Fall armyworm in maize; Understanding In Detail
Fall armyworm is larvae stage of fall army worm moth. It is a migratory pest native to America and was first reported in Africa in 2016. The army worm causes damage by feeding on leaves and stems of maize, rice, sorghum, sugarcane, vegetable and cotton.
Fall army worm damages maize in all stages. In maize seedlings, it causes damage by feeding within the whorl. Larger larvae can cut the base of the plant. Mature plants suffer attack on reproductive structures. Symptoms of army worm damage in maize are a mass of holes on leaves and the whorl (funnel), ragged edges and larval frass.
Young larvae skeletonize the leaf lamina. Severe feeding damage to young plants can kill the growing point; causing ‘dead heart’ in maize. Maize cobs are attacked by larvae boring through the kernels. At high densities, large larvae may act as armyworms and disperse in swarms, but they often remain in the locality on wild grasses, if available.
Eggs are laid on the underside of lower leaves (less specific when FAW populations are large) and newly-hatched larvae feed on the leaves, removing green material and leaving skeletonised areas, small to medium hole and windows. If plants are infested very early in the vegetative stage, larvae may leave the plant and shelter in the soil at the base of the plant, where they can cause damage to the stem, or kill the young plant. These larvae may move up onto the plants at night and feed on foliage.
Established larvae prefer to hide in the whorl, or at least in sheltered sites on the plant (leaf axils, under wrapper leaves on the cob). Leaves damaged in the whorl reveal the extent of the feeding damage as they emerge and expand. Where damage is severe, the leaf may break off or be so damaged that no new leaves emerge from the whorl.
If medium-large larvae are in the whorl when the tassel is emerging, they can feed on the tassel. Severe and prolonged feeding on the tassel can result in significantly reduced tassel size.
What to look for on fall armyworm
Early detection is essential, particularly during crop emergence and establishment. Regularly check seedling crops for damage, eggs and caterpillars. Look on the underside of leaves for egg masses and small larvae and check the whorl for larger larvae. Destructive sampling is essential to adequately assess an infestation.
Record the number of eggs and larvae per plant. Recording the size of larvae is also useful in decision making. Where infestations are patchy (a cluster of damaged plants here and there), be careful to randomly select plants rather than focusing on the damaged areas.
Ensure you have correctly identified any larvae found as Helicoverpa armigera and native armyworm species may also be present in crops.
Management decisions cannot be made on the presence of damage alone. Leaf damage will be visible long after larvae have finished their development. The condition of newly emerged leaves, and the condition of the whorl (damaged/undamaged) are better indicators of an active infestation than damage to older, fully expanded leaves. Dissection of plants is essential to get an accurate estimate of the FAW infestation.
Plant death, caused by larvae burrowing down inside the stem from the whorl to the growing point, has not been observed in the field. Plant death has been observed in establishing crops (younger than 4-5 leaves) when large larvae feed at the base of seedlings whilst sheltering in the soil. This damage occurs when infestations have not been detected early enough post-emergence and large larvae eventuate.
Vegetative crops have the potential to compensate for defoliation, but just how much defoliation can be tolerated at different growth stages is the focus of current research into FAW management in maize. Persistent defoliation, and particularly feeding that damages the growing point of seedlings, can result in plant death. This type of damage is unlikely in crops older than 4-5 leaf stage, and more likely when FAW densities are high and repeated egg lays are occurring.
Feeding damage to tassels prior to emergence is not uncommon, and at high FAW densities, this feeding can reduce the size of the tassel. However, in almost all instances where this damage has been observed, sufficient pollen is still produced to effect pollination. FAW are more damaging to cobs than Helicoverpa.
In addition to burrowing in at the terminal end of the cob (silk end), FAW larvae will burrow through the side of the cob, increasing the amount of grain loss and increasing the risk of subsequent disease establishment and weather damage to the cobs. Assessing the risk of cob damage requires inspection under wrapper leaves to find larvae that may burrow into the cob when sufficiently large.
Management of fall armyworm
Fall armyworm: FAW is managed in maize when first noticed, usually when the plants are around knee high. Because the pest can be so devastating, some farmers reported treatment after seeing just one larva. Others said they wait to spray until most leaves are damaged. Busia extension workers reported around two of every five plants are found to be infested by FAW at this stage.
Farmers and extension workers reported multiple management strategies, none used alone. Cultural methods are used in combination with pesticides.
Cultural control and non-pesticide inputs:
- Farmers spray a combination of soap (“Omo”) and water using knapsack sprayers; they reported no efficacy.
- Farmers apply ash to the whorl (2kg/ha, 2 days in a row), which has farmer-reported efficacy
against other insects including aphids; some farmers reported efficacy against FAW, including more vigorous regrowth without FAW damage.
- Rain is known to suppress FAW.
- Pepper extract is used but was reported to have no efficacy.
- Soil paste is used but is not considered effective.
- Chickens are brought to fields to eat FAW.
- Cooking oil is used to attracts ants which are thought to kill FAW.
Biological control and biopesticides:
- None reported.
Chemical control: Up to 60% of farmers are commonly treating FAW with pesticides. Anywhere from one to three treatments might be applied. Most farmers reported two sprays, 14 days apart, regardless of the product.
- Alibas (alpha-cypermethrin): this pesticide is not known (and was not reported) to have efficacy against FAW.
- Belt (flubendiamide): used and reported as efficacious; not widely available; very expensive.
- Coragen (chlorantraniliprole): used and reported as efficacious; not available.
- Duduthrin (lambda-cyhalothrin): this product is widely used, with no efficacy reported. It is commonly mixed in an open bucket and splashed onto plants with brooms. The use of rags or cloths for application was also reported.
- Escort (emamectin benzoate): used and reported as efficacious; not widely available.
- Furadan/Rocket (carbofuran): reported as used; illegal in Kenya.
- Match (lufenuron): used and considered effective, but not widely available
- Orthene/Ortran (acephate): used and considered effective.
- Vantex (gamma-cyhalothrin): not widely used
Fall Armyworm Pest What to do.
- Preventive measure: Intercropping maize with drought-resistant greenleaf desmodium and planting Brachiaria grass on the farm’s edge helps curb fall armyworms see under Push-pull
- Deep ploughing to expose larva to sun and to predators
- Weeding to enhance crop vigour
- Plant varieties with hard husk cover
- Biological control – use of parasitic Hymenoptera to act as larval parasitoids
- Scientists have already confirmed the effectiveness of using ash and chilli powder to control the fall armyworms in maize: Here is how farmers can do it: Buy ripe chilli powder (pepper) from the market or prepare your own using ripe pepper. Dry the pepper and make powder by either grinding or pounding, remove the big particles and leave the fine powder. Sieve cold wood ash from the fireplace.Get 1 gorogoro (2kg tin or plastic) of ash. Mix 1 gorogoro of wood ash with 5 teaspoonfuls of chilli powder. Mix the chilli and wood ash properly by shaking them in a container. Put the mixture in a used pesticide container that has small holes. Apply the mixture from the container by shaking it once into each plant funnel. For good results, apply the mixture immediately you see the worms in the maize and repeat the same if you notice any pests in the maize or pest damage to your crop.
- Spray plant extracts.