The Bogong moth story is a fascinating example of how complex insect life cycles don’t translate well to simplified sound bites.
Recent observations that there are fewer Bogong moths (Noctuidae: Agrotis infusa) in the Alps this summer made the news. One of the researchers credited with the observations found no moths in three caves he had visited last year, but he did find some in other caves in the region. There are limited long-term data on Bogong moth populations, and all of this news appears to be based on anecdotes, so it is impossible to verify if the species is truly in decline.
A few fact checks:
- Not all Bogong moth populations are migratory, some stay where they are all year.
- Not all Bogong moths commute between western New South Wales and the Alps. There are populations throughout much of south-eastern Australia and some locations in South and Western Australia.
- Bogong moths don’t fly to the Alps to breed. The populations found in alpine caves breed and lay eggs on the lowlands of western New South Wales and Queensland, where larvae develop, and then adults fly to the Alps to hibernate, before returning to the lowlands again to breed. Reproduction appears to be affected by higher temperatures, so it’s likely that moths from warmer areas move to the Alps for the summer to delay their breeding time until it is cooler.
- Adult moths are predominantly nocturnal, but may occasionally be seen flying during the day.
Most people think of A. infusa as the iconic adult moths seen around the Alpine Region & surrounds over summer. Adult Bogong moths are a cultural keystone species for indigenous people in the southern alpine regions, and an important traditional food resource.
On the other hand, the larvae (common cutworms) are a minor crop pest in most eastern states. Agrotis species collectively cause millions of dollars annual damage to grain crops in their lowland range. There is very little information on native host plants for A. infusa larvae, perhaps because they were never noticed until they began chomping on crops; they’ve been recorded on lucerne, potatoes, cabbage, cauliflower, peas, linseed, clover and many other crops, and also on introduced weeds, particularly capeweed.
When adult Bogong moths develop and migrate to the Alps, they transfer large amounts of nutrients and energy between distant ecosystems. In 2008, a study by Ken Green found they may also transport arsenic or other agricultural chemicals from their larval stages, and concerns about potentially lethal effects on the alpine food chains were raised. However, subsequent research by Dr Pettina Love and Dr Susan Lawler found no evidence that arsenic concentrations presented a threat to wildlife or human health.
Bogong moths sure get around. They have regular appearances and occasional breeding events in New Zealand and Tasmania. But it’s unclear if these are true migrations, or just blowin’ in the wind. This 1964 natural history note lists a few sightings of butterflies and moths at sea, including Bogong moth remains found in seagull droppings when sailing through the Bass Strait.
So what do these recent observations mean for the long-term future of Australian Bogong moths? They are unlikely to go extinct any time soon, but their population dynamics may be changing and we need to understand why. Unfortunately, without long-term data on larvae in their breeding grounds, adults in their hibernation locations, or detailed information on the non-migratory populations, it’s hard to answer that question.
Why do we know so little about our native insects? Most insect species have complex life histories, they interact with multiple different systems throughout their life cycles, and their populations fluctuate naturally from year to year. Yet research of many insect species has traditionally focused on life stages separately. And larvae are a lot harder to find in their natural habitat, unless they’re causing damage. In general, we have extremely limited knowledge of larval stages of most insect species in the world. For species like Bogong moths, that are considered pests as larvae but harmless or beneficial as adults, this history of research silos becomes problematic. Labelling animals as ‘goodies’ and ‘baddies’ might make it easy for us to identify our relationship with them, but it’s an unrealistic and potentially dangerous approach to knowledge. Relationships are necessarily complicated. We need to recognise this in how we manage our landscapes and direct our research efforts.
Regardless of whether we consider the moth a pest or an icon, it’s an important piece of the ecological jigsaw we live in and we need to understand its patterns. Bogong moths (adults and larvae) are high-energy food for many animals, including birds, mammals and predatory insects, so fewer moths can have knock-on effects on other parts of the system. And don’t forget the parasites that depend on the moths to complete their own life cycle. Two tachinid flies (Chaetopthalmus bicolor, Tritaxys milias), two nematodes, and an ichneumonid wasp (Netelia producta) have been recorded parasitising A. infusa, but there are likely to be many more species. There are plenty more questions to answer….
- Why do some populations migrate and others don’t?
- Are non-migratory populations experiencing increases or declines?
- How does pesticide use and land clearing in the larval grounds affect the number of adults arriving in the Alps?
- How do climate extremes affect the number of adults arriving in the Alps? And how do they affect the non-migratory populations?
- How do Bogong moth parasites respond to environmental drivers and affect moth populations?
- What other roles does A. infusa play in the ecosystem? Noctuid moths are common flower visitors and pollinators, but there is no research testing the role of Bogong moths as a pollinator of the many native flowering plants it has been observed visiting (nocturnal pollination in general is seriously understudied).
There is so much we don’t know about a species as famous as the Bogong moth, imagine how little we know about all the thousands of other native insect species you’ve never heard of. Many of these knowledge gaps can only be filled through dedicated natural history research and observational ecology – the sort of research that is rarely funded or considered high-impact, but is vital to understand the networks of interactions that structure our ecosystems.
© Manu Saunders 2019