Armyworms are the larvae of Spodoptera moths. There are over 30 different species of Spodoptera, which are found all over the world. Although structurally the same, there are differences between species in colour, host plants and geographical distribution.
During the caterpillar stage, armyworms attack a wide variety of field and vegetable crops and can be extremely damaging to crop yields. Depending on the species they may eat the stems, leaves and fruit of the host plant. Understanding which species is attacking a crop is important when deciding on management and control methods.
Spodoptera damage symptoms
The name ‘armyworm’ comes from their practice of moving in ‘armies’, feeding on plants, pastures and crops. Typical signs of an armyworm infestation include holes in leaves and ragged leaf edges. Younger larvae feed on the under surface of leaves where they eat the lamina so that all that remains are the thin epidermis and veins.
Larger caterpillars can eat whole leaves, leaving only major veins. On maize, armyworms can cause ‘shot’ holes in the whorl, visible when the leaves unfurl.
Which species of armyworm?
Different species of armyworm caterpillars can look very similar. What’s more, armyworm larvae can also display a variety of colours within a species. But there are some features to look out for when trying to identify the species attacking your crops.
Correct identification is important to apply the most effective management or control methods. Here we focus on four species of armyworm.
Young armyworms, less than 2cm long
Spodoptera frugiperda (fall armyworm): Greenish with black heads, soon turning light orange. Feed on young leaves emerging from the whorl causing window-pane damage.
Spodoptera exempta (African armyworm): Green or light brown. Also causing window-pane damage when feeding on young leaves, and whorl.
Spodoptera littoralis (cotton leafworm): Greyish or dark green body with a brownish head. Feed on the lower side of all leaves, and whorl.
Spodoptera exigua (beet armwormy): Pale green or yellow. Feed on the upper surface of all leaves, and whorl.
Older, up to 3 cm or even 4 cm long armyworms
Fall armyworm: Light green to dark brown; backside often has thin white lines and darker lines on its sides. Inverted light “Y” on front of the head. Small black spots on the body shaped like a trapezium on each body segment, and 4 spots in a square at end of the body. Feeds on young leaves, whorl, tassels, silk, and cob.
African armyworm: Shiny black; thin dark blue lines on the top of the body; several greenish-yellow lines at the side. Inverted light “Y” on the front of the head, but doesn’t have the 4 spots on the second-to-last segment like fall armyworm. Feeds on young leaves, and whorl, and cob from the top downwards.
Cotton leafworm: Dark green to reddish-brown or blackish-grey. Dark and light stripes run along the sides of the body, usually a bright yellow stripe along the sides. 2-4 dark semi-circular to triangular spots on the back. Feeds on all leaves, whorl, and cob.
Beet armyworm: Green to dark brown, sometimes almost black, with green and dark brown stripes and spots running along the body. The darker stripe down the back and paler stripe along each side. Pink or yellow underneath. Feeds on all leaves, whorl, and cob.
Prevention, monitoring, control
Prevention, monitoring, and control options depend on the species of armyworm identified. The Plantwise Knowledge Bank Pest Management Decision Guides (PMDGs) provide useful information for farmers and extension workers on how to tackle fall armyworm outbreaks.
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