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Potential impact of fall armyworm on pastures

Fall armyworm is reported to feed on tropical and subtropical grasses in grazing systems overseas. It is likely that tropical and subtropical pastures in Queensland will support large populations of fall armyworm in suitable seasons.

It is unclear what impact fall armyworm might have on pasture productivity, but outbreaks of dayfeeding armyworm (Spodoptera exempta) have caused significant short-term defoliation in buffel pastures. This defoliation can kill seedlings and retard the growth of established plants. Infested pasture may be reinfested by the offspring of the first infestation.

Grasses that fall armyworm infest overseas include:

  • Sorghum haleepense, common name Johnson grass
  • Chloris gayana, common name Rhodes grass
  • Agrostis spp., collectively called bent grasses
  • Digitaria spp., collectively known as pangloa, digit or finger grasses
  • Cynodon dactylon, common names include: couch, green couch, couch grass
  • Poa spp., common names in Queensland include: annual Poa, winter grass
  • Panicum spp., collectively known as panics.

Most of these are common across Queensland, in grazed pastures, hay production, and urban situations.

The potential for fall armyworm to infest other important native (e.g. Mitchell grass, spear grass, blue grasses) and sown pastures (e.g. Buffel grass, Rhodes grasses) is unclear, but it is possible these could host.

Hay production, including Lucerne, may be at risk of production loss caused by fall armyworm infestation, including irrigated production over winter in warmer regions.

What to look for

Look for egg masses and small larvae. Larvae may be more active at night. Small larvae, and their damage, can easily be overlooked. The majority of feeding occurs over a period of around 3–5 days as the final 3 larval instars (4–6) complete their development. If smaller larvae in the crop are not detected, the defoliation done by these later instars can appear to happen suddenly.

How to manage an outbreak

Early detection is essential. Regularly check your pastures for signs of damage or loss of productivity.

Use a sweep net or sweep a bucket through the grass to dislodge larvae. Larvae may be active during the day in thick stands, but more often at night in less dense, or damaged areas. Once all foliage has been removed, fall armyworm will move en masse to a new area.

In hay production in the United States of America, it is recommended that control actions be implemented at 2–3 larvae per square foot (~30 cm2). In Queensland, there are currently no registered insecticides for this insect in pasture situations, but check the Australian Pesticides and Veterinary Medicines Authority (APVMA) for permits (see below on how to do this).

Graziers should assess their pastures on a regular basis, and determine the abundance and impact of fall armyworm, and other armyworm species. If a loss of production seems likely, seek advice on what options are available to limit loss of production. A useful non-chemical approach is to heavily stock the infested paddock(s) to utilise the pasture before it is eaten by armyworm, and physically reduce armyworm populations through stock movement.

Key to the control of any pest is an integrated pest management approach. With industry, we are working to identify strategies and tactics for the medium to long-term response.

Some insecticides used for the control of Helicoverpa armigera, other armyworms and caterpillar pests may provide some level of control of fall armyworm. Biocontrol agents released for Helicoverpa are also expected to have an impact on fall armyworm.

It is critical that following the application of any permitted insecticides, the subsequent cutting of hay or grazing of stock is done in accordance with labelled withholding periods, and export slaughter intervals.

The APVMA is currently assessing, as a priority, applications for permits for the use of chemicals against fall armyworm in pasture. Check for the latest chemical permits applying to fall armyworm by using the APVMAs permit portal—search for 'fall armyworm' and tick the 'pest/purpose' button.

Be on the lookout and if you suspect fall armyworm, report it immediately by phoning 13 25 23.

Further information

Read the pastures fall armyworm fact sheet.