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The exotic fall armyworm in Australia: a south-eastern perspective


By Julia Severi, Extension Scientist
Last updated: March 30th 2020

The exotic fall armyworm, Spodoptera frugiperda, has been detected in several locations in northern Queensland since February 2020, and the national Consultative Committee on Emergency Plant Pests has agreed that it is not technically feasible to eradicate the species from Australia.

Given that the fall armyworm is a tropical-subtropical species, you may be asking, will this pest impact south-eastern grain regions?


What is the fall armyworm?

Despite its common name, fall armyworm is not a ‘worm’ but a moth species (lepidoptera) whose larvae feed on a wide range of plant species, including several economically important agricultural crops and pastures.

Maize, sorghum and rice are recorded as crops frequently fed on by fall armyworm. However, while many grass species (Poaceae) are preferred hosts, some broadleaf crops such as cotton and various vegetable crops are also impacted.

Fall armyworm originates from the tropical and sub-tropical areas of the Americas where it is warm enough to breed and persist year-round.1 But it can also be a seasonal pest in more temperate climatic regions of the Americas. For example, migrations occur to northern areas of the United States during late summer and early autumn.2

In 2016 fall armyworm was detected in west and central Africa, and has since established across most of sub-Saharan Africa, and in more recent years, many regions of Asia, and now in parts of Australia.3

 

Will fall armyworm impact crops in south-eastern Australia?

At the time of writing this article, fall armyworm detections in Australia have been limited to northern Queensland.

While its geographical range is like to expand, there are two reasons why fall armyworm may not be an issue during most of the winter cropping season in south-eastern Australia.

Firstly, fall armyworm requires temperatures to be above a certain threshold for development. This has been studied by several researchers and experiment results have varied between studies with lower development thresholds estimates between 7.4°C to 17°C.2,4 Therefore, winters in south-eastern Australia are likely too cold for fall armyworm development.

Secondly, the fall armyworm doesn’t have a diapause phase; it cannot survive cold winters in an ‘inactive’ state.1

This thinking is supported by a study using CLIMEX modelling to predict fall armyworm potential distribution across the globe, and suggests that south-eastern Australia is not suitable for fall armyworm to establish permanently.4

However, fall armyworm is highly migratory and capable of dispersing several thousands of kilometres. This is a handy behavioural adaptation and how it outsmarts its lack of a diapause phase during winter.

South-eastern Australia could receive seasonal migrations of fall armyworm (like in much of the United States) during summer when susceptible crops like maize and sorghum are grown.

Ultimately, how Australia’s climatic and vegetation zones will influence the timing and magnitude of migrations is still unknown.

Keeping track of distribution expansion from currently affected areas in Queensland will help us stay ahead of the curve. An important action to take at the moment is for those inspecting crops regularly to become familiar with how to identify fall armyworm and to report suspect sightings in south-eastern Australia to the relevant state department of agriculture (Exotic Plant Pest Hotline - 1800 084 881).

 

Armyworms in south-eastern Australia

‘Armyworm’ is a catch-all term for several species of moth larvae spanning several genera (Mythimna, Leucania, Persectania and Spodoptera), which can cause economical damage in crops.

There are several armyworm species in Australia, and their distribution and presence depend on climatic conditions. For example, in south-eastern Australia there are fewer species of armyworm that damage crops than in northern, sub-tropical and tropical regions.

In south-eastern Australia, the three main species are native: the common armyworm (Mythimna convecta), the southern armyworm (Persectania ewingii) and the inland armyworm (Persectania dyscrita). They are difficult to distinguish, however, correct species identification in the field is generally not critical because their habits, type of damage, and control are similar.

During warmer months, the lesser armyworm (Spodoptera exigua) is also present in south-eastern Australia.

Generally, the tell-tale feature of these armyworm larvae is the three light parallel body stripes, commonly visible on the ‘collar’ behind the head (cervical shield) and usually continuing down the length of the body.

A native armyworm species (Mythimna or Leucania sp.). Note the three stripes on its ‘collar’ behind the head that continue down its body. Image: cesar

 

How to tell fall armyworm apart from other armyworm species

Like other armyworm larvae in south-eastern Australia, the fall armyworm can grow to approximately 35 - 40 mm and has the following general features:

  • A hardened head capsule
  • Three pairs of true legs on the region behind its head (on its thorax)
  • Leg-like structures on its ‘belly’ known as abdominal prolegs (four pairs).
  • One pair of prolegs at the end of the larvae known as anal prolegs

Note that like many immature moths, fall armyworm larvae have three pairs of true legs, four pairs of abdominal prolegs, and one pair of anal prolegs, circled from left respectively. Image: John C. French Sr., Retired, Universities: Auburn, GA, Clemson and U of MO, Bugwood.org (CC BY-NC 3.0 US)

 

Images of fall armyworm from the United States show that the fall armyworm can also have the three light parallel collar stripes, and they can extend the length of the body (but not always).

Fall armyworm larvae images taken in the United States. Note that sometimes three parallel light stripes are present/obvious on its body. Images: Phil Sloderbeck, Kansas State University, Bugwood.org (CC BY-NC 3.0 US); Clemson University - USDA Cooperative Extension Slide Series, Bugwood.org (CC BY 3.0 US); John C. French Sr., Retired, Universities: Auburn, GA, Clemson and U of MO, Bugwood.org (CC BY-NC 3.0 US)

 

So, if the fall armyworm can have three parallel light stripes, how can you tell them apart from other armyworm larvae in south-eastern Australia?

The key feature is its ‘spottiness’.

The body of fall armyworm larvae features several obvious spots or ‘pinaculae’. These are the hardened areas that bear their body hairs. These spots are very pronounced compared to other armyworm species in Australia.

On their apparent second last body segment (eight abdominal segment), four of these spots are particularly enlarged and are in a square arrangement.

Fall armyworm larvae are quite spotty! Note the four enlarged spots on their eighth abdominal segment arranged in a square. Image: Russ Ottens, University of Georgia, Bugwood.org (CC BY 3.0)


The lesser armyworm and a native armyworm species that already occur in south-eastern Australia. See how they are not as spotty as the fall armyworm? Images: cesar

 

Note that as larvae, the fall armyworm goes through approximately six instars. As they progress through each stage they grow in size and their appearance changes with later instars. Identifying larvae less than 10 mm in size is likely to be difficult in the field.

Also note that fall armyworm, and armyworm in general, can come in a wide variety of colours, which is why we won’t focus on colour for identification.

The fall armyworm could be confused with other moth larvae, particularly, Helicoverpa punctigera and Helicoverpa armigera. The video below shows the key differences between Helicoverpa sp. and fall armyworm.

The fall armyworm could also be confused with weed web moth larvae that are currently present in many broadleaf crops in southern and central New South Wales at the moment. Please feel free to contact us for further moth larvae identification assistance via pestfacts@cesaraustralia.com.

 

More information on fall armyworm

Fall armyworm – should you be concerned?

Fall armyworm identification

Fall armyworm – the essentials

Fall armyworm surveillance or monitoring


References

1 Luginbill P, The fall army worm, US Dept. of Agriculture (1928).

2 Early R, González-Moreno P, Murphy ST, Day R, Forecasting the global extent of invasion of the cereal pest Spodoptera frugiperda, the fall armyworm. NeoBiota 40: 25-50 (2018).

3 CABI, Spodoptera frugiperda (fall armyworm), in Invasive Species Compendium, CAB International, Wallington UK. www.cabi.org/isc [accessed 24 March 2020]

4 du Plessis H, van den Berg J, Ota N, Kriticos DJ, Spodoptera frugiperda, CSIRO-InSTePP Pest Geography (June 2018)

 

Acknowledgements

Thanks to Jessica Lye (cesar) for edits.

Header image: Frank Peairs, Colorado State University, Bugwood.org (CC by 3.0)

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