Identifying the Fall armyworm
The Fall armyworm’s life cycle moves from egg to caterpillar (larva) to pupa to moth. (See photo below)
Eggs are round, and change colour from green to light brown before hatching after 2-7 days. The female lays “egg masses” on a host plant, about 150-200 tiny eggs. They are found on the lower leaves and covered in a felt-like layer of grey-pink scales. Each female can lay more than 1000 eggs in her lifetime. (See photos 1 and 2 below.)
The caterpillars are the stage that causes damage to plants by feeding on soft plant tissues. Caterpillars have stripes down the length of their bodies and dark heads with a pale, upside-down Y-shaped marking on the front. They also have four dark dots on the eighth segment of their bodies. As they mature, caterpillars change from light green to dark brown. They are at their most damaging when they are 3-4 centimetres long. Caterpillars take 2-3 weeks to mature, and then change to pupa. (See photos 3-7 below.)
The pupa is shiny brown and is usually found underground. If the soil is too hard, larvae may web together leaf debris and other material to form a cocoon on the soil surface. The pupa spends 9-13 days inside a loose cocoon underground, then emerges from the cocoon as a moth. (See photo 8 below.)
Females are slightly bigger than males. The male forewing is mottled (light brown, grey, and straw-coloured), and the female has light colouring. Adults emerge at night, and females use the period before egg-laying to fly for many kilometres before they settle to lay eggs. On average, adults live for 12-14 days. Breeding can be continuous, with four to six generations per year. (See photo 9 below.)
Differentiating Fall armyworm from other armyworms
It is difficult to tell the difference between Fall armyworm and other armyworms in the field. But there are differences if you look closely. Check:
- Does it have a dark head with a pale, upside-down, Y-shaped marking on the front (see circle in diagram and photo 6 below?
- Does each of the body segments have a pattern of four raised spots when seen from above (see circle in diagram)?
- Does it have four dark spots that form a square on the second-to-last body segment (see circle in diagram and photo 5 below)?
- Is its skin smooth to the touch?
If the answer to these questions is yes, then it is a Fall armyworm caterpillar.
Symptoms of Fall armyworm feeding and damage
Fall armyworms start by eating plant leaves, then move to the growing points of the plant. During the day, small caterpillars hide in the joints between the leaves and the stem and whorls of the maize and move out during the night to feed on leaves. They may cut the stems of young plants.
As they develop, Fall armyworm move permanently into the whorl (see photo 13 below). This means that it is difficult to detect early infestations. On young maize plants, damage to the whorl can kill the growing points, which prevents any cobs from forming.
Feeding can cause the whorl and upper leaves to be a mass of holes, ragged edges, and excrement (called “frass.”) Deep feeding in the leaf whorl can destroy developing tassels. (See photos 14 and 16 below.)
Maize plants can recover from some leaf feeding by caterpillars, particularly when they are young, as long as the caterpillars do not attack the growing point of the plant.
When the plant is large, the Fall armyworm can enter the maize cob directly. The caterpillars usually burrow into the side of the cob (see photo 15 below), causing damage to grains which can lead to rot.
Fall armyworm infestation causes stunting and destruction of developing tassels and kernels, which reduces grain quality and yield (see photo 16 below).
How Fall armyworm spreads
- Adult moths can fly for long distances, hundreds of kilometres on the wind. This is often how they are introduced to a new area. Also, the large number of eggs they lay enables the pest to quickly establish itself in a new area.
- They are also spread by movement of infested plant materials. For example, in Kenya, transporting green maize for roasting is a popular business. If this maize is infested, it contributes to the spread of the pest.
- The expansion of maize monoculture across Africa may also help the pest to spread, and contribute to it affecting all farmers, both large-scale and small-scale.
Advice for farmers
Starting one week after maize germinates, farmers should monitor for the presence of the pest or symptoms of feeding.
- Cream-coloured or grey egg masses on the lower leaves, covered in a felt-like layer of grey-pink scales.
- Light green to dark brown larvae with three thin yellowish white stripes down the back and a distinct white inverted “Y” on the head.
- Larvae covered with yellowish brown frass inside the leaf whorl.
- Patches of small “window panes”—which is where the young caterpillars have chewed on one side of the leaf (see photo 12 in the linked document)—and large ragged and elongated holes in the leaves that emerge from the whorl.
- Monitor damage on 10 consecutive plants in 10 randomly selected sites, for a total of 100 plants.
- When your plants are in the first half of the vegetative phase of growth (the time between germination and flowering), use control practices only if at least 1 in 5 plants show signs of recent damage.
- If less than 1 in 5 plants show damage, the cost of using control products outweighs the economic benefit of reducing the pest population.
- When your plants are in the second half of the vegetative phase of growth, use control measures only if 2 in 5 plants show signs of recent damage.
- If less than 2 in 5 plants show damage at this stage, the cost of using control products is higher than the economic benefit of reducing the pest population.
Where possible, check with your local extension agent to confirm that these thresholds are correct for your location
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The above article was adapted from @Farm Radio International resource pack @CABI. All rights reserved