Meteorologists across eastern New South Wales have spent recent nights staring at radar imagery showing widespread rain, only to look outside and find not a drop. The culprit, it turns out, is the bogong moth - back in big numbers and apparently determined to make the weather bureau look silly.
The phenomenon is not new. During the Sydney Olympic Games in 2000, a swarm of bogong moths drawn to stadium lights was so enormous that meteorologists mistook it for rain. Fast forward 26 years and the bogongs have returned in force, this time firmly on the radar - every single night, particularly across eastern NSW and around Sydney, according to the ABC's meteorologist, who noted the imagery shows rain where there hasn't been a drop on the ground.
Weather radars, designed to detect rain, hail or snow, may be picking up swarms of the moths on their long-distance migration to the Australian Alps. Radio pulses meant to bounce off water particles and return as echoes on the rain radar screen may instead be reflecting off insect bodies and wings. A Bureau of Meteorology spokesperson confirmed it is possible radars across eastern NSW are detecting bogong moths, as large numbers of insects can resemble very light widespread rain, particularly near sunrise and sunset. The spokesperson added that while radars have been showing increased non-rain echoes, completely attributing the effect to moths is challenging, given other factors such as temperature inversions can cause similar effects.
For scientists, this accidental entomological surveillance could be "absolute gold." Dr Kate Umbers, an associate professor in zoology at Western Sydney University and managing director of Invertebrates Australia, has been working to understand where bogong moths breed and the routes they take. Researchers have tried numerous tactics - from tagging tiny moths to collecting stories and newspaper reports - to trace the moths' journey across hundreds of kilometres. Umbers says the radar data poses all sorts of interesting possibilities and questions, including: "How many bogong moths equals heavy rain?"
Moths are far from the only flying animal able to be picked up by radar. Prof Justin Welbergen, an ecologist at Western Sydney University who specialises in flying foxes, has been using radar data to track their movements in near real time and retrospectively. It began, he says, because he had been researching the coordinated nightly departure of bats from their roosts and happened to check the weather radar while in Melbourne. "I decided to look at the weather radar to see if I could see any signal of the flying foxes in the radar - and there it was. And the rest was history."
Flying fox emergences are highly organised events, where the hungriest individuals leave first, Welbergen says. "Adult females were followed by their young, and then the adult males would be the last ones to leave, only after all the females had left." The animals show up clearly in radar data as "a big bag of water," emerging at about the same time each night around dusk and flying in predictable directions. With those insights, researchers built a working prototype that allows them to track and count flying foxes using radar data - in near real time with the help of a computer algorithm, and even reconstruct historical movements over several decades with data from weather stations. A similar initiative in the United States, called BirdCast, relies on weather radar to track bird migrations.
Welbergen hopes to expand the flying fox prototype, known as "Aerocast," in collaboration with other researchers and organisations, for tracking other aerial animals, such as birds and even bogong moths. Although dependent on radar coverage and quality, it is a promising tool for conservation, which could be used to detect signals of important ecological events, such as mass flowering or extreme heat. Like bogong moths, flying foxes are difficult to track across the landscape because of their extreme mobility. "Flying foxes travel all over the place, all the time. They congregate in these roosts, and the individuals that are there may not be there the next day."
It is not the only thing the two species have in common, Welbergen says. "They often emerge from these central locations at predictable places, and predictable times of the day and year. So our techniques for detecting flying foxes, I expect, are readily transferable to moths as well, because [on the radar] flying foxes are effectively just giant moths."
In the meantime, Umbers has been delighted to have the help of millions of research assistants across Sydney and Canberra as the latest bogong wave passes through. "For us, having that many eyes on this species at one time is the best possible scenario we could hope for in trying to understand them." It has been a worrying few years since populations collapsed, but the small, brown critters are not out of the woods yet. Their alpine habitat remains threatened by global heating, and an abundance of sightings in cities may not necessarily equate to a population boom overall. Still, Umbers says it is "wonderful" that a new cohort of Australians are getting acquainted with a bogong moth storm. "There's a lot of people alive that have never seen this," she says. "The moths are saying: don't forget about us, look after us, look how amazing and impactful and epic we can be."