FAW identification

Many leaf-eating caterpillars share similar visual characteristics and damage symptoms. FAW larvae can vary widely in appearance, and are at their most distinctive in later instar stages; distinguishing younger larvae can be difficult.

Collecting a sample of young larvae and leaves, and rearing them to later instars may help confirm FAW presence if initial identification efforts are inconclusive.

How can I tell it’s FAW?

At a glance, FAW larvae look similar to other armyworm species and other caterpillars that grow to a similar size (3-4cm), such as helicoverpa and cutworms. Larger FAW larvae have both a pale ‘Y’ at the head end and a prominent set of four raised spots on their second last body segment. The spots can also be present in helicoverpa larvae, but FAW have a more rounded body shape.

FAW lifestages

The pale yellow eggs are laid in ‘furry’ clusters of 100-200. The scales covering the mass may give it a silvery or grey appearance.

As larvae develop, they become darker with white lengthwise stripes and develop dark spots with spines. Older larvae have an inverted ‘Y’ shape on the head and a distinctive pattern of four large raised dark spots on the second to last body segment. Larve can grow to up to 40mm long.

The red-brown pupa is found in soil under the host plant.

Adult moths have a brown or grey forewing, a white hind wing, and a wingspan of 32–40 mm. Male fall armyworms are more patterned and have a distinct white spot on each forewing.

Fall armyworm take about 2-3 weeks to develop from egg to pupae (depending on temperature), and another 2-3 weeks before moths emerge.

Fall armyworm characteristics include the inverted ‘Y’ on head capsule and four raised spots in a square at the rear (1)

It is important to not rely on a single characteristic when identifying larvae. Because there can be an overlap in characteristics between species, a correct identification is more likely when multiple points of identification are considered.

Some of the identification characteristics that may be found on a large larva. Click on the image to view a full-sized version of this guide.

More information:

Download Identifying armyworm larvae (May 2020 – 4MB A3 pdf)
Download a Fall armyworm diagnostics presentation by Mark Schutze (DAF, Plant Biosecurity Laboratory) – 2MB pdf
Where live insect collection is impractical, high quality photos, including close-ups are essential to provide the best chance of identifying the specimen. Download a Caterpillar identification: taking photos factsheet (600kB PDF).
The FAW pheromone traps currently used have the potential to attract a range of insects.