Drain flies are not particularly seasonal but, like all insects, they are cold-blooded so breed more in warmer temperatures.
Their total lifespan from larvae to death is one to two months, with their final 20-odd days spent as the adult fly you see in your home.
After Sydney’s unseasonably warm start to autumn, there are probably more drain flies around now than usual.
“I would expect to see higher numbers during summer, in warmer weather,” said Dr Dan Bickel, principal research scientist at the Australian Museum. “But if your house is warm, the seasons probably do not make a huge difference.”
Latty and Lessard agreed.
“They also prefer stagnant water, so if you go on holiday and don’t run your taps for a while you might notice an increase when you come back,” Lessard added.
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The entomologists agreed people wanting to rid their house of drain flies should avoid pouring insecticide down the sink. Instead, a kettle of boiling water will kill the larvae feeding on the sludge in your pipe. And cleaning those pipes will discourage new flies from laying their eggs there.
“The first thing to do is to try to figure out where they’re living, and keep that clean,” Latty said. However, she also urged people to take a more sympathetic approach.
“They’re not a health problem; they don’t vector diseases. Most are just vaguely annoying, but I actually think they are kind of furry and cute,” she said.
Bickel similarly opposed the senseless killing of drain flies (“people get really upset about these, but they are harmless”), while Lessard argued the drain fly was actually a good housemate, helping to tidy things up around your home.
“The larvae actually help clean your sinks because they eat those kitchen scraps that might get stuck in the drains,” he said, adding that the adults do not bite or feed: like other flies, they feast as larvae and then live until their energy runs out (or they meet a swatted demise).
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