For many Australians, a sign that Christmas time has truly arrived is an encounter with a bright, sparkly beetle.

There are different variations right around the country, according to Simon Fearn, the collections officer at Launceston's Queen Victoria Museum and Art Gallery.

"They are a fairly large and diverse group and the males typically have long mandibles that they fight each other with."

In northern Australia, beetles can have mandibles as long as 44 millimetres.

Tasmania's Christmas beetle is a bright green and gold stag beetle, although colours vary from place to place; on the coast, the beetles can be dark purple or bronze.

"No-one knows why," Mr Fearn said of the colour.

"It could be something to do with the chemical content in the soil."

This Christmas beetle is the largest ever documented in Tasmania. ( Supplied: Queen Victoria Museum and Art Gallery Collection )

Mr Fearn recently found the largest Christmas beetle ever recorded in Tasmania.

"One got flung off the tree and onto the lawn," he said.

"I went down and got him and he turned out to be the largest one we've ever documented."

Life starts as a grub

Christmas beetles aren't born festive-looking and start life as a white grub.

And while they are common across mainland Australia, Tasmania is a Christmas beetle stronghold.

The beetles spend about two years in rotting trees as grubs. ( Supplied: Queen Victoria Museum and Art Gallery Collection )

Mr Fearn said this was because they did not have to compete with as many other types of beetles.

He said they love to live in rotten wood and the females laid their eggs in the dead roots of tree stumps.

"The grubs bore into the roots and stay as large white grubs for about two years," he added.

"They turn into a beetle in March or April, then they stay in the roots as a fully formed beetle until Christmas time.

"The Christmas beetle that everyone gets excited about is in the final stage of a three-year cycle."

The beetles then fly into the tops of gum trees, where the males use their mandibles as pruning sheers to cut off shoot tips and drink the sap.

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Creating a beetle-friendly garden

In Tasmania especially, there will be hundreds of thousands of beetles feeding in the tops of trees.

"Most people don't even notice them," Mr Fearn said.

"The other giveaway sign is when there's a lot of beetles feeding in a tree, on the ground there's lots and lots of shoot tips.

"Every time someone cuts down a tree and leaves the stump in the ground, they're providing a food source for many hundreds of Christmas beetle grubs."

It was rare for the beetles to harm gardens, Mr Fearn said, and would only be a problem if they were in plague proportions.

"Normally in your garden they are just a great thing to have," he said.

"They are a species that thrives where humans are.

"If you cut a tree down, try and leave the stump in your lawn because you're going to provide a really important food source for these beetles."

Beetle Bachelorette

Mr Fearn said male Christmas beetles were much larger and outnumbered females by about three to one.

This imbalance has brutal consequences for the males and their long mandibles.

"The males have to compete to get access to females," he explained.

"They are constantly fighting each other in the tops of the trees."

The losers might fall to the ground, which is when most people notice them.

Male Christmas beetles must fight each other to win over the females. ( Supplied: Queen Victoria Museum and Art Gallery Collection )

Are there fewer?

Mr Fearn said he was frequently asked whether Christmas beetle numbers had dropped, with many people recalling seeing more when they were younger.

They haven't, but he said it's probably seen that way because most adults were not outside as much as they were as a child to notice.

However in Tasmania in the early 1980s the beetles were around in plague proportions; they made homes in untreated power poles and hatched when they were replaced.

"So older people probably do have a point," Mr Fearn said, adding that Christmas beetles were still around in healthy numbers.