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]

armyworm

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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]

The fall armyworm is widely distributed in eastern and central North America and in South America. It cannot 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]

It was first reported in Africa in 2016, where it is causing significant damage to maize crops and has great potential for further spread and economic damage.[16] It has since spread to 28 countries in Africa.[17] In December 2018,[18] it began to spread widely in India.[19] In January 2019, a heavy infestation of fall armyworm was recorded in corn plantations in Sri Lanka.[20]

The pest was first detected in China in the southwest province of Yunnan in January 2019[21] (or June 2019).[18] 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".[22] Also in June 2019 it was detected in Southeast Asia.[18]

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).[31] 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.[6] 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.[32]

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.[4]

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.[3] 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.[6] Because larvae cannot enter into diapause they cannot survive cold temperatures.[33]

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.[34]

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.[36]Cotesia icipe is another African braconid wasp suitable for the biological control of this lepidoptera.[37]

Fifty-three different parasite species have been discovered in fall armyworm larvae, spanning ten different families.[31] 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.[31]

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.[38] Samples of this were sent to Maree Crawford, the insect pathologist at the Queensland Department of Agriculture for further analysis.[38] Australian entomologists have said the finding is reassuring and that laboratory tests have been promising.[39] 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.[40] 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.[40] 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.[39] The N. rileyi research has given them hope that this can be avoided.[39]

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]

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.[34] 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.[31] 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 (19.510^6 short tons)/annum if not successfully controlled.[43] The fall armyworm have proved to be a pest in many regions, and methods of control continue to be developed.

The fall armyworm was identified in Africa in 2016. In early 2017, armyworms infested large swathes of corn crops across southern Africa, devastating the livelihoods of many farmers. It is thought they arrived as an invasive species from the Americas as eggs in imported produce.[44] This is causing immense concern among agricultural experts, due to the potentially huge amount of damage this invasive species will do to African food crops if allowed to spread.[45][46] Many African countries have agreed to take urgent actions against armyworms.[47]

After being first reported in India in May 2018 in Tamil Nadu, then the Sri Lankan Ministry of Agriculture issued a warning notice to farmers in the northwestern and north central provinces about possible fall armyworm invasion. At the time of warning, crop destruction had already been reported from the Ampara, Anuradhapura, and Polonnaruwa areas.[48] The larvae are known among the local people as Sena dalambuwa (armyworm caterpillar).[49] Not only corn, but also sugarcane plantations were attacked by the caterpillars in Anuradhapura, Ampara, and Monaragala districts.[50] 041b061a72