Millions of moths have descended on the suburbs. When will they leave?

If you're in New South Wales right now you may have noticed an unusual increase in bogong moths in and around the house. 

The bogong moth is currently migrating in the millions over a 1,000-kilometre stretch to cool mountain caves for a summer hibernation. 

In recent weeks swarms of the moths have been appearing in any crevice they can find and staying far longer, or so it seems.

With a long journey ahead, this is why their numbers are at levels "that haven't been seen for a long time" and how long they will stick around for.

Why am I seeing so many moths?

Caitlyn Forster, a lecturer in entomology at the University of Sydney said there were a number of factors behind this year's influx of moths.

"Bogongs actually use the stars to navigate, which means they are impacted by lights, and particularly in urban areas where we've got a lot of lights on our streets and houses," Dr Forster said.

She said in other instances weather could play a role in causing large moth numbers to appear, particularly strong winds. 

"They can actually get blown off their migration course and then more of them can end up close to the coast ... We're still trying to work out exactly what happens here."

Another point of interest for Dr Forster is the generational gap of visually seeing the insects, which were listed as endangered in 2021.

She said in theory the number of bogong moths for this time of year was normal, "we just haven't seen it for quite a while".

"There has been a drop in their populations that was really noted around 2017 and 2019," she said.  

"[But] something that's really interesting is the idea of a shifting baseline centre.

"A lot of younger people don't actually know a point where there were lots of insects around because there's not been a lot of insects around for quite a long time."

Senior conservation biologist at Zoos Victoria, Marissa Parrott told 702 ABC Sydney that 99.5 per cent of the bogong moth population had been decimated, making them almost "undetectable" in recent years.

Dr Parrott said in NSW and the ACT there had been more rain and food attracting them in numbers "that haven't been seen for a long time".

"We are so excited to see the numbers of bogong moths right now, it's the best year in nearly a decade for them," she said.

"It's a really great start of a recovery ... and we do hope it continues."

Where are they going?

Bogong moths breed in southern Queensland, northern NSW, western Victoria and parts of South Australia.

In spring, they migrate in large numbers to the Victorian Alps where they enter a summer hibernation.

But the moths' exact route to Victoria's High Country is largely unknown, making this year's surge all the more remarkable. 

Researchers from Western Sydney University and Invertebrates Australia have painted moths with a small, blue dot in the Blue Mountains to understand their distribution and flyways.

They are asking citizen scientists to report sightings of the marked moths as part of conservation efforts.

The group were also monitoring 14,000 bogong moths from Mount Kosciuszko, which had tiny paper tags attached to their wings in February.

Meanwhile, online observations of untagged moths are being used to create heat maps showing where the moths appear, which will be compared with weather patterns to see if there is a correlation. 

When will they leave?

Dr Forster said it could be days or weeks before the moths disappear from urban areas. 

"It's an interesting question because it could be pretty soon ... but we're sort of just waiting out to see how long they'll hang around," she said. 

She said in the meantime she was working on conservation education by getting people out in nature. 

"It's really important that while we've got all these bogong moths around that we actually go out in nature, particularly with our kids, and give them a chance to actually see these huge numbers and appreciate what nature has to offer," she said. 

How can I help their journey?

There have been sightings of swarms as far as Gunnedah near Tamworth, to Balmain in the Sydney's inner west and Kurrajong Heights in the Blue Mountains. 

Zoe Elliot from Bomaderry, near Nowra on NSW's south coast, told the ABC in recent weeks the nectar from her grevillea and callistemon trees had been attracting the moths.

"They haven’t been an issue for me, but I made the mistake of leaving a door open one evening," she said. 

To get the moths back outside she fashioned a trap using a large jar, a funnel and battery-powered fairy lights, sharing the idea online.

"I just thought, if people had a simple way to help the moths, even if they were busy, it may save a few and help them on their migration."

Dr Parrott said the best way to avoid a bogong moth problem was to turn off unnecessary lights, which may lure the moths away from their path.

She encouraged the public to take a photo of any moths they see and upload it to the moth tracker website, Bogong Watch. 

"That way we can see where they are being drawn away from their migration, what their numbers are doing and how best to help them keep recovering into the future."