Moth flies by night and lays around two thousand eggs in batches of 200-300 in April or May on plants. The eggs are small and creamy white and hatch into tiny pale brown headed caterpillars which remain close to where eggs were laid. They come out at night to feed and bury themselves near the damaged plant during the day. When disturbed they curl up. They turn green with age and may become quite dark in dense infestations. They prefer broadleaf plants such as Capeweed, Medics and Atriplex saltbushes. However they will eat grasses or each other during periods of food shortages. The larvae feed at night and are occasionally seen on garden plants. During the day they burrow into the soil or seek refuge under litter. They normally have five instars which take about 4 months to develop. Under poor conditions they may have up to 8 instars. The main generation pupate in the soil for about 3 weeks from late August to October. The adult moths soon emerge and feed on spring nectar. There may be several generations per year. The most prolific generation is in winter when broadleaf weeds are more abundant. These emerge in spring and form mass night flights to congregate in summer moth camps in the mountains. These sites have rock crevices or caves and the first moths to arrive move deep into the crevice and subsequent moths pack the crevices by placing their heads under the wings of those in front. Up to 17,000 moths per square metre have been recorded on cave walls. Some of these sites are temporary and the moths move onto permanent camps near peaks in the mountains in a day or two. Most moths stay in the permanent camps and aestivate (sleep) from November until April. Some may fly each evening around dusk and swap camps or occupy temporary camps. The have large fat reserves to support them over this summer period and predators such as birds (magpies, crows and ravens), lizards, spiders, mountain pygmy possums (Antechinus spp.), bats, fish and nematodes feed on them. Aboriginals used them as a food source by lightly flaming them to remove the wings and scales or crushing them into a paste to make cakes. (NB. Most people are sick after consumption if they are not accustomed to eating them.) Some feed on their evening flight but most of them don't and many die from starvation. The floors of some caves are littered with the bodies of dead moths to a depth of up to 50 cm. They spend about 100 days in the caves and crevices. From February to April the moths leave the camps, feed on nectar, migrate north west back to the plains and return to the same area where they were born. They mate and the male dies as his intestines are pulled from his body with the spermatophore which is taken by the female. The females lay eggs on broadleaf plants and die soon after. Undertaker ants soon consume most of the spent bodies.
This sometimes causes a public nuisance problem in Sydney and Canberra when swarms from the western plains are blown over the mountains into the cities and as far east as New Zealand. Great numbers may be washed up on beaches. They are attracted to lights in their millions and have blocked air conditioning shafts and interfered with automatic lifts. During the day they rest in crevices or exhausted moths die and may form heaps under lights. Towns to the west of the Great Divide are affected nearly every year. Occasionally they are seen as far south as Melbourne.
Pastures and crops.
Origin and History:
The caterpillar is a major pest of broadleaf plants including lucerne, linseed, cabbage and cauliflower. They cut irregular sections from the leaves and drag them into its underground tunnel or cuts young seedlings off at ground level. Occasionally it will damage wheat and barley seedlings.
2 or 3 large caterpillars per square metre will cause serious damage.
Management and Control:
The food supply and weather are important in determining the abundance.
Biological control by fungi, wasp and fly parasites prevent serious infestations in many years.