Fall armyworm

Witch Hazel

The armyworm's diet consists mainly of grasses and small-grain crops such as corn, but the species has been noted to consume over 80 different plants.[17] 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.[4] The resistance comes from a unique 33-kD proteinase that the corn produces when it is being fed on by fall armyworms or other larva. This protein was found to significantly decrease fall armyworm larva growth.[18]

When possible, larva will cannibalize the larva 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.[19]

Adult moths sip nectar from flowers such as that of witch hazel (Hamamelis virginiana).[20]

The armyworm's egg is dome-shaped, and measures around 0.4 mm in diameter and 0.3 mm in height.[4] Females prefer to lay eggs on the underside of leaves, but in high populations they will lay them just about anywhere.[21] In warm weather, the eggs will hatch into larvae within a few days.

The larva 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.[4] The mature caterpillar is about 1.5–2.0 inches (38–51 mm) in length.[1] 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.

The larva 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.[21]

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.[4] Adults are nocturnal and fare best during warm and humid nights.[4]

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.[4] The larva's main defense against enemies is their ability to reach large numbers and migrate before seasonal conditions are suitable for predators.[21]

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.[4]

Fifty-three different parasite species have been discovered in fall armyworm larvae, spanning ten different families.[17] Often larvae can survive through much of their crop consumption despite outbreaks of disease, because of the larva's fast life cycle.[4] 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.[17]

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.[22] 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.[21]

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 larva will not be noticed until they have destroyed almost everything in as little as a night.[21] 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.[4] It is estimated that almost 40 percent of those species that armyworms target are economically important.[17] Because the larva 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.[4]

The fall armyworm was identified in Africa in 2016. In early 2017, armyworms infested large swathes of corn crops across southern Africa, devastating the livelihood of many farmers. It is thought they arrived as an invasive species from the Americas as eggs in imported produce.[23] 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.[24][25] Many African countries have agreed to take urgent actions against armyworms.[26]

The CABI-led programme, Plantwise and partners have several recommendations for managing fall armyworm, these include; planting early, avoiding staggered planting, intercropping with crops that aren't 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.[27][28][29][30][31]

Intercropping with the 'push-pull' technique with crops such as Desmodium and Napier grass can be used to control fall armyworm.[28]

For some crops including wheat, maize, sorghum, millet and rice it is recommend by Plantwise partners recommend planting short maturing and less preferred varieties.[28][29][30][31]

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. Another strategy is to plant crops earlier to avoid the increase in armyworm numbers as the summer progresses.[4] 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.[32]

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.

First reported in India in May 2018, from Tamil Nadu, Ministry of Agriculture of Sri Lanka 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.[33] The moth is known among local people as Sena dalambuwa (armyworm caterpillar).[34] Not only corn, but also sugarcane plantations were attacked by the caterpillars in Anuradhapura, Ampara, and Monaragala districts.[35]

In December 2018, heavy infestations in corn cultivation were identified. The spreading of the moth leads to attack corn all around the country within weeks.[36] On 6 January 2019, caterpillars spread to Monaragala district and devastated corn crops.[37][38] At the end of January 2019, the armyworm is present in all districts of Sri Lanka except Nuwara Eliya and Jaffna.[39]

The Sri Lankan Department of Agriculture recommended 12 pesticides under three categories, to be used alternatively in every seven days.[40] Organic farming expert, Thilak Kandegama said that the threat can be overcome by sprinkling ashes as a repellent.[41] Agricultural Ministry also decided to use drone technology for the spraying of insecticides to control the spreading of caterpillars.[42][43]

On 29 December 2018, armyworms were recorded from paddy cultivations in the Sinhapura area of Polonnaruwa. In January 2019, caterpillars were also recorded from paddy cultivations of Nochchiyagama area in Anuradhapura district.[44]

The agriculture drone developed by XAG has a spraying efficiency of 14 hectares per hour, which can attack the infected farmland and kill pests on a large scale, thus reducing the degree of moth migration. The agriculture drone has a special atomisation spraying mechanism, micron-level droplets and powerful downdraft, is designed to achieve uniform applications of ultralow-volume pesticides. Both sides of the leaves and the central part of crops, where the pests conceal themselves, can be more precisely targeted. This not only increases fall armyworm’s exposure to chemicals but also cuts down 30% pesticide use and 90% agricultural water. At present, this technology has been extended from China to Vietnam, Zambia and other regions.[45]