You roll out of bed on a lovely spring morning, make yourself a cup of coffee, and gaze out across the tree-lined street full of soft, white blossoms.

There's a light breeze that brings along wafts of warm … wait, what is that smell … does the air smell like semen?!

That's a question people have been asking on social media, and as the ABC Science video presenter-producer (aka the nerdy doctor who lives inside the TikTok and Insta screen prison you carry around in your pocket), it's my job to answer that. 

The culprit of the unexpected odour is the blooms of the Pyrus calleryana tree, also known as the ornamental pear tree.

If you don't recognise the name, maybe you've heard their Aussie nickname: "cum trees".

You can find them mostly growing in south-east Australia, especially in Victoria and NSW, but they're also found further afield in Queensland, South Australia and Western Australia.

The pungent aroma comes from their blossoms — the odour plume tickles the odorant receptors in your nose and the combination of activated receptors helps your brain work out the scent.

If you've smelt semen before, then your brain likely recognises an odour-evoked memory, and there you have it — suddenly you realise these flowers smell an awful lot like ejaculate.

Because semen can smell like ammonia, several experts have suggested that the similar smell from the ornamental pear blossoms comes from volatile amines.

But I went hunting through scientific journals and discovered there aren't any peer-reviewed studies demonstrating this, so take that with a grain of salt.

What's the smell really for?

"Flowers function as a sort of advertisement, like a billboard", says Patricia Lu-Irving, a botanist at the Australian Institute of Botanical Science.

White flowers tend to be pollinated by insects, while coloured flowers are more likely to be pollinated by animals, she says.

"To humans, white petals might look boring, but to an insect, each petal looks like a simmering runway because they can see the UV light reflected," says Bryan Lessard, an Australian entomologist with Parks Australia and better known as Bry the Fly Guy.

The white flower of the ornamental pear attracts insects — but to make sure they attract the right type of insect, the flowers produce a unique smell.

"Most floral scents function to attract pollinators," Dr Lu-Irving confirms.

The dank smell of the ornamental pear blossom attracts honey bees and flies, specifically, hover flies.

Honey bees are well known for being fantastic pollinators, but what you might not expect is that hover flies are too.

Hover flies are basically bee cosplayers; they copy the adorable yellow and black stripes of our better-known pollinators — much like these performers from RuPaul's Drag Race.

The real question is: who wore it better, Jan or hover flies?

Their bee mimicking doesn't stop there; Dr Lessard says they even buzz around flowers, and that's why they're sometimes called flower flies.

Hover flies use their long straw-like mouth parts to slurp up the ornamental pear blossom's sweet nectar and chow down on that protein-rich pollen.

Their adorable hairy little legs rub against the pollen and collect it, which they transfer to different flowers as they buzz between each meal. The end result? Pollination!

Hover flies may look like they're hatching a diabolical plan when they rub their hands together, Dr Lessard says, but these cuties are actually scraping the pollen off their eyes so they can see, putting it into a little ball with their feet and then eating it. 

"Hover flies are super cute and adorable," Dr Lessard says.

"In Australia, there are 200 species of hover flies that are native to the country." 

Hover flies aren't the only underrated Australian pollinator — blow flies are also excellent at pollinating.

"Blow flies, or blowies as they are affectionately called, can actually carry twice as much pollen than the European honey bee," Dr Lessard says.

Dr Lessard's research has even found that blow flies can carry pollen from "up to 37 different species of native plants".

Commercialised fly populations are also being developed, so we're not just reliant on bees for crop pollination.

So, next time you see the ornamental pear blossoms lining your street and catch a whiff of their pungent scent, remember that the smell isn't for you — it's for honey bees and their biggest fans, hover flies.

Flies are essential pollinators, they are doing important work, and that smell will soon pass.