The bogong moth (Agrotis infusa) is a temperate species of night-flying moth, notable for its biannual long-distance seasonal migrations towards and from the Australian Alps, similar to the diurnal monarch butterfly. During the autumn and winter it is found in southern Queensland, western New South Wales, western Victoria, and also in South and Western Australia. Adult bogong moths breed and larvae hatch during this period, consuming winter pasture plants during their growth. During the spring, the moths migrate south or east and reside in mountains such as Mount Bogong, where they gregariously aestivate over the summer until their return towards breeding grounds again in the autumn.
The moth's name, bogong, is derived from an Australian Aboriginal language; the Dhudhuroa word bugung describes the brown colouration of the moth. It is an icon of Australian wildlife due to its historical role as an important food source and because Aboriginal peoples would come to where the moths spend the summer to feast on them and hold intertribal gatherings. In recent years, it has invaded major cities like Canberra, Melbourne, and Sydney due to strong winds during its spring migration.
Starting around 1980 and accelerating rapidly after 2016, the Bogong Moth population has sharply declined as a result of increasingly severe droughts, along with increased temperatures in caves used by the moths for aestivation, both primarily resulting from anthropogenic climate change. In December 2021 the bogong moth was added to the IUCN Red List as an Endangered Species.
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TerrestrialTerrestrial animals are animals that live predominantly or entirely on land (e.g., cats, ants, snails), as compared with aquatic animals, which liv...
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OviparousOviparous animals are female animals that lay their eggs, with little or no other embryonic development within the mother. This is the reproductive...
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MigratingAnimal migration is the relatively long-distance movement of individual animals, usually on a seasonal basis. It is the most common form of migrati...
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starts withAdult bogong moths have an overall dark brown colouration, with a dark stripe interrupted by two light-coloured spots on the wings, distinguishing it from other moths. There are visual differences between the migratory and nonmigratory forms of the moth; migratory moths have brown hind wings while nonmigratory moths have paler hind wings. Bogong moths have a wingspan ranging between 40–50 mm (1.6-2.0 in), and a body length of around 25–35 mm (1-1.4 in). The average weight of an adult bogong moth is 0.326 grams.
Bogong moth eggs are dome–shaped in appearance and are vertically ridged. They are 0.7 mm (0.03 in) in diameter and 0.4 mm (0.02 in) in height.
Caterpillars initially start out with a pale colouration, but as they grow and consume food they become green with pale and dark stripes and spots. Larvae achieve a maximum length of 50 mm (2.0 in).
Bogong moth populations are primarily located across southern Australia, west of the Great Dividing Range. The regions contain populations of nonmigratory and migratory moths of this species, distinguished by their differing seasonal presences in each region. The adult bogong moth lays eggs across New South Wales, southern Queensland, and northern parts of Victoria, where larvae hatch and grow until adulthood. During the spring season and subsequent summer aestivation, bogong moths migrate south or east towards the Australian Alps, and can be found in the Australian Capital Territory and Bogong Mountains. However, bogong moths can also be found in locations as far as Tasmania and New Zealand due to strong winds that blow them off their path.
Bogong moths undergo whole scale long-distance migration biannually, in which they can travel up to 965 km (600 miles). The spring migration begins in early September and occurs from the lowlands of Southern Australia south towards the Australian Alps for purposes of reaching aestivation sites. During the summer, the moths remain in their aestivation sites until autumn, when they migrate back towards the breeding grounds of the lowlands as early as February, but primarily in April. The bogong moth utilises particular aestivation sites repeatedly throughout migrations, as seen with the development of parasites that depend on the regular arrival and departure of the moths from caves. The population within each aestivation site fluctuates throughout the summer due to moth mortality and the departure and arrival of moths either migrating further south for aestivation or north to return to breeding grounds. However, this differs from changes during migration periods, when the populations rapidly increase with arrival or decrease with departure.
Bogong moths are nocturnal migrants, but the exact mechanism for long-distance navigation is not clear. It is possible that they are oriented by light, as seen in the influence of light intensity on activity during aestivation. Additionally, bogong moths may use an internal magnetic compass to aid in navigation, as seen in the similar monarch butterfly. A study published in 2018 concluded that the Bogong moth uses a combination of the Earth's magnetic field and recognisable landmarks to calibrate their route.
However, there are also nonmigratory populations; this generally occurs in areas with favourable conditions, where migration to avoid harsh conditions such as seasonal changes in larval food crop abundance is not necessary. Some populations of bogong moths in areas such as Tasmania and in coastal populations of New South Wales do not migrate, and reach sexual maturity at a faster rate than migratory populations.
Bogong moth larvae subsist on winter pasture crops and wild crop weeds such as cape weeds within bogong moth breeding grounds, primarily depending on annual dicotyledons that grow during the winter. Attacks on a wide variety of cultivated crops have been seen, with plants such as Medicago species, wheat, cabbages, cauliflowers, silver beet, peas, and potatoes all recorded being consumed by bogong moth caterpillars. However, the larvae avoid grasses, which overtake pastures during the summer, making summer unfavourable due to lack of larval food sources. Adult bogong moths feed on the nectar of flowers such as Epacris, Grevillea and Eucalyptus while breeding or migrating, but will not actively feed during aestivation.
Adult bogong moths lay up to 2000 eggs in the soil or on plants near the soil after returning from aestivation sites in the autumn migration. Incubation times vary depending on temperature, with eggs hatching after a period of 4–7 days in laboratory conditions. The larvae of bogong moths undergo six instars. Caterpillars grow slowly throughout the first three instars, taking until June to develop over the winter. However, the larvae go through fast growth during the spring, reaching the final instar in late August to September, soon before migration. They are active at night, when they feed on plants within breeding grounds. Larvae undergo pupation in soil chambers at a depth of 20–150 mm (0.8-7.9 in). Pupation can last between 3–11 weeks depending on the temperature and environment. Pupae are 20 mm (0.8 in) in length and have a shiny, brown appearance. Adults emerge from the soil chambers and shortly after begin migration. Adult bogong moths are active at night, and have different seasonal behaviors. During the spring, bogong moths feed and migrate south, where they aestivate during the summer. Adult bogong moths are not sexually mature yet, and do not actively seek food during this period of dormancy. In autumn, the moths migrate again and return to their breeding grounds, laying eggs and dying.
In the spring/summer aestivation seasons of 2017-8 and 2018-9, dramatic drops in numbers of the moths in the Alpine caves have been observed. Millions of the moths have usually lined the walls of these caves over summer, but for the 2017-8 and 2018-9 aestivation seasons there were none in some caves, according to Professor Eric Warrant of Lund University in Sweden. He says that the drop in numbers was probably caused by a lack of rainfall due to winter drought in their breeding areas and climate change, the lack of rain producing insufficient vegetation to feed the caterpillars. Some caves still harboured thousands of moths, which suggested that the moths "came from different breeding grounds, where one location wasn't as badly affected by the drought as the other. I don't think they will disappear completely, but there's certainly a possibility that they could become locally extinct at their breeding grounds.", Warrant said.
Other biologists and ecologists have pointed to the dramatic effect on animals which feed on the moths, which are an important source of protein for wildlife, including the threatened mountain pygmy possum as well as other insectivorous mammals and birds. "The vulnerability of the Australian Alps to climate change is the worst in the world because we've got these short little mountains so when it gets warmer, there is nowhere for these cold-adapted species to go", according to Euan Ritchie, a wildlife ecologist at Deakin University.
In December 2021 the bogong moth was added to the IUCN Red List as an Endangered Species, based on a February 2021 assessment.
