Dr Penelope Greenslade, School of Science, Psychology and Sport, Federation University
The Bogong moth has just been listed as endangered by the International Union for Conservation of Nature and put on their Red List, but why? On a first impression the moth is not particularly attractive, unlike the endangered Ulysses swallowtail butterfly, as it is smaller and a nondescript brown in colour. Nor does it provide economic benefits as does another moth, the silkworm. Its existence is not threatened by any disease nor is it harvested for food any longer. Quite the opposite, as the moth is considered by some as a nuisance, as at certain times of year, large numbers were attracted to lights in cities like Canberra, entering houses and offices where they cluster in nooks and crannies to the consternation of the inhabitants who hasten to destroy them. It also can be a pest of crops such as cotton and wheat where the caterpillars cause damage and are controlled by the application of insecticides.
No, the reason for the listing is that in the past, moth numbers were estimated to be in the billions, but a massive reduction has been observed over the last few years. Now why is this particularly important? It is because of an intricate web of interconnected and dependant other species in its summer sites that rely on high moth numbers for food, one of which is the endangered Mountain Pygmy Possum. A study of the moth’s ecology has revealed a complex network of ecological relationships and shows that the loss of one species can result in a population collapse of several other species, all part of the interconnected, dependent species in the network. I note below six animal groups which are known to be dependent, at least partially, on these moths and there may be more. They include three mammals, two parasitic nematode worms, springtails and two birds.
In spring, adult moths undertake a long-distance nocturnal migration from the northwest plains of New South Wales and the downs of southern Queensland to the mountain ranges of the southeast, with the reverse journey in autumn. It is for this remarkable long distance migration pattern that the Bogong Moth justifies protection. Its caterpillars live in the north east plains in winter where they feed on a range of native leguminous and other broad-leaved plants as well as on crops such as cotton and even grasses such as wheat. When conditions in its feeding grounds warm and dry out in spring, the fully grown caterpillars transform into pupae in the soil, from which adults later emerge and migrate south to the highlands of the Great Dividing Range. Here they aestivate (pass the summer in a state of dormancy), in cool, moist rock cavities in granite outcrops. They aggregate in close formations on flat upper surfaces of the crevices. This migration brings a massive influx of energy and nutrients to the mountains each year, crucial to the health of the alpine ecosystem (Green 2021). Here they are easy prey for other animals as they are relatively immobile.
Early settlers not only observed migrating moths but also recorded Indigenous tribes gathering and moving each spring to the foothills of the mountains for ceremonial purposes which included taking part in corroborees and feasting on the nutritious moths. This cultural activity is described by Josephine Flood in her book “The Moth Hunters“ where she suggests the gatherings had probably taken place annually for thousands of years. Even though large numbers of people gathered to feed on “moth cakes” prepared up the mountain, it is unlikely that this activity had much impact on population numbers of moths as when it was still being practised, moth numbers allegedly remained in the billions.
Ian Common, a local entomologist, meticulously documented population sizes in aestivation sites in the early 1950s. He noticed that some moth mortality occurred when adult parasitic mermithid nematodes emerged from the aestivating bodies. The adult worms fell to the cave floor where they laid eggs and died. It was assumed that the eggs hatched and parasitised a new population of moths the next spring by climbing the cave walls in rivulets of water. Inevitably the cave floor accumulated a deep layer of decomposing moth litter which was a source of food for other organisms. A species of Springtail, Triachanthella sp., a fungus feeder, was found abundantly in this habitat but not outside the cave. Three species of marsupial, one the endangered Mountain Pigmy Possum and two Antechinus species are known, from a study of their scats, to feed on the moths in summer. In winter, when moths were absent, the Pygmy Possum had no need of food as it hibernates. Not least of course, the live moths were of considerable cultural significance as food to Indigenous peoples. Birds such as the Little Raven and Pied Currawong have also been observed entering caves to feed on the moths. Other, less desirable, predators are feral cats and pigs, as well as foxes.
So, what caused this sudden population collapse of the Bogong moths? It is not well understood, but the most likely cause was the prolonged drought in eastern Australia, that started in the year 2000. It arrested the growth of food plants so caterpillars hatching from such eggs as were laid, had no food, and could not survive. Other suggestions are that the expansion of croplands at the expense of the natural grasslands, plus any accompanying pesticide use, might have also contributed to the decline. Arsenic has been detected in the vegetation outside the aestivation sites supposedly leached out from those moths that failed to survive the summer. It is unlikely that the arsenic originated from insecticide use against the caterpillars but is more likely to have accumulated in in the vegetation consumed by the caterpillars and passed into the adult moths. Human actions may also be partly to blame because of an increase in artificial light in cities, especially those with a high UV component that attract moths and disrupt their nightly behaviour.
There is some recent evidence that populations are again on the increase at least in some areas. This follows recent drought-breaking rains in northern NSW that started in 2020 and have continued into 2022. Moreover, it is suggested that Bogong moths are a typical “boom and bust” species capable of rapid responses to either favourable or unfavourable extreme variations in weather. If this is the case, the IUCN listing of the species as Endangered may be premature. The species is only undergoing its normal behaviour, being highly sensitive to extreme weather events of relatively short duration. However, there are advantages for conservation in having a species listed as it is more likely to attract grants and, in this case, draws attention to the importance of montane faunas. The documented warming climate could also play a part but a more minor one.
The management of Victorian National Parks is aimed at promoting “Healthy Parks for Healthy People” which can translate to an emphasis on recreational activities rather than conservation. For instance, last year (February 2021) a single expedition of 90 senior schoolboys hiked up Mount Bogong where expedition leaders pointed out a moth hibernation site among boulders at the summit, mentioning that the moths were good to eat. Inevitably some interference to populations resulted. Victorian Parks authorities are negligent in not placing informative signage at sensitive sites such as these.
Acknowledgements
Thanks are due to Roger Farrow and Michael Nash for their improvements to the text.
References
Common, I. F. B. (1954). A study of the ecology of the adult bogong moth, Agrotis infusa (Boisd) (Lepidoptera: Noctuidae), with special reference to its behaviour during migration and aestivation. Australian Journal of Zoology.2 (2): 223–263. doi:10.1071/zo9540223.
Green, K. (2011). The transport of nutrients and energy into the Australian Snowy Mountains by migrating bogong moths Agrotis infusa. Austral Ecology. 36 (1): 25–34.doi:10.1111/j.1442-9993.2010.02109.x
Flood, J. (1980). Studies, Australian Institute of Aboriginal (1980). The moth hunters: Aboriginal prehistory of the Australian Alps. Canberra Australian Institute of Aboriginal Studies. ISBN 978-0391009943.
Green, K., Broome, L., Heinze, D., Johnston, S. (2001). Long distance transport of arsenic by migrating Bogong moths from agricultural lowlands to mountain ecosystem. The Victorian Naturalist.18 (4): 112–116.
National Environmental Science Program Threatened Species Research Hub (2019) Threatened Species Strategy Year 3 Scorecard – Mountain Pygmy-possum. Australian Government, Canberra. Available from: Search Results – DAWE mammals-by-2020/mountain-pygmy-possum.
Warrant E., Frost B., Green K., Mouritsen H., Dreyer D., Adden A., Brauburger K., Heinze S. (2016). The Australian Bogong Moth Agrotis infusa: a Long-Distance Nocturnal navigator. Frontiers in Behavioral Neuroscience, 10:77.
Warrant, E.J.; Whitehouse, M.E.A.; et al. (11 February 2021). IUCN Red List of Threatened Species. Retrieved 29 December 2021.
You must log in to post a comment.