Lounging on the tulips, roosting in the eucalyptus, humming around the bottlebrush – a butterfly swarm has invaded a rural Australian town to the delight of locals and tourists alike.

Thousands of monarch butterflies have taken over Nannup, in south-west Western Australia, although experts warn their high numbers are evidence of the advance of invasive weeds.

Horticulturalist Chris Barlow said the butterflies, which have a life cycle of about six weeks once hatched, feed as larvae on one of two weeds, varieties of cotton bush and milkweed, which are prevalent in surrounding pine plantations.

A monarch butterfly settles on tulips at the Nannup flower and garden festival. Photograph: Angela Rasmussen/Nannup flower and garden festival

“The reason why it was so good this year is because they had a very good food source,” he said.

Barlow said the species is one of few that can eat the plants, adding that the monarch’s distinctive, bright orange colouring is a warning of their potential toxicity. “There’s only a couple of species of birds that can actually eat the butterflies.”

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But the adult butterflies are also interested in other plant species. Maggie Longmore, who runs Nannup’s annual flower and garden festival in August, said they were particularly drawn to the tulips, as well as native bottlebrush and eucalyptus plants.

“It has been an added attraction for the festival,” she said. “It’s gone a bit viral on social media … People have been coming to see the butterflies and the tulips.

“I’ve lived in Nannup for 25 years. We see a few butterflies around, but this year they’re in the thousands.”

The window has also been particularly long this year – more than two months, Longmore estimates. Burana Skelton, from Mandurah to the north, was driving through Nannup with her family during the July school holidays when they started to notice the insects.

“We were, like, counting the butterflies as we were driving down the main street, going: ‘Oh, there’s one, two.’ And we got up to 20,” she said.

“As we got further down towards the little local park area … it was literally covered in [them].

“My husband and the kids got out the car … We had all our windows down and even those dogs were looking at it.”

Monarch butterflies roost by the roadside in Nannup. Photograph: Burana Skelton

Monarch butterflies are native to North America and are believed to have been blown across to Australia via the Pacific islands during cyclonic weather in the early 1870s.

Barlow says the species only took hold in the last 30 years when the invasive weeds also gained a footing, describing the situation as “a delicate balance”, but still “a beautiful sight”.

“What we recommend is people to control the milkweed and the cotton bush inside of bushland areas, and in and around roadsides, so it doesn’t spread.”

Skelton said the phenomenon’s complicated backstory did not detract from its beauty.

“It’s hard to put into words,” she said. “It felt magical.”