Biology
The fall armyworm (Spodoptera frugiperda) is a plant pest that can damage a wide variety of crops. The larvae predominantly feed on crops and pastures from the Poaceae (grass) family, in particular maize, but also sorghum, and forage grasses. It has also been noted on other grass crops including cereals but these are considered secondary hosts. The pest can also feed on non-grass crops such as cotton, vegetables and some fruit crops. Fall armyworm is known for its ability to disperse and migrate long distances, which enables it to exploit new habitats and expand its range when favourable host plants and climatic conditions are present.
Life cycle and development
Fall armyworm has four life stages: egg, larvae (caterpillar), pupae and adult (moth). At its ideal temperature (28º Celcius (C)), fall armyworm can complete its life cycle within 30 days, from egg laying to the emergence of adult moths.
After mating the female moth will lay eggs in clusters of 50 to 200 eggs, with clusters covered in hairs or scales. Female moths lay most of their eggs within four to five days of mating but can continue laying for up to two weeks. The eggs hatch within two to four days after being laid on the lower leaf surfaces.
After hatching, fall armyworm larvae complete five to six growth stages and reach maturity in 14 to 22 days. Once mature, larvae drop to the ground, where they pupate for around eight to nine days in warmer months, and 20 to 30 days in cooler areas, before emerging as a moth.
Fall armyworm do not diapause (suspend development) during any stage. They live year-round in places where there are available hosts and favourable temperatures. The ideal range for development is 23-30ºC. The insect can continue to to develop slowly below this range but do not survive year-round where temperatures fall below 9-12⁰C and where frosts occur.
Dispersal and migration
Fall armyworm is well known for its capacity to disperse within a crop locally and migrate over large distances. The adult moths are good flyers and can travel hundreds of kilometres in a short period of time. In North America, fall armyworm can survive year-round in southern Florida and Texas. As temperatures rise, populations build up and moths migrate northward along the eastern seaboard as far north as Canada.
Fall armyworm has been accidentally introduced to other parts of the world and was found in the African continent in 2016, the Indian subcontinent in 2018 and was found in Southeast Asia and China in 2019.
For Western Australia, this means that fall armyworm will establish in areas in the north with favourable conditions and host plants and migrate southward as host plants and climatic conditions remain suitable. If conditions are favourable, it could complete a few generational cycles in southern areas.
In addition to the adults, fall armyworm larvae also disperse. The neonates spin a silk thread that they use to balloon away from the egg mass to nearby host plants. Larvae of all ages are able to quickly crawl from one host plant to another.
This means both adults and larvae are able to move quickly within crops and from one crop to another or to nearby host plants after harvest.
Natural enemies
A number of natural enemies present in crops that attack the eggs, larvae and pupae of other lepidopteran pests are likely to also attack fall armyworm in the same crops.
Generalist predators, such as spiders, beetles, ants, sucking bugs and predatory wasps, will likely reduce the numbers of eggs, larvae and pupae. The adults and pupae may be fed upon by birds and other animals. A study conducted in Georgia, United States, demonstrated that 60 to 90 per cent of pupae were consumed by predators. Soil pathogens, such as nematodes, may infect the pupae and drive numbers down.
Overseas, egg parasitoids are very important for biological control. Parasitoids such as Trichogramma pretiosum have been shown to reduce the number of larvae hatching from the egg masses and thereby reduce damage to the host plant. These parasitoids have been easily mass-reared and are available in Australia where they have been used for management of Heliothis and other moth pests. They have been found parasitising fall armyworm eggs in northern WA. Other parasitoids, such as Telenomus remus, are also present in Australia and reduce fall armyworm numbers overseas. This parasitoid has been found in fall armyworm in the Northern Territory.
Larval parasitoids have also reduced fall armyworm numbers overseas. Cotesia marginiventris, which is present in Australia, is an important natural enemy in several countries. Cotesia ruficrus and other species have been found attacking fall armyworm in Western Australia.