African armyworm

Introduction

The African armyworm is a migratory moth, the larvae (caterpillars) of which are important pests of pastures and cereal crops, predominantly in Africa south of the Sahara, Yemen, and certain countries of the Pacific region.

Normally, only small numbers of this pest occur, usually on pastures. However, periodically the populations increase dramatically and mass migration of moths occur, covering many thousands of square kilometres and traversing international boundaries. They travel from field to field in great numbers, hence the name "armyworm". 

Outbreaks follow the onset of wet seasons when dry grasslands produce new growth and cereal crops are planted. The severity and extent of outbreaks are increased by extended drought followed by early season rainstorms, which concentrate egg-laying moths and provide flushes of new grass as food for newly hatched caterpillars, and dry and sunny periods during the caterpillar development, which promote survival and rapid development. Therefore, major upsurges occur in seasons of sporadic rainstorms and long sunny periods throughout the outbreak period.

Caterpillars are major pests in outbreak years, causing significant losses on a local, national and regional scale. During outbreaks, caterpillars occur in such high numbers that they have to travel in masses from one field to another in search of food to complete their development, devastating crops as they move. Significant losses are most consistently reported from eastern and southern Africa. However, in recent decades, the frequency of reports from West Africa has increased, possibly due to the extension of suitable grassland habitats following forest and bush clearance for agriculture. 

The economic importance of the African armyworm is due to its rapid development (short life cycle), high reproductive capacity, and mobility by migration. Moreover, there is little time to react as infestations frequently go unnoticed, since young caterpillars are difficult to detect. When caterpillars become conspicuous (at the fourth instar), they cause a lot of damage in a very short time. 

Damage

The degree of damage to crops depends on: 

• Stage of development of the crop

• Prevailing weather conditions

• Density of caterpillars present and area affected

In areas with high and localised rainfall during armyworm seasons, plants can regenerate provided the growing tips are not damaged, with little or no yield reduction. In contrast, in areas of erratic rainfall, farmers may lose their crop completely. The degree of damage by armyworms varies from year to year. In East Africa, a severe outbreak can cover several square kilometres at very high densities, while in non-outbreak years, caterpillar density is often low, and the size of attacked areas is negligible. The first armyworm outbreaks appear in Tanzania and Kenya, and are serious in nine years out of ten, causing up to 90% losses of crops and pasture in bad years. They covered 157,000 hectares of crops and pasture in 2001. In major outbreak years the moths subsequently migrate to cause extensive damage in Uganda, Ethiopia, Somalia and Eritrea and may travel as far as Yemen, or south to Malawi, Mozambique and Zambia.

Grasslands, even maturing crops can be totally destroyed. If drought conditions follow an outbreak, plants may not recover from defoliation and replanting may fail to produce a crop.

Damage to cereal crops results principally from attack on young plants by young caterpillars hatching or dispersing into the crop from adjacent wild grasses. Weed-free maize crops taller than 50 cm are unlikely to become infested by newly hatched caterpillars because the leaves are too tough to allow them to establish. However, when older caterpillars moving from heavily infested grasslands invade fields, even maturing crops can be totally destroyed. Reported yield losses caused by defoliation in maize range from nine percent in plants attacked at the early whorl (four leaves) stage to 100% in those damaged at the pre-tassel stage. The ability of young maize plants to recover from armyworm damage depends on the position of the growing point at the time of attack and the amount of root development when the caterpillars stop feeding. Damage is always serious if the growing point is affected but, as it remains at the base of the plant until near to the pre-tassel stage, it may be below ground during the outbreak and remain undamaged.

Replanting maize after armyworms have destroyed the first-sown plants is frequently unsatisfactory, as the optimum planting dates will have been missed. Yield losses of 6% have been estimated for each day's delay after the optimum planting date in high-rainfall areas in Kenya. Late planting may result in much higher losses in areas with less rainfall; yield losses of up to 92% have been recorded in such areas in Malawi and Kenya. If drought conditions follow an outbreak, plants may not recover from defoliation and replanting may fail to produce a crop.

In sorghum, millet, rice and teff, armyworm damage may stimulate growth of tillers (lateral shoots on or just under the surface of the ground), which can increase yield in favourable conditions. If subsequent rainfall is adequate for crop growth and development, yield losses may be limited, providing the damage occurs before the critical grain-initiation stage has been reached.

Damage to pasture and rangeland can be extensive and severe. Good rainfall after infestation is an important factor in pasture recovery. In Kenya, vegetation changes in infested pastures have been reported to persist for many years before good grass cover has been restored by management of dicotyledonous weeds.

Indirect losses to livestock due to armyworm outbreaks in pastures are sometimes severe, due to a combination of starvation and poisoning. The latter occurs when cattle graze on pastures recently infested with armyworm. Deaths among cattle grazing recently infested pasture have been reported by herdsmen in southern Ethiopia, Somalia (where 100 cattle were reported to have died on one occasion), and Kenya, as well as in southern Africa. Speculations as to the causes of death include high cyanide levels induced in Cynodon grasses by armyworm damage, and ingestion of caterpillars or fungal mycotoxins on armyworm faeces.

Host Range

Symptoms

The symptom of African armyworm attack is gross feeding damage to foliage, growing points and young stems. Young caterpillars scrape out the tissue of one side of the leaves creating a 'window' effect; leaves may dry up and assume a scorched appearance. Older caterpillars feed on leaves starting at the margins and moving inwards, leaving the leaves with a ragged appearance. They may eat whole leaves leaving only the midrib. Severe infestation results in total defoliation or destruction of the plant to ground level. Older caterpillars drop to the ground if disturbed. With large populations the ground may be literally covered with the gregarious band of caterpillars. 

Affected Plant Stages

Flowering stage, fruiting stage, seedling stage and vegetative growing stage.  

Affected Plant Parts

Growing points, leaves and stems. 

Symptoms by affected plant part

Growing points: external feeding. Leaves: external feeding, windowing, ragged leaves. Stems: external feeding. 

African armyworm lifecycle

A related species, Spodoptera exigua (Lesser armyworm or beet armyworm) feeds not only on Gramineae but also on many other crops, including cotton, tobacco, tomato, groundnut and beans. Unlike S. exempta, this species does not migrate over long distances, but the caterpillars are gregarious and move in swarms. They are about 1.2 cm long with a wingspan of 2.5 cm. They are light grey with a small, round, light orange spot and a small, kidney-shaped spot on the forewing. Newly hatched caterpillars are light green with black heads. The fully grown caterpillars are about 3 cm long, light green to dark brown with conspicuous stripes along the sides of the body.

Figure 1: Adult moth

Ⓒ A.M. Varela, icipe

Figure 2: Egg mass on upper leaf surface: note eggs under characteristic woolly cover

Ⓒ A.M. Varela, icipe

Figure 3: Eggs at 'pinhead stage' (about to hatch).

Ⓒ A.M. Varela, icipe

Figure 4: First instar larvae

Ⓒ A.M. Varela, icipe

Figure 5: Advanced larval stage.

Ⓒ A.M. Varela, icipe

Figure 6: Fully grown larva.

Ⓒ A.M. Varela, icipe

Figure 7: Pupal cell in soil: note pupa inside the cell. Close-up of pupa (inset).

Ⓒ A.M. Varela, icipe

A female moth (Figure 1) can lay 500 - 600 eggs, in clusters of 50 - 150, covered with scales and hairs, which gives them a woolly appearance (Figure 2). Eggs are greenish cream coloured when freshly laid turning dark in colour before hatching (Figure 3). The eggs hatch after 2-4 days depending on temperature. Newly hatched caterpillars are about 1 mm long, light green in colour with black heads (Figure 4). Young larvae feed on the underside of leaves, causing characteristic 'windowing' (Figure 5). The fully-grown caterpillar is about 30 mm long, light green with conspicuous stripes along the sides of the body (Figure 6). The average larval period is about 11 days. The larvae pupate in the soil (Figure 7) and the adult moth emerges after about six days.

Pest and disease Management: General illustration of the concept of infonet-biovision

Cultural practices

Monitoring

To monitor the presence of armyworm, conduct visual inspection by going around all your fields. Armyworms feed at night and hide under debris during the day. Solitary forms are usually sparsely distributed and difficult to find. Consequently, armyworms are not usually noticed until severe damage occurs. 

However, they can be monitored in late evening or early morning as they may still be actively feeding. Some caterpillars may be seen feeding on overcast days, especially during a severe outbreak

• Hand-pick the caterpillars and feed these to chickens and ducks

Regular monitoring is vital for timely action. Look in field margins, low areas where plants have lodged, beneath plant debris around the base of plants, on the ground, and underneath the plant leaves.
If conditions are known to be favourable to the pest, a close watch daily should be kept on grassland and young cereal crops. The earlier the infestation is noticed, the more effectively can control methods be carried out.

A recommendation for monitoring armyworms is to examine 100 plants at random by examining 20 plants from five locations.

Tentative nominal action thresholds for control measures have been determined for maize. To avoid yield losses of over 15%, action thresholds for early whorl maize should be taken as 200 second (L2), 80 third (L3), or 20 fourth (L4) instar caterpillars per 100 plants. Serious damage develops rapidly once caterpillars reach the L4 stage (CABI, 2000). As a general rule, control measures for the protection of pasture are not recommended unless caterpillar densities exceed 10/m² (CABI, 2000).

 Control of armyworms is a large-scale venture and requires international collaboration. It usually involves early warning based on light traps or pheromone catches, or forecasts based on prevailing meteorological conditions. Accurate monitoring and prompt reporting of armyworm outbreaks is essential for forecasting and control.

 A forecasting system for armyworms has been in operation in East Africa since 1969. National crop protection services have departments with special responsibility for control of migrant pests, including armyworm, such as the Plant Protection Services in Tanzania and the Crop Protection Branch in Kenya. In Tanzania, the National Armyworm Control Programme based at Tengeru-Arusha, runs a network of traps distributed throughout the country. Farmers are advised to inspect their fields for signs of infestation immediately the forecast warns of expected outbreak in the area. Recently, a community-based monitoring system has been implemented successfully in several high-risk districts, where armyworm forecasters have been trained to monitor male moth numbers through the use of pheromone traps (Mushobozi et al., 2005).

In Kenya, radio dissemination is provided by Kenya Agricultural Information and Resource Centre. The Desert Locust Control Organization for Eastern Africa (DLCO-EA) and the International Red Locust Control Organization for Central and Southern Africa (IRLCO-CSA) have regional responsibilities for armyworm.

Armyworm attacks are notifiable in the region. This means that if anyone spots armyworms, it should be reported immediately to the authorities (Ministry of Agriculture/ National Crop Protection Services), which will then send an eradication team, depending on the severity of the outbreak.

Listen to radio announcements to prepare yourself for armyworm outbreaks.

Field sanitation

Cut grass weeds from bordering fields. Remove weeds regularly to reduce breeding sites and shelter for armyworm. However, if fields do become infested leaving grass weeds until the caterpillar have pupated or been controlled may help to reduce damage to the crop, since caterpillars may feed on weeds. Remove all plant debris after harvesting.  

Variety selection

Some maize varieties are more susceptible to attack than others, e.g. Katumani, a dryland variety grown widely in Eastern Kenya. These varieties are most at risk where probabilities of armyworm infestation are high. 

Tillage

Plough and harrow field thoroughly. Turning the soil exposes armyworm pupae to desiccation and natural enemies.  

Habitat Management

Biological pest control

Natural enemies

Natural enemies should be encouraged by maintaining natural surroundings with plenty of breeding places for them, including trees and shrubs.

Many birds, toads, lizards, small mammals, insects and spiders prey on the African armyworm at different stages of its life cycle:

• Lacewings, predatory wasps, parasitic wasps, flies, and spiders attack armyworm caterpillars.

• Night birds and bats feed on the African armyworm moths.

• Birds (storks and crows) may decimate small outbreaks but have little influence on larger ones.

Viruses and fungi

Biopesticides and physical methods

Biopesticides

Biopesticides (including botanicals/plant extracts and microbials) such as Neem, Pyrethrum and Bt should be applied if larvae are at or above threshold levels and preferably when caterpillars are approximately 12 to 20 mm long, namely before most damage has occurred. Once caterpillars are mature, that means they are 30 to 35 mm long, they will have done most of their feeding damage and it would no longer be economical to apply a biopesticide. In Namibia, the quoted threshold is 25 armyworms per trap. Traps are checked weekly (Namibian Crop Pests No. 28).

Biopesticides should be applied in the evening since armyworms prefer to feed at night.

Precautions: It is important to follow all precautions and directions listed it on the label when using a commercial biopesticide (or a pesticide) and ensure that the product is registered for armyworms on the specified crop. Pay particular attention to the required water volume to be used. Best control is achieved when using the highest water volumes. This applies to pesticide and bio-pesticide use for all pests/crops. 

Neem

Trials carried out in Tanzania showed that both neem seed and leaf extracts could be used to kill armyworms. Even though neem extracts are as effective as SpexNPV and synthetic pesticides, their use is only practicable in small holdings.The high bulk of neem needed and high transport costs means it is not feasible to use it on a large scale.

How to prepare neem solution:

Fallen neem fruits are collected from underneath the trees. The flesh is removed from the seeds and any remaining shreds washed away. The seed is carefully dried in airy conditions (in sacks or baskets) to avoid formation of mould. When needed, the seeds are shelled, finely grated, and then soaked overnight in a cloth suspended in a barrel of water. Dosage: 50g of neem powder per litre of water. This solution is then sprayed on infested plants.

• Grind 500 grams (g) of neem seed kernels in a mill or pound in a mortar.
• Mix crushed neem seed with 10 litres of water. It is necessary to use a lot of water because the active ingredients do not dissolve easily. Stir the mixture well.
• Leave to stand for at least 5 hours in a shady area.
• Spray the neem water directly onto the crop using a sprayer or straw brush.

About 20 to 30 kg of neem seed (an average yield from 2 trees), prepared as neem water can treat one hectare of crop.Neem water can be stored and will remain effective for 3 to 6 days if it is kept in the dark.

For more information on neem click here  

Pyrethrum

Recipes for pyrethrum in pest control

• Pick the flowers on a warm day when they are fully open.
• Pile them up into small heaps in the sun to warm through.
• Then spread them out to dry on thick mats in a shady area.
• If they are to be stored, they need to be kept in an airtight container in the dark. Light reduces the effectiveness of the flowers.

Pyrethrum powder:Grind flowers to a dust. Use pure or mix with a carrier like talc or lime. Sprinkle over infested plants.Pyrethrum liquid:Mix 20g pyrethrum powder with 10 litres water. Soap can be added to make the substance more effective but it is not vital. Apply immediately as a spray.

For more information on pyrethrum click here.  

Bt (Bacillus thuringiensis)

Commercial formulations of several Bt strains are available in East Africa. These include Dipel® , Thuricide® and Xentari® . Use an application rate indicated on the product label. How to use Bt:

• Spray thoroughly, covering all the plant surfaces.
• Apply when larvae are less than 5 mm long or when the eggs begin to hatch. Bt works best on young larvae.
• In the hot tropics, it is more effective to spray Bt in the late afternoon as there are longer and cooler hours ahead. This enables Bt to remain longer surfaces of the leaves. Bt survives better in cooler temperature. Whereas, spraying in the morning provides a shorter and hotter environment.
• Do not mix the Bt concentrate with alkaline water (pH 8 or higher). Alkalinity reduces its effectiveness. To make the water acidic, add a few tablespoons of white vinegar in a gallon of water before adding Bt.

For more information on Bt click here.

Physical methods

There are different physical methods mainly practicable in small holdings:

• Plough a deep ditch. Keep it filled with water. This method is helpful, when caterpillars are found to be moving towards your field from the adjacent fields.
• Another method is to dig a deep ditch with vertical sides to trap the caterpillars and prevent them from crawling out. Dig a hole, a diameter of a fence post, in every 10 meters within the ditch. Caterpillars are lured to congregate in the holes. Collect and properly dispose the trapped caterpillars
• Make pitfall traps - see image below
• Use light traps. They can provide useful information about the population of moths and therefore of caterpillars. Light traps help to predict if there is going to be an outbreak. However, light traps attract many other insects, including other moths. Therefore, it is very important to be able to recognise moths of the African armyworm. Use of light traps is primarily a tool in monitoring. In addition, a wooden tripod with a kerosene lantern is a ''light trap'' locally improvised.
• A tripod made of wooden poles (bamboo) is constructed with a lantern (kerosene) hanging in the middle over a bowl of water. The lantern is a fire hazard so the tripod must be secure, and the lamp must be hung so that the wood does not catch fire.
• Hand picking of caterpillars. This is only practicable in very small plots

For more information on traps click here