Tropical armyworm

Scientific name: Spodoptera litura

Tropical armyworm adult

Tropical armyworm caterpillar

Tropical armyworm larvae

Tropical armyworm eggs

Damage by young caterpillars

Tropical armyworm damage

Life cycle
  • Key characteristics
  • Biology
  • Impacts
  • Control

  • Feed on a wide range of plants, including broad leaved plants
  • Known only in the North Island, causing sporadic damage in Northland, Waikato, Bay of Plenty, Taranaki and Manawatu regions
  • Occasional outbreaks occur but are rare and are likely to be initiated by climatic disruption to natural enemies and crop weed control, followed by above average summer and autumn temperatures
  • Egg batches are found on the upper surfaces of leaves and are covered by hair scales from the moth’s body
  • Young caterpillars are green but as they grow, colour varies from dark green to brown or grey, with black and white markings on the back and usually with two conspicuous longitudinal yellow stripes
  • The caterpillars grow up to 50 mm long
  • Typically the head of the tropical armyworm has a white V marking
  • The moths are strongly attracted to lights. They have dark forewings with white markings and their hind wings are pearly white.

Differentiating tropical armyworm caterpillars from cosmopolitan armyworm

  • Cosmopolitan armyworm feed only on cereals and grasses
  • Tropical armyworm egg batches are found on the upper surfaces of leaves whereas cosmopolitan armyworm egg batches are usually between the grass blades in or near the sheaths
  • Young caterpillars of both species are green. Cosmopolitan armyworm caterpillars grow up to 40 mm long and are a pale greenish-brown to pink with a thin whitish line down the back and dark lines on each side
  • The caterpillar head of the tropical armyworm has a white V whereas cosmopolitan armyworm has a honeycomb pattern (see illustration)
  • Cosmopolitan army-worm moths have dull yellow-brown to reddish-brown forewings and grey hind wings.

  • Tropical armyworm overwinter as larvae and sometimes pupae. Eggs are laid from late December, soon after the moths emerge.
  • The female can lay as many as 2000-2600 eggs. She lays them in masses of 100-300, covered with hair scales from her body. The egg masses are 4-7 mm in diameter and cream to golden brown. They hatch in 3-5 days
  • Development takes approximately 25 days from egg to moth in summer and there can be 3 generations from January through to March
  • The newly-hatched larvae skeletonise leaves, whereas older individuals eat entire leaves and may severely damage buds and flowers.  The larvae generally hide during the day and become active during dusk to dawn. The larval stage usually takes about 18 days in summer
  • The full grown caterpillars burrow into the soil to pupate. The pupal stage lasts either a few weeks or several months, depending upon time of year.

  • Tropical armyworm can eat the entire foliage from broadleaf plants such as clover, lucerne, plantain and chicory and in severe cases only the grasses remain in pastures. It is also a pest of crops such as kumara, cabbage, silverbeet, beans and celery in New Zealand and is known to attack maize and sweetcorn overseas
  • Outbreaks of tropical armyworm are rare as a range of generalist parasitoids and predators prey on all stages
  • Epidemic outbreak populations, when caterpillars move ‘like an army’ through crops and pastures, are likely to be initiated by a combination of events. These events will be more common under predicted climate change scenarios and include:
    • Dry spring soil conditions resulting in poor weed control in crops such as maize. The protected environment aids survival
    • Flooding during summer causing harm to predators and parasitoids stages living in or near the soil
    • Above average summer and autumn temperatures allowing for additional generations to develop.

Insecticides, applied at first sign of caterpillars and damage, remain the most effective way to control tropical armyworm. Those currently registered for use include, alpha cypermethrin chlorpyrifos, diazinon, methomyl, methamidophos, and carbaryl. The latter three are currently under review by the EPA. Refer to the NZ Agrichemical manual or your local agrichemical representative for more information on insecticidal control.