Spodoptera frugiperda (Smith), also known as autumn armyworm, belongs to the order Lepidoptera, noctuidae. Native to tropical and subtropical regions of the Americas, it is widely distributed on the American continent and is an important local agricultural pest. With the increasing frequency of international trade activities, the grassland nightcrawler has now invaded 44 African countries in sub-Saharan Africa, as well as India, Bangladesh, Sri Lanka and Myanmar in Asia, and has further invaded and spread to other parts of Asia, mainly southeast Asia and southern China.
1. It is a harmful situation
Larvae can cause leaf defoliation by feeding on the leaves, which are then transferred to the pest. Sometimes a large number of larvae are harmed by cutting roots, cutting off the stems of seedlings and young plants; the larvae can burrow into the ears of pregnant plants, eat buds and growth points such as tomatoes, and burrow into the fruit. When the population is large, the larvae spread in groups like marches. On corn, 1-3 instar larvae usually come out at night to infest, mostly hidden on the back of the leaf to feed, and form a translucent film "window hole" after feeding. Young larvae also spit silk and spread to the surrounding plants with the help of wind to continue to be harmful. 4-6 year old larvae are more seriously harmful to corn, and irregular long holes are formed after eating leaves, and the leaves of the whole corn can also be eaten lightly, which can cause the death of the corn growth point when severe, affecting the normal development of leaves and fruit ears. In addition, the elderly larvae feed on male and fruit ears of maize.
Second, morphological characteristics
Adult: Wingspan 32-40 mm. The forewings are grey to dark brown and the females grey to greyish brown; the males have dark brown forewings with dark spots and light dark markings, and the wing moles have distinctly gray tail-like protrusions. The hindwings are off-white , with brown and transparent veins. The male external genitalia hold the flap square. The rim of the hugger at the end of the gripper is missing. Female mating sacs do not have mating tablets.
Eggs: The eggs are domed, 0.4 mm in diameter and 0.3 mm high, usually 100-200 eggs are stacked in a lumpy shape, the eggs are covered with scales, light green or white at the beginning of birth, and gradually turn brown before hatching.
Larvae: 6 ages, occasionally 5. The body is green at the beginning of the incubation, with black lines and spots. When grown, it remains green or pale yellow, with a black dorsal midline and valve line. The mature larvae are 35–50 mm long, with yellow inverted Y-shaped spots on the head (distinguished from the heads of Oriental armyworms and cotton bollworms in the figure below), and the black dorsal hairs are covered with raw primordial bristles (2 bristles on either side of the dorsal midline of each segment). The terminal segment of the abdomen has 4 dark spots arranged in a square arrangement. For example, when dense (when the population density is large and food is scarce), the end-age larvae are almost black during the migration period.
The larvae are 6 years old, the body color and body length vary with age, and the young larvae have a green or yellow body color, a body length of 6-9 mm, and a black or orange head. The larvae are mostly brown, but also exist in black or green individuals, 30–50 mm long, with a black, brown or orange head with white or yellow inverted "Y" spots. The surface of the larvae has many longitudinal stripes, the dorsal midline is yellow, and there is a yellow longitudinal stripe on each side of the dorsal midline, and the outer stripes are black and yellow longitudinal stripes. The most obvious feature of the larvae of the grass moth is that it has 4 dark spots arranged in a square arrangement at the end of the abdomen, and the head has a distinct inverted "Y" pattern.
Pupae: The pupa is oval, reddish brown, 14–18 mm long and 4.5 mm wide. Mature larvae fall to the ground to borrow shallow (usually 2-8 cm deep) soil to make a pupal chamber, in which the cocoons wrapped in sand particles pupate. Pupate can also be found on harmful host plants such as the ears of corn.
Third, biological learning
Host extensiveness. The grassland nocturnal moth is a polyphagous moth that can harm more than 80 species of plants, and is fond of grasses, and is most likely to harm weeds such as corn, rice, wheat, barley, sorghum, millet, sugarcane, rye grass and Sudan grass; it is also harmful to common crops such as cruciferous family, cucurbitaceae, mallow family, legume, leguminacea, solanaceae, beet, onion, soybean, phaseolus, potato, sweet potato, alfalfa, buckwheat, oats, tobacco, tomato, pepper, onion and other common crops, as well as chrysanthemums, carnations, geranium and other ornamental plants (genus), It even causes harm to apples, oranges, etc.
Severity of the hazard. The grassland moth is most harmful to corn. According to statistics, in Florida, usa, the damage of the grassland moth can cause a 20% reduction in corn yield. In some economically disadvantaged areas, the loss of maize production caused by its harm is even more serious, such as honduras in Central America, which can cause a 40% reduction in maize production, and in Argentina and Brazil in South America, it can cause 72% and 34% of the loss of production, respectively. In September 2017, the International Centre for Agriculture and Biological Sciences reported that in 12 maize-growing countries in invaded Africa alone, the infestation of the grassland moth could reduce annual maize yields by 8.3 million to 20.6 million tonnes and economic losses of up to US$2.48 billion to US$6.19 billion.
Eco-polymorphism. The grassland moth is divided into two haplotypes of maize strains and rice strains, the former mainly eating for maize, cotton and sorghum, and the latter mainly feeding on pest rice and various pastures. The two haplotypes are basically identical in external morphology, but have obvious differences in sexual pheromone composition, mating behavior, and host plant range. The meadow moth completes a generation to go through four worm states of eggs, larvae, pupae and adult, and its generation length is related to the ambient temperature and host plant.
Suitable for a wide range of life. The suitable development temperature of the grassland moth is 11 to 30 °C, and under the condition of 28 °C, a generation can be completed in about 30 days, while under low temperature conditions, it takes 60 to 90 days. In the absence of diapause, in the United States, the meadow moth can only survive the winter in the mild climate of South Florida and Texas, while in Central and South America, where climate and host conditions are suitable, and in most parts of newly invaded Africa, it can reproduce annually.
Migratory diffusivity. Adult grassland moths can use wind to carry out long-distance directional migration at altitudes of several hundred meters, flying 100 km per night. Adults usually migrate up to 100 km before spawning, and if the wind direction is suitable, the flight distance will be longer, and it has been reported that adult grassland moths can migrate from Mississippi in the United States to southern Canada within 30 h, up to 1600 km.
Other biological habits. Adults are phototropic and generally migrate, mate and lay eggs at night. Egg masses are usually laid on the underside of the leaves. Adults can live up to two to three weeks, during which time female adults can mate and lay eggs multiple times, laying 900 to 1,000 eggs in a lifetime. At the right temperature, the eggs hatch into larvae in 2 to 4 days. The larvae have 6 ages, and the older larvae have a habit of cannibalism.