with  water and mosquito larvae
A discarded pot that previously contained pot biryani. Pots like these often end up on roadsides and in garbage heaps. This one was found in an empty site. You can see mosquito larvae floating on the surface. Pic: Bhanu S
Mosquito larvae in a discarded pot of water.
Close up of the larvae. Pic: Bhanu S
Discarded flower pots.
Discarded flower pots are great sources of stagnant water for mosquitoes. This pile was left behind by a plant nursery that shifted from the area. Pic: Bhanu S
A large discarded vessel
Water lilies were once kept in this large vessel. Studies show that dengue mosquitoes prefer stagnant water, which previously had some leaves or biological matter that was broken down by bacteria. Pic: Bhanu S
An erstwhile lily pond
Another erstwhile lily pond. Pic: Bhanu S
Old and broken flower pots
People often leave behind old and broken flower pots while moving homes. Pic: Bhanu S

Read more: In Kolkata, filthy fountains spread dengue fear

Water bowls for street dogs

It is common practice in Bengaluru for people to place bowls of water outside shops and houses for street dogs. The dogs may drink from them frequently in summer. However, in monsoon, with cloudy weather and cooler temperatures, the bowls are not used as often. If water is not replaced regularly in these bowls they can become hotspots for dengue mosquito larvae.

Bowl of water
Bowls or pots of water placed for street dogs. Pic: Bhanu S
Unemptied water bowl outside a house
This water bowl for dogs placed outside a home in a residential street has not been emptied in a week. Pic: Bhanu S

Water storage containers, drums

Low-income communities or informal settlements in our city often don’t receive running water. Citizens are therefore forced to store water in drums, pots and buckets. Even small amounts of water leftover in these containers are enough for mosquitoes to breed.

Bucket with collected rainwater
Forgotten bucket with rainwater collected. Pic: Bhanu S
Construction workers in the upcoming Rajiv Gandhi Layout in North Bengaluru live in makeshift homes on empty sites, adjacent to houses they are building. They store water in these large drums. The drum above is kept covered. Pic: Bhanu S
Drums with collected water
But often such drums are kept uncovered. And if the water in these is unused for a while, studies show they have potential to attract dengue mosquitoes. Pic: Bhanu S
Puddles created by washing clothes and utensils
Washing clothes and utensils creates such puddles in places with poor drainage. These puddles can become viable habitats for dengue mosquito larvae. Pic: Bhanu S

Potholes and poor drainage

Speaking of poor drainage, mosquito larvae can breed in as little as 5ml water; potholes and poor drains provide ample water sources for mosquitoes. According to media reports from Gurgaon, potholes that have not been repaired for a long time and were collecting stagnant water were discovered to have mosquito larvae.

Frequent rain and lack of sunlight contribute to potholes holding water for extended periods of time without drying out, like the ones shown in the above and below pictures. Pic: Bhanu S
Pic: Bhanu S
Rainwater flowing into a gutter behind a building
Several parts of Bengaluru, particularly periphery areas, still don’t have a well-developed underground drainage system or clear rain gutters. Here, rain water from the flooded basement of this apartment building is being drained into an adjacent gutter. Pic: Bhanu S
Gutter with construction debris and mud
The gutter is clogged with construction debris and mud. The water stands stagnant for several days before drying out. Pic: Bhanu S

Papers cups, solid waste

Bengaluru’s garbage crisis feeds into the dengue crisis. Reports show that solid waste accumulated in an area collects and holds stagnant water, making it a potential breeding site for mosquito larvae.

Empty sites with garbage
Empty sites such as this one are commonly used as dumping grounds in the city. Pic: Bhanu S
Discarded plastic and paper cups
Discarded plastic and paper cups accumulate enough water for mosquito larvae. Pic: Bhanu S
Discarded paper cups and paper plates
Pic: Bhanu S
Discarded food packets and containers
Discarded food packets and containers are similarly potential breeding grounds for mosquitoes. Pic: Bhanu S

The way forward

It is important to note that while not every photo may contain dengue mosquito larvae or eggs, the photos are a good representation of the urban environment in which dengue mosquitoes thrive.

Mosquitoes on human legs
A final image of dengue mosquitoes feasting on my legs, foolishly left exposed, as I walked through my neighbourhood. 80% of people infected with dengue virus will show no symptoms, but mosquitoes biting us and then biting another person can spread the infection. Pic: Bhanu S

Researchers predict that as the planet sees warmer weather and unpredictable rainfall, vector borne diseases like dengue will become much more prevalent. Some reports even suggest that dengue in India could attain COVID 19-like pandemic levels.

Our built environment, infrastructure and waste management will have to improve significantly to cope with this problem.

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