[Smart Farmers] Understanding the “Army Bugs”: Why is the Fall Armyworm so lethal?

What is the Fall Armyworm?

The fall armyworm was discovered and named by scientists in 1797. It is a species of insect in the order Lepidoptera, family Noctuidae, genus Spodoptera. Its English name is fall armyworm, commonly known as fall armyworm, and is listed as one of the top ten global warning pests by the Food and Agriculture Organization of the United Nations. The fall armyworm is a holometabolous insect, and it goes through four stages: egg, larva, pupa and adult to complete one generation.

The eggs of the fall armyworm are dome-shaped, usually 100 to 200 eggs piled up in a mass, covered with yellow scales, light green or white when first laid, and gradually turn brown before hatching. There are six instars of the larvae, and the body color and length change with the instars. The most obvious feature is the white or yellow inverted "Y" pattern on the head and four black spots arranged in a square on the last segment of the abdomen.

The pupa of the fall armyworm is reddish brown and shiny, 14 to 18 mm long and about 4.5 mm wide. The typical characteristics of male adults are yellow-brown annular patterns on the forewings, white patches at the top corners, a black spot at the base of the wings, white hindwings, and a gray stripe on the rear edge; the typical characteristics of female adults are gray-brown annular and kidney-shaped patterns on the forewings, yellow-brown outlines, distinct horizontal lines, white hindwings, and gray stripes on the outer edge.

Fall armyworm larvae

Why should we be wary of the fall armyworm damage?

The fall armyworm is native to tropical and subtropical regions of America. In 2016, it first appeared in West Africa and Central Africa. In 2018, it spread to India and South Asia and Southeast Asian countries. In January 2019, the fall armyworm was introduced to China and quickly spread throughout the country.

Adult fall armyworms can migrate long distances with the help of wind at an altitude of several hundred meters, and can fly 100 kilometers every night. The larvae are the main stage in which they cause damage. In their native habitat, they can feed on more than 76 families and 350 species of plants. They particularly like corn, rice, cotton, peanuts, sorghum and vegetables. They can eat up a whole corn field in a day, and are nicknamed "army bugs." Each female moth can lay about 1,500 eggs in its lifetime, and can lay up to 2,000 eggs. This biological characteristic of "being able to fly, eat and reproduce" makes it extremely harmful to crops.

Can corn that has been damaged by the fall armyworm still be eaten?

Fall armyworm mainly harms corn. In the seedling stage of corn, fall armyworm larvae like to curl up in the heart leaves of the plant to cause damage. While the leaves are growing, they are also chewing, causing the leaves to be uneven; when corn is tasseling, the larvae will gather on the tassels that have not been fully drawn out to cause damage. At this time, the tassels are wrapped by the top two leaves, and the larvae have a better hiding environment. Young larvae cause damage to the spikelets, while old larvae directly bite off the lateral branches and main axis of the tassels; after the tassels are drawn out to scatter pollen, they lose their hiding environment and the larvae move; after the female ears are silked, the larvae will damage the filaments, bite off part or all of the filaments, and enter the ear tip along the filament channel to feed on the grains, or they can directly bore through the bracts to feed on the grains.

Corn ears that have been damaged by fall armyworms are usually not eaten by humans. Although these "little moths" do not directly affect the safety of corn, their damage can make corn more susceptible to mold damage and produce toxins such as aflatoxin.

Damage status of corn at different stages

What are the methods to prevent and control fall armyworm?

At present, there have been many reports on the prevention and control of fall armyworm at home and abroad, and the methods mainly include monitoring and early warning, agricultural prevention and control, physical and chemical control, biological control and chemical control.

Monitoring and early warning:

Population monitoring and early warning are the basis of prevention and control work. Three methods can be used: sex pheromone trapping, light trapping and field survey. Sex pheromone trapping has a high sensitivity and is suitable for monitoring populations at low densities in the early stages of occurrence. Since the phototaxis of the fall armyworm is significantly lower than that of other noctuid pests such as cotton bollworm, the light trapping method is not sensitive enough, but it can be used for population monitoring at high densities. Its advantage is that the nature of insect migration and egg-laying dynamics can be determined by dissecting the ovaries of female insects. The methods of adult sex trapping and light trapping can be used to make short-term predictions of field population occurrences. In production, the actual field occurrence investigation can be promptly initiated based on the short-term prediction results, and prevention and control practices can be guided based on the survey data.

Agricultural control:

Agricultural prevention and control mainly takes measures from the aspects of cultivation management, crop layout, and variety resistance to create a farmland ecological environment that is not conducive to the occurrence and damage of fall armyworms, and focuses more on early prevention. For example, late-sown corn is planted around corn as an isolation belt to concentrate on attracting fall armyworms to lay eggs and kill them; corn-soybean intercropping can significantly reduce the survival rate of fall armyworms on corn. In production, reasonable intercropping of corn and soybeans can be used to reduce the damage of fall armyworms in corn fields.

Physical and chemical control:

A female fall armyworm lays about 500 to 1,000 eggs. Killing one adult that has not laid eggs is equivalent to protecting 667m2 of crops. During the active period of adults, taking advantage of their chemotaxis and phototaxis, using food traps, sweet and vinegar liquid, sex traps and insecticidal lamps to kill adults can reduce egg laying.

Biological control:

Use biological agents such as Beauveria bassiana, Metarhizium muscardine, Cabbage armyworm nuclear polyhedrosis virus, Bacillus thuringiensis, etc. to prevent larvae at an early stage, protect and utilize natural enemies such as the black egg wasp of the noctuid armyworm, Trichogramma yellow-eyed wasp, and caterpillar stink bug, and promote sustainable management.

Chemical control:

Chemical pesticides are the main method and emergency measure for controlling the outbreak of fall armyworm in the short term. They should be applied in the early morning or evening after sunset, from the peak period of egg hatching to before the spread of young larvae. Use spinetoram, emamectin benzoate, chlorfenapyr and other pesticides for prevention and control. Pay attention to the alternation and rotation of pesticides and their safe use to delay the development of resistance and improve the prevention and control effects.

Author: Xu Lina (Researcher at the Institute of Plant Protection and Agricultural Product Quality and Safety, Anhui Academy of Agricultural Sciences)

Scientific review: Qi Rende (Researcher at the Institute of Plant Protection and Agricultural Product Quality and Safety, Anhui Academy of Agricultural Sciences)

Planning: Xie Yun, Luo Yuchen (internship)