Fall armyworm
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.[1] 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.[2] Another remarkable trait of the larva is that they consistently practice cannibalism, despite its fitness costs.[3][4]
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.[5]
Geographic range[edit]
Native range[edit]
The fall armyworm is widely distributed in eastern and central North America and in South America. It cannot survive overwinter in below freezing temperatures,[7][8] 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.[6]
Introduced range[edit]
The potential global distribution of S. frugiperda[9] has been modelled using CLIMEX.[10][11] 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.[12]
Migration[edit]
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.[35][6] Their migration rate is remarkably fast, estimated at 300 miles (483 km) per generation.[35] Some scientists speculate that this fast migration is aided by the movement of air in weather fronts.[35]
Neurochemistry[edit]
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.[36]
Enemies[edit]
Predators[edit]
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.[6] The larva's main defense against enemies is their ability to reach large numbers and migrate before seasonal conditions are suitable for predators.[35]
Parasitoids[edit]
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.[6] In 2018, egg parasitoid wasps of the genera Telenomus and Trichogramma were discovered to attack army worm eggs in East Africa.[37] Cotesia icipe is another African braconid wasp suitable for the biological control of this lepidoptera.[38]
Parasites and disease[edit]
Fifty-three different parasite species have been discovered in fall armyworm larvae, spanning ten different families.[32] Often larvae can survive through much of their crop consumption despite outbreaks of disease, because of the larva's fast life cycle.[6] 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.[32]
Fungi[edit]
In February 2021, it was reported that an Australian agronomist Georgia Rodger had found at a property near Beaudesert (southern Queensland) the tropical fungus Nomuraea rileyi which was known to be effective in killing and consuming fall armyworms.[39] Samples of this were sent to Maree Crawford, the insect pathologist at the Queensland Department of Agriculture for further analysis.[39] Australian entomologists have said the finding is reassuring and that laboratory tests have been promising.[40] This is substantiated by various studies including a 2018 journal article which looked into the effectiveness of N. rileyi had on infestations of armyworms in Indian maize crops.[41] The study concluded N. rileyi could potentially be a cost-effective tool in combating the pest, compatible with eco-friendly management practices, although further studies were required.[41] Farmers in Australia have struggled to control the pest which has been destroying crops, prompting concerns about potential food shortages which could cause an increase in food prices for consumers.[40] The N. rileyi research has given them hope that this can be avoided.[40]
Subspecies[edit]
The fall armyworm may be presently undergoing a divergence into two separate species. These two strains have major genetic differences that are connected to the plants they feed on, even though both still exist in the same area (sympatric speciation). These two strains can be loosely categorized into a rice strain and a corn strain. This separation is occurring because of differences in habitat (preferred host plant), and differences in reproductive behavior. The reproductive differences can be divided into two categories: difference in the timing of mating at night, and difference in female sex pheromones.[2]
Mating[edit]
Mate searching behavior and male–male conflict[edit]
A female attracts males by perching atop the 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.[42] 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.[35]
Interactions with humans[edit]
Research use[edit]
S. frugiperda cells (Sf9 and Sf21 cell lines) are commonly used in biomedical research for the purpose of recombinant protein expression using insect-specific viruses called baculoviruses.[43]
Pest of crop plants[edit]
Because of their food preferences, fall armyworm larvae can wreak havoc on a wide range of crops. The first historical account of the fall armyworm's destruction was in 1797 in Georgia. Destruction can happen almost over night, because the first stages of a caterpillar's life require very little food, and the later stages require about 50 times more. Because of this rapid change in food consumption, the presence of larvae will not be noticed until they have destroyed almost everything in as little as a night.[35] Some examples of targeted crops include cotton, tobacco, sweet corn, rice, peanuts, and even fruits such as apples, oranges, and many more. The list of possible food sources for the worms is extensive, so crop damage is wide-ranging.[6] It is estimated that almost 40 percent of those species that armyworms target are economically important.[32] Because the larvae eat so much of the plant, they are very detrimental to crop survival and yield. In corn, larvae will even burrow into the corn ear to eat the kernels.[6]
The UN Food and Agriculture Organization estimates that S. frugiperda will reduce maize/corn yields by 17.7 million metric tons (19.5×106 short tons)/annum if not successfully controlled.[44] The fall armyworm have proved to be a pest in many regions, and methods of control continue to be developed.
Management and control[edit]
Because of the fall armyworms' great destructive power, farmers must go to great lengths to deter the larvae. Insecticide is a widely used form of protection; in southern regions, farmers may have to apply insecticide to corn every day.[6] Agricultural drones have been used to apply pesticides, used in China, Vietnam, Zambia and other regions.[61]
The CABI-led programme, Plantwise and partners have several recommendations for managing fall armyworm, these include: planting early, avoiding staggered planting, and inter-cropping with crops that are not susceptible to fall armyworm, such as cassava or yam. They also recommend conserving shelters and flowering plants on the edges for beneficial insects such as ground beetles and parasitoids.[62][63][64][65][66]
Inter-cropping with the "push-pull" technique with crops such as Desmodium and Napier grass can be used to control fall armyworm.[63]
For some crops, including wheat, sorghum, millet and rice, it is recommend by Plantwise partners to plant short maturing and varieties that are less preferred by S. frugiperda.[63][64][65][66]
Another strategy is to plant crops earlier to avoid the increase in armyworm numbers as the summer progresses.[6]
In South Africa, farmers are using pheromone lures with a combination of Dichlorvos blocks to trap and eliminate male armyworms, with the intention of disrupting mating cycles.[67]
CIMMYT and its partners are using forward genetics to breed for better S. frugiperda resistance in maize.[68] Genome-wide association studies (GWAS) are the most effective method for associating S. f. resistance to the responsible genomic region, especially used in maize/corn but also wheat, sorghum, millet, rice, and legumes.[69][70] The first uses of conventional breeding in the first decade of the 1900s were reported by Gernet 1917 and Hinds 1914, improving resistance in maize/corn, sorghum, millet, Cynodon dactylon, and Arachis hypogaea.[69]
In Australia, a caterpillar-specific virus packaged as Fawligen biopesticide was approved under emergency regulations in 2020 to help control the armyworm, and the parasitoid wasp Trichogramma pretiosum is also used.[71]
Directorate of plant protection Quarantine and storage, Ministry of Agriculture, Govt of India regularly issues advisories from time to time to manage the menace of Fall Army Worm in India.