A farmer plants corn, and two or three weeks later he walks between the rows and notices scattered holes in the outer leaves and wet, sawdust-like frass inside the whorl. He assumes it is a passing insect and puts it off, and a week later the field looks as if someone went through it with scissors.
This is the usual scenario with the fall armyworm (Spodoptera frugiperda), a nocturnal moth of the order Lepidoptera that originates in the tropical regions of the Americas. It was detected in West Africa in 2016, then spread within a few years across most of Africa, Asia and the Middle East, and has become one of the most dangerous corn pests in the world.
Pest: Fall armyworms are voracious larvae that attack corn in particular and other crops, feeding on the leaves and boring into the heart of the plant.
Symptom: Holes and scratches on the leaves and bran-like droppings in the whorl, and it may infest the developing ears.
Activity: It multiplies quickly in warm weather and moves from one field to another over long distances on the wind.
Common mistake: Delaying control until the larvae grow. The young stages are the weakest and the easiest to control.
Rule: Early detection with pheromone traps, checking the whorl, and immediate action against the young stages protect the crop from major losses.
Fall Armyworm Overview
What makes this pest frightening is not a single trait but the combination of three traits that let it arrive quickly, multiply heavily and hide from spraying:
- Long-distance flight: The moth travels very long distances with the help of the wind, and may cover around a thousand kilometers in its lifetime.
- Large egg numbers: The female lays her eggs in large, repeated masses.
- Hidden larva: The larva hides inside the corn whorl, where ordinary spraying does not easily reach it.
That is why your field can be infested even when the neighboring fields are clean, and the fight against it is a battle of timing before it is a battle of pesticides.
The fall armyworm has been recorded on more than 80 plant species, but it clearly prefers grasses:
- Main hosts: Maize (field corn, grain corn and sweet corn), sorghum, millet, rice, sugarcane and forage grasses.
- Secondary hosts: Cotton, peanuts, cowpeas, bananas and some vegetables.
Corn remains the main target of damage, especially in late and overlapping plantings.
Damage Symptoms: What to Look for in Your Field
The damage changes with the age of the larva and the age of the plant, and knowing this sequence lets you detect the infestation in its first days.
By Larval Age
- Newly hatched larvae (first and second instars): They feed in groups on the leaf surface and scrape it, leaving the transparent epidermis, so clear "window" patches appear on the leaves, usually near the egg mass.
- Third instar and beyond: They spread out and enter the whorl, eating the rolled leaves before they open. When the leaves unfold they show repeated holes with ragged edges, lined up in the same pattern. The larvae also become cannibalistic, so often only one or two larvae remain in a single whorl.
- The last two instars: They consume most of the food of their larval life, so the damage suddenly accelerates. This is the secret behind "the field that changed within a week."
By Corn Age
- From emergence to mid-whorl: The most dangerous period and the easiest to control. In small seedlings the larva may chew the growing point, killing the heart.
- From mid-whorl to tasseling: The damage is more visible, but the larger plant tolerates more.
- Tassel and ear stage: Feeding on the male tassel usually causes no notable loss, but a hole in the ear and eaten kernels make the ear unmarketable.
The clearest sign of all is the wet, sawdust-like frass inside the whorl and on the upper leaves. Fresh frass means an active larva right now, while old dry frass with old holes may mean the larva has pupated and gone. During scouting, also look for egg masses on the underside of the leaves.
Life Cycle and Appearance: How to Tell Each Stage Apart
In hot weather the cycle from egg to moth is completed in about 30 days, and it lasts much longer in the cold. The insect does not enter winter dormancy in warm regions, so generations follow one another almost all year round.
Egg
- Shape: Small and round, about 0.4 mm in diameter, cream to gray in color.
- Where it is laid: Laid in masses that usually contain between 100 and 200 eggs, often on the underside of the leaves, and sometimes in two layers.
- Distinguishing mark: The mass is covered with a layer of pinkish-gray scales from the female's body, looking like fuzz or felt.
- Number: The female may lay several masses during her life, and her total eggs may reach about 1,500 to 2,000.
- Hatching: The eggs hatch within 2 to 5 days depending on temperature.
Larva
- Duration: It passes through six larval instars over about 14 to 21 days in warm weather.
- Shape and color: The newly hatched larva is light green with a dark head, and the full-grown larva reaches about 3.5 to 4 cm in length, with a color between light green and dark brown, and lengthwise stripes on the back and sides.
- Distinguishing marks in older instars: A light-colored inverted Y on the front of the dark head, and four black dots arranged in a square on the second-to-last abdominal segment, while the dots on the other segments are arranged in a trapezoid.
Some other Spodoptera species look similar, so at the first sign of an unfamiliar infestation in your area, take a sample to an agricultural advisor. If the larvae feed heavily on vegetable and cotton leaves and hide in the soil during the day, it may be the cotton leafworm, and you can compare the two pests in the Pest Library.
Pupa
- Pupation site: The mature larva drops to the soil and pupates at a depth of 2 to 8 cm inside a soft cocoon of soil and silk.
- Appearance: The pupa is shiny reddish-brown, about 13 to 17 mm long.
- Duration: It lasts about 8 to 13 days in warm weather, and much longer in the cold.
Adult Moth
- Size: Body length about 1.5 to 2 cm, and wingspan about 3.2 to 4 cm with the wings spread.
- Wings: The male's forewing is brownish-gray mottled with clear light markings and a white spot near the tip, while the female's wings are duller and less distinct. The hind wing is silvery white with a thin dark margin.
- Behavior and lifespan: Active at night, especially on warm, humid evenings. It lives about two to three weeks and starts laying eggs a few days after emerging.
Monitoring and Biological Control
Every correct spraying decision starts with weekly scouting. It is not enough to walk along the edge of the field. The method recommended in corn research institutes' guidelines goes like this:
- Route: Walk through the field in a W shape, skipping the first 4 to 5 rows from the edge.
- Sample: Stop at five points and check 10 consecutive plants at each point.
- Calculation: Calculate the percentage of plants with fresh infestation (wet frass or live larvae).
Among the thresholds used in some extension programs: intervene at about 10% freshly infested plants from emergence to mid-whorl, and about 20% after that. These figures are indicative and vary with crop value and the recommendations of your local extension service.
Start scouting from the first week after emergence and repeat it at least weekly, and crush any egg masses you find. Every mass removed means a hundred or more larvae that will never hatch.
Pheromone traps: Fall armyworm traps attract males, and their main benefit is early warning: a rise in moth numbers in them means a wave of eggs is coming within days, so you intensify scouting. Some guidelines recommend about 5 traps per roughly 4,000 square meters for monitoring, and larger numbers for mass trapping. They are a warning tool and do not replace inspecting the plants.
Biological control carries great weight: in its native range, a large share of the eggs and larvae is killed by natural enemies, the most important of which are:
- Egg parasitoids: Such as Telenomus remus and Trichogramma, released commercially in some programs from the start of emergence.
- Larval parasitoids: Such as Cotesia and Chelonus, and some parasitic flies.
- Predators: Ladybugs, earwigs, ants, predatory bugs and birds.
- Biopesticides: The bacterium Bacillus thuringiensis (best effect on young larvae), the fall armyworm nuclear polyhedrosis virus (SfMNPV), and the fungus Beauveria bassiana.
Every unnecessary broad-spectrum spray kills this free army, which is why selective pesticides are preferred whenever possible.
Chemical Control: Timing Before Pesticide Type
A good pesticide is useless if it arrives late or does not reach the larva. Most complaints of "I sprayed and the worm did not die" come down to one of three causes:
- Spraying too late: A larva in its first and second instars is small and exposed, so it dies easily. After the third instar it shelters in the whorl and needs a larger dose and more time.
- The spray does not reach the whorl: Frass blocks the entrance. Direct the spray from above into the whorl with a suitable nozzle and enough water.
- Wrong time of day: Larvae are most active in the morning and evening. Spraying in the early morning or before sunset raises the chance of hitting the pest and reduces evaporation of the solution.
At the ear stage, spraying becomes of little use because the larva is protected inside the husks, so success is made at the whorl stage.
Rotating Active Groups
The fall armyworm has developed resistance to older groups such as carbamates, organophosphates and pyrethroids, so relying on a cheap pyrethroid alone often disappoints.
The Insecticide Resistance Action Committee (IRAC) recommends a "windows" system: one group for a period of roughly one generation (about 30 days), then a different group in the next generation, without going straight back to the first. Effective groups include:
- Group 28 (diamides): Such as chlorantraniliprole and cyantraniliprole.
- Group 5 (spinosyns): Such as spinetoram.
- Group 18 (molting hormone mimics): Such as methoxyfenozide, which is gentler on natural enemies.
- Group 6: Such as emamectin benzoate.
- Group 11A: Bt bacteria.
Note: Switching between two trade names from the same group is not rotation, and a pesticide mixed from two active ingredients counts toward both groups.
Suitable Products From Our Store
At Orkida Agri we have selected products from different active groups so you can build a proper rotation program on them, along with a sprayer that helps you get the solution into the whorl:
- Coragen 20 SC: Chlorantraniliprole (Group 28). It paralyzes the larva's muscles so it stops feeding within hours of contact with the treated leaves, and its effectiveness is high on lepidopteran larvae in corn. We rely on it as a first spray when fresh infestation appears on the young larvae, before they go deep into the whorl, and keep to the number of sprays allowed per season.
- Radient 120 SC: Spinetoram (Group 5) for leaf worms, a suitable option for the next window after Group 28.
- Runner 24 SC: Methoxyfenozide (Group 18), a growth regulator that stops larval feeding quickly and suits programs that preserve natural enemies.
- Eco Spray 20-liter backpack sprayer, Ultra model: With its multiple nozzles it helps you direct the solution into the whorl instead of spraying the leaves from the side, and that is half the success of control.
Our advice: follow the dose and pre-harvest interval written on the label for the treated crop, because an insufficient dose is the shortest route to resistance.
Prevention and Farming Practices
Prevention reduces the number of sprays and delays the pest's arrival at the damage threshold. These practices are applied before planting, during it and after harvest:
- Deep plowing before planting: To expose the pupae in the soil to the sun and birds.
- Synchronized planting dates: In the area as far as possible. Late planting beside an older planting receives the accumulated generations.
- Removing corn residues and grass weeds: After harvest and along the field edges.
- Intercropping with legumes: Which has shown a reduction in infestation in several trials.
- Hybrids with tight ear husks: Especially in sweet corn.
- Balanced fertilization and irrigation: They make the plant grow faster and better able to get past the danger stage.
- A field notebook: In which you record scouting results, sprays and the groups used, so you can base your rotation decision on it.
Frequently Asked Questions
Is the fall armyworm the same as the cotton leafworm?
No. Both belong to the genus Spodoptera, but the fall armyworm (S. frugiperda) prefers corn and grasses and feeds inside the whorl, and is distinguished by an inverted Y on the head and four square dots near the end of the abdomen. The cotton leafworm (S. littoralis) attacks cotton, vegetables and clover more.
When should I spray corn against the fall armyworm?
When the percentage of freshly infested plants reaches the threshold recommended by the extension service (about 10% in the early stages in some programs), in the early morning or before sunset, directing the solution into the whorl.
Why doesn't the worm die after spraying?
Usually because it is old, or protected inside the whorl by frass, or because the pesticide belongs to a group it resists, such as some pyrethroids. Review the timing and spraying method and change the active group.
Can it be controlled without chemical pesticides?
In small areas it can be greatly reduced by collecting egg masses and larvae, using Bt products on young larvae, and protecting natural enemies. In widespread infestations it usually needs an integrated program that includes selective pesticides.
How long does the fall armyworm live and how many generations does it have per year?
The cycle is completed in about a month in hot weather, and the moth lives two to three weeks. In warm regions generations follow one another without winter dormancy, so it may reach several generations a year.
Do pheromone traps help eliminate it?
They attract males only, and their main benefit is early warning of the moths' arrival and timing when to intensify scouting. Relying on them alone is not enough.