Humidity, lack of rain — reasons for glut of flies in Delhi

This is not an isolated case — an unusually high number of flies are being reported from different parts of the city this monsoon.

While the monsoons are indeed the season for flies to breed, this year, they are breeding in extremely large volumes, with experts pointing to a combination of four factors — high humidity, several days going with light or no rain, delayed desilting of drains, and inadequate waste management.

Faiyaz Khudsar, scientist in-charge of DDA’s Biodiversity Parks Programme, said warmer months are conducive for breeding of house flies, and the July-August period is generally their peak time to breed. “The air is moist and there is regular rainfall and water accumulation. When combined with rotting or leftover food on the streets, this makes it an ideal ground for these flies to breed. This rotting and wet food also makes it easier for flies to feed through proboscis. This is what is possibly leading to a surge in their numbers at this time,” he said.

Depending on the weather, the housefly cycle may take as little as a week to complete, or take up to four weeks. The egg-laying stage consists of multiple single millimetre-long eggs laid down by the fly in a group, with over 500 laid down in a single cycle. Housefly maggots or larvae, upon hatching, prefer to feed on decaying food, which experts said is abundant during the monsoon season. This then is followed by a pupal stage, where the maggot encloses itself in a casing, before the housefly emerges. Each house fly can live up to four weeks.

This month, southwest Delhi has recorded a 32% rain deficiency till July 14, according to the India Meteorological Department (IMD). The figure for the deficit increases to 37% in New Delhi, 38% in south Delhi; 70% in north Delhi, 85% over northwest Delhi, and 99% over west Delhi.

Delhi’s relative humidity meanwhile ranged from 91-94% on Saturday; 85-63% on Friday; 71-56% on Thursday, and 98-100% on Wednesday.

Entomologist Mohammad Faisal said heavy showers can wash fly maggots away. “But that is not happening. Humidity is high, and there is light, scattered rain at times. This, combined with desilting muck lying unattended, and delayed garbage disposal gives them ideal conditions to breed in large numbers. We may see a reduction in their numbers if there is a very heavy spell of rain,” he said.

Other experts said housefly numbers also shoot up in February. “What flies need is an ideal temperature of 27-35 degrees Celsius, which is what we are seeing right now. This ideal temperature range speeds up their breeding cycle from egg to maggots, pupa, and finally flies and allows them to breed quickly,” said Sumit Dookia, assistant professor at Guru Gobind Singh Indraprastha University (GGSIPU) and part of Delhi’s Biodiversity Council.

The United Residents Joint Action (Urja) — an apex body of over 2,500 resident welfare associations in Delhi — said improper management of waste has emerged as the biggest problem to high house fly numbers.

“We receive the highest number of fly-related complaints in neighbourhoods where the local market has restaurants that leave some leftover food in back-lanes or in the open. This decaying food becomes a breeding ground for flies. At the same time, we have seen high fly numbers in areas where household waste is left outdoors, but is not regularly picked up,” said Urja president Atul Goyal.

One of the major concerns and indirect damage by these flies is by the potential transmission of pathogens (viruses, bacteria, fungi, protozoa, and nematodes). Pathogenic organisms are picked up by flies from garbage, sewage and other sources of filth, and then transferred on their mouthparts, through their vomitus, faeces and contaminated external body parts to human and animal food, according to Institute of Food and Agricultural Sciences at University of Florida, the US.

Pathogens commonly transmitted by house flies are salmonella, shigella, campylobacter, Escherichia, enterococcus, chlamydia, among others. These flies are most commonly linked to outbreaks of diarrhoea and shigellosis, but also are implicated in transmission of food poisoning, typhoid fever, dysentery, tuberculosis etc.

Dr Pulin Gupta, professor, medicine at Ram Manohar Lohia hospital, said that over the last 15 days, there has been a surge in acute gastroenteritis, apart from viral infections.

“In the last 15 days we are seeing an upsurge not only in viral infection but also in acute gastroenteritis. These are not looking like typhoid which of course happens when sewage water mixed with drinking water. This is the breeding season for mosquitoes and flies. The numbers are more this year. The flies sit of faeces and then carry the infection to food/cut fruits etc. there is definitely an uptick now,” said Dr Gupta.