HortFACT
Identification
Adult tropical Armyworm moths have dark forewings with whitish irregular markings and marginal lines and a span of approximately 40 mm. The hind wings are pearly white. The moths are nocturnal, and are strongly attracted to light.
The eggs are laid in batches on the upper surface of leaves, and frequently on other flat surfaces such as the walls of houses. They are covered with a felt of pale brown scales detached from the abdomen of the female moth.
Young larvae [caterpillars] are pale green, but older larvae are grey with black and white markings and usually with a conspicuous yellow line down each side of the back. There is always a white dot on each side of the second and third segments of the body from the head. The fully grown larvae, which may be up to 50 mm long, burrow a short distance into the soil to pupate [change from a larva to a pupa].
The pupae are dark reddish brown.
Host plants
There is a wide range of host plants. Larvae feed on the foliage of beans, celery, silverbeet, and cabbage, and have been found to burrow into stored kumara tubers. They can also be a serious pest of pasture by feeding on clover and broad-leaved weeds. Overseas, the tropical Armyworm is recorded as a pest of taro, cotton, and tobacco.
Damage
Newly-hatched larvae feed only on the superficial tissues of the lower leaf surfaces, but older individuals eat entire leaves and may severely damage buds and flowers. Damage to pasture is characterised by the complete defoliation of clover and broad-leaved weeds, though grasses remain virtually untouched.
Distribution
Tropical Armyworm is established in Australia, where it is a pest of cotton, and in the Pacific Islands, where it is a pest of taro. Adult moths have been recorded in New Zealand occasionally before 1974, but these were almost certainly migrants blown over from Australia. In the summer of 1974-75, however, the moth became established in New Zealand, and its larvae reached epidemic proportions in parts of Northland and Auckland. It now appears to be established in at least the northern part of New Zealand, and can be expected to cause significant damage from time to time, depending on seasonal conditions.
Life cycle
| Egg |  |
| Larvae | |
| Pupae | |
| Adult | |
| Month | |
| Note: Coloured bars indicate periods of peak activity in each of the life cycle stages | |
The times spent in each life stage vary considerably according to the time of year, but in summer the eggs hatch in 4-5 days and the larval stage lasts about 18 days. The pupal stage lasts about 2 weeks and the female moth lays eggs about 4 days after it emerges from the pupa.
Several generations occur during the summer, depending on the average temperature; the exact number is not known. Overwintering is known to take place at the larval stage, although pupae may also overwinter.
JENNIFER COX