Mothmaggedon: Where the Bogongs came from and where they have gone

It came seemingly out of nowhere. Heralded by the occasional flutter around the porch light, promptly followed by a determined and recurrent thwack against the kitchen window by one, two and then dozens of moths and then the final step of any local event, the inevitable posting to social media: the Bogong Moths had arrived.

If you were thinking it had been years since we had an influx of Bogongs like the numbers experienced over the weekend, you’d be right.

The project manager at Bogong Watch, Linnea Vilhelmsson Emneby, said: “It seems to be the biggest number of moths we have seen since their numbers drastically declined by 99.5 % after a few years of drought starting in 2016/2017.” 

But she warned to not read too much into the sightings just yet:  “…compared to what the Bogong numbers used to be a decade or two ago, it is unclear how today's number compares, and we have to be careful about drawing conclusions without the data to properly support it.” 

And getting that data has proven to be a tad tricky. I wrote earlier this year of the tagging of some 10,000 Bogong Moths (turned out to be closer to 14,000 individuals!) in an effort to learn more about these endangered insects. Linnea explained that while none of the tagged individuals were spotted again in the wild, the project still proved valuable.

“We didn't see any of the tagged moths again. That's okay though. 14,700 tagged moths out of approximately 4 billion is such a teeny proportion that we would have needed a lot of luck to catch a tagged moth,” she said.

“The project was incredibly worthwhile, however, as we made a lot of strong connections to other moth buffs. We received heaps of observations from the Bogong Sentinels (volunteers using light traps to assist the project), the number of bogong observations on iNaturalist increased tenfold in only a few months, and the project shone light on the plight of the bogong.”

The moths that the region encountered over the past few days are not, however, from the same generation of tagged moths that migrated down from the alps. 

“From what we know of their life-cycle, they breed, lay eggs and die after having returned to their breeding grounds in the autumn, so the ones emerging now will be the offspring of the tagged moth,” Linnea said.

There are a few theories as to what has led to the arrival of so many moths this year, including westerly winds blowing them off their usual migratory course and onto the coast, along with a decent amount of rainfall over autumn.

“A rainy autumn in their breeding grounds might have helped boost their numbers,” Linnea said. “There have been a few pretty good moth years recently, which leads to more moths laying more eggs creating more moths and so on. Exponential growth! … But again – not certain. There's a lot to still learn.”

And where did those hundreds of moths that appeared so suddenly disappear to? Well, the ones that did not end up in the stomachs of local birds, bats, frogs and lizards are continuing their migration. 

The NSW mothmaggeddon was just a brief stop-over and they are on their way back to the cool of the Australian Alps over summer.

You can help provide valuable data by submitting your sightings of these ecologically valuable insects to Bogong Watch