Johannesburg residents and business owners have been battling a noticeable increase in flies, with restaurants, homes and public spaces reporting swarms that are becoming harder to control. According to Dr Nina Parry, a fly and waste bioconversion specialist who recently completed her PhD, the city is currently experiencing a surge in two main species, the house fly (Musca domestica) and the green bottle fly (Lucilia species), both of which thrive in warm, wet and dirty environments.
"These flies breed wherever there is warm, moist organic matter, that includes rotting food, compost, open drains, animal manure and poorly managed waste," explained Parry.
Her doctoral research focused on black soldier fly larvae, insects known for their ability to break down large amounts of organic waste such as food scraps and manure. Her work was about figuring out the best way to use fly larvae to turn organic waste into animal feed and fertiliser, in a way that can work at factory scale, not just in a lab.
Why is it getting worse?
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Johannesburg's summer weather creates perfect breeding conditions. In warm and wet conditions, it takes as little as three to four days and only half a teaspoon of organic matter for a fly egg to develop into a maggot. Within days, that maggot crawls away to a dark, dry place and becomes a fly ready to lay hundreds more eggs.
"The warmer it is, the faster they multiply," said Parry, adding that is the reason: "why summer is the peak season for flies, and why March is usually the worst month".
A single fly can live for up to two months, continuously breeding throughout that time. The more food waste, open drains and animal waste that exist, the more opportunities flies have to reproduce.
House flies feed on almost anything organic, from fruit scraps and food waste to open drains and compost. Green bottle flies, which are slightly less common, prefer protein-rich waste, especially meat and animal matter.
Flies pick up bacteria from the waste and organic matter they feed on and can transfer it to human food. However, Parry said they are more irritating than dangerous.
"They actually clean themselves a lot, so they don't carry as much bacteria as people think," said Parry.
Winter naturally reduces fly populations because cooler temperatures slow down their breeding cycle and causes many flies to die. But once warmer weather returns, so does the infestation.
What the City can do
According to Dr Parry, the fly problem in Johannesburg is largely linked to waste management and infrastructure.
"The City can reduce flies by improving rubbish collection, closing open drains, fixing sewer leaks and encouraging residents to separate waste properly," she said.
Organic waste, especially food scraps, should be placed in small airtight bags to limit the amount of exposed material flies can breed in.
According to Dr Parry, household insecticide sprays can quickly reduce fly numbers but offer only short-term relief. She noted that these products are often non-specific, meaning they may also kill beneficial insects that naturally control pests. She added that chemical build-up in the environment could have negative ecological consequences.
Dr Parry said that when flies gather in large numbers indoors, particularly around ceilings and light fittings, targeted use of households' insecticides sprays can help deal with the immediate problem. However, she explained that prevention is far more effective than repeatedly killing adult flies.
"Preventative measures are much more effective because they remove the things that attract flies and provide breeding space," she explained.
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She advised residents to keep kitchens and eating areas clean by washing dishes promptly after use and ensuring food particles are not left exposed. Food waste should be sealed in a separate bag and disposed of outside the home as soon as possible.
Moisture control is also important. Drying sinks and basins and fixing leaks can help reduce insect activity, especially during hot, dry weather. Pet waste should be removed daily, as it can attract flies.
For additional control, she suggests the use of hanging sticky traps indoors, red-top fly traps near refuse bins, and insect-zapper lights placed outdoors. These tools can help reduce fly numbers in specific areas.
As Johannesburg faces rising temperatures and waste challenges, Parry said tackling the fly infestation will require both better city management and responsible household and business behaviour.
