The fall armyworm (Spodoptera frugiperda) is a species in the order Lepidoptera and one of the species of the fall armyworm moths distinguished by their larval life stage. The term "armyworm" can refer to several species, often describing the large-scale invasive behavior of the species' larval stage. It is regarded as a pest and can damage and destroy a wide variety of crops, which causes large economic damage. Its scientific name derives from frugiperda, which is Latin for lost fruit, named because of the species' ability to destroy crops. Because of its propensity for destruction, the fall armyworm's habits and possibilities for crop protection have been studied in depth. It is also a notable case for studying sympatric speciation, as it appears to be diverging into two species currently. Another remarkable trait of the larva is that they consistently practice cannibalism, despite its fitness costs.
The fall armyworm is active at a different time of year from the true armyworm, another species in the order Lepidoptera and family Noctuidae, but of the genus Mythimna. Outbreaks of the true armyworm usually occur during the early part of the summer; the fall armyworm does most damage in the late summer in the southern part of the United States, and early fall in the northern regions.
Description
The adult moths are wing tip to wing tip, with a brown or gray forewing, and a white hindwing. There is slight sexual dimorphism, with males having more patterns and a distinct white spot on each of their forewings. The first larval instar is light colored with a larger dark head. As they develop through instars, they become browner with white lengthwise lines. They also develop dark spots with spines.
Geographic range Native range The fall armyworm is widely distributed in eastern and central North America and in South America. It cannot survive overwinter in below freezing temperatures,Murúa MG et al. (2009) Natural distribution of parasitoids of larvae of the fall armyworm, Spodoptera frugiperda, in Argentina Journal of Insect Science 9(20) so it only survives the winter in the most southern regions of the United States, namely Texas and Florida. Because of this, the fall armyworm is a more prominent pest in southeastern states. However, seasonally it will spread across the eastern United States and up to southern Canada, inhabiting areas with suitable food supplies.
Introduced range The potential global distribution of S. frugiperda has been modelled using CLIMEX. The modelled global potential distribution reflects the marked seasonal range dynamics experienced in North America, with much of the potential range in Europe, South Africa, China and Australia consisting of habitat that is only climatically suitable during the warmer months.. A more recent physiologically-based population dynamics model was developed for assessing the potential distribution of S. frugiperda in Europe. The model showed that the Mediterranean coastal areas of Southern Europe might be particularly suitable for the establishment of the species.
Africa S. frugiperda was first found on the African continent in 2013 in Sao Tome, then spread through Nigeria, Benin, Togo, and was found in Ghana in February 2017. In December 2020 S. frugiperda was first found in Syria in Daraa on the Jordanian border and is believed to have arrived from there without human assistance, having just been found in that country also. The fall armyworm is causing significant damage to maize crops in Africa and has great potential for further spread and economic damage. It has since spread to 28 countries in Africa.
Asia S. frugiperda was first detected in Bangladesh in late 2018. it has reached 37 districts. As a result of the introduction of S. frugiperda and lumpy skin disease within a few months of each other, the FAO, the World Food Programme, Bangladesh Government officials, and others agreed to begin improving Bangladesh's agricultural emergency response capabilities. The use of two biopesticides – Spodoptera frugiperda nuclear polyhedrosis virus/SfNPV (the SNPV/single nuclear polyhedrosis virus specific to S. frugiperda) and Habrobracon hebetor – is recommended.
In December 2018, the Fall armyworm began to spread widely in India. In January 2019, a heavy infestation of fall armyworm was recorded in corn plantations in Sri Lanka.
The pest was first detected in China in the southwest province of Yunnan in January 2019 (or June 2019). Through 2019, the pest infested a total of 26 provinces. The armyworm is expected in 2020 to hit China's Northeast wheat belt. A report issued by the Ministry of Agriculture and Rural Affairs rates the situation as "very grave".
The fall armyworm was first reported in Southeast Asia in late 2018 in Thailand and Myanmar and its presence is now confirmed in almost all Southeast Asian countries.
Oceania In January 2020 S. frugiperda was detected on the Torres Strait Islands, in February in North Queensland, and then continued into the rest of Queensland, and the Northern Territory, Western Australia, and then in September was found in New South Wales between Moree and Boggabilla (and later in Narrabri, Wee Waa, Dubbo, Breeza, and Maitland). S. frugiperda is expected to severely impact Queensland's wool industry because it feeds on all major grazing plants. It was observed in traps baited with a male pheromone lure, firstly on Darnley Island and Saibai islands in the Torres Strait, and subsequently on the mainland near Croydon. Within a week it was officially declared ineradicable. In April 2020, it was detected in Papua New Guinea, spreading across the Torres Strait.
Fall armyworm was first detected in New Zealand in February 2022. Biosecurity New Zealand and sector partners ran a biosecurity response to limit the spread of Fall armyworm and try to eradicate it from New Zealand. This included surveillance and research to better understand the moth, its spread, and potential impacts in New Zealand. By April 2023, it became clear that Fall armyworm was widespread, particularly in the North Island, and that eradication was unlikely because it had been windblown from Australia, and this is likely to repeatedly occur.
Diet
Larvae
The armyworm's diet consists mainly of grasses and grain crops such as corn, but the species has been noted to consume over 80 different plants (50 non-economic and 30 economic plants). Armyworms earned their common name by eating all plant matter they encounter in their wide dispersals, like a large army. A few sweet corn varieties have partial, but not complete, resistance to armyworms. The resistance comes from a unique 33-kD proteinase that the corn produces when it is being fed on by fall armyworms or other larvae. This protein was found to significantly decrease fall armyworm larva growth.
Cannibalism When possible, larvae will cannibalize the larvae of smaller instars. A 1999 study showed that cannibalism only benefits the caterpillar when other food is scarce. Despite this, the caterpillars will cannibalize others whenever they can, even though it was found to decrease their own fitness in many cases. One known reason why cannibalism is detrimental to the fall armyworm is because of disease transmission to the cannibal. In nature, the negative effects of cannibalism may be balanced by the fact that cannibalism removes competitors, thereby making more resources accessible and indirectly increasing fall armyworms' fitness.
Adults Adult moths drink nectar from flowers such as that of witch hazel (Hamamelis virginiana).
Life history The fall armyworm's life cycle is completed within 30 days during summer, and 60 days during the spring and autumn seasons; during the winter, these caterpillars' life cycle lasts about 80 to 90 days. The number of generations a moth will have in a year varies based on climate, but in her life span a female will typically lay about 1,500 eggs. Because larvae cannot enter into diapause they cannot survive cold temperatures.
Egg The armyworm's egg is dome-shaped, and measures around in diameter and in height. Females prefer to lay eggs on the underside of leaves, but in high populations they will lay them just about anywhere. In warm weather, the eggs will hatch into larvae within a few days.
Larva The larvae go through six different instars, each varying slightly in physical appearance and pattern. The larva process lasts from 14 to 30 days, again depending on temperatures. The mature caterpillar is about in length. This is the most destructive life stage as the larvae have biting mouth parts. The larvae have a distinctive inverted Y suture on the forehead.
Pupa The larvae then pupate underground for 7 to 37 days in a cocoon they form of soil and silk. Duration and survival of the pupal stage depend on the temperature of the environment.
Adults Once emerged, the adults live for about 10 days, and sometimes up to 21 days, with the female laying most of her eggs early in life. Adults are nocturnal and fare best during warm and humid nights.
Reproduction A female attracts males by perching atop a host plant feeding area and releasing a sex pheromone as the signal that she wishes to mate. The pheromone has been studied and found to contain the components Z7-12 and Z9-14. Each female only mates once per night; this creates a physical conflict between the multiple males that will fly towards a ready female. There is an order to which the females call and mate: virgin females do first, females who have mated once next, and females who have already mated multiple times call and mate last during the night.
Migration Adults are capable of flying long distances, so even though they are unable to overwinter north of the southern region of the United States, the moths can migrate as far north as southern Canada in warm months. Their migration rate is remarkably fast, estimated at per generation. Some scientists speculate that this fast migration is aided by the movement of air in weather fronts.
Neurochemistry Allatotropin and allatotropin+allatostatin C – neuropeptides – extracted from Manduca sexta were both found to suppress feeding in all life stages, increase larval mortality, and reduce adult lifespan, by Oeh et al 2000.
Enemies
Predators Fall armyworm caterpillars are directly preyed upon by many invertebrates and vertebrates. Common predators include birds, rodents, beetles, earwigs, and other insects. It has been shown that direct predation can cause significant losses to caterpillar populations. The larva's main defense against enemies is their ability to reach large numbers and migrate before seasonal conditions are suitable for predators.
Parasitoids Fly and wasp parasitoids target the fall armyworm, most commonly Archytas marmoratus, Cotesia marginiventris, and Chelonus texanus. The armyworm is also vulnerable to additional parasitoids, varying with location. In 2018, egg parasitoid wasps of the genera Telenomus and Trichogramma were discovered to attack army worm eggs in East Africa. Cotesia icipe is another African braconid wasp suitable for the biological control of this lepidoptera.Deux nouveaux insectes identifiés au Kenya peuvent aider à lutter contre les ravageurs du maïs.
Parasites and disease Fifty-three different parasite species have been discovered in fall armyworm larvae, spanning ten different families. Often larvae can survive through much of their crop consumption despite outbreaks of disease, because of the larva's fast life cycle. Despite this, parasites of the fall armyworm are being studied extensively as a means of fighting armyworm attacks on crops. One suggested approach would be to introduce parasites from South America to North American fall armyworms, and vice versa.
Fungi