Although over the years measures have been taken to contain fall army worms in Zambia, these pests remain a high priority. When fall army worms move in, they can cause destruction to maize fields, leading to severe financial losses for farmers. In this article we discuss everything you need to know about the fall army worm and how to eradicate them.
Fall army worms, also known as Spodoptera frugiperda, are destructive caterpillars known for their large-scale invasive behaviour. They often appear in the late summer and early fall. They are native to the tropical and subtropical regions of the Americas, particularly South America. However, these caterpillars have migrated into parts of Africa and Asia, and can be found in Zambia as well.
They first arrived in Africa in 2016 and were detected in Zambia in December that same year. These caterpillars quickly spread to cover ten provinces by January 2018. An adult fall army worm moth can travel up to 160 km in one night, and this has contributed to their rapid spread.
 Eggs of the fall army worm, Spodoptera frugiperda (JE Smith), hatching. Photo: James Castner, University of Florida. |
 Egg mass of the fall armyworm, Spodoptera frugiperda (JE Smith). Photo: James Castner, University of Florida. |
 Newly hatched larva of the fall armyworm, Spodoptera frugiperda (JE Smith). Photograph by Lyle J. Buss, University of Florida. |
 This image shows what the fall armyworm larva looks like when it is mature. Photo: James Castner, University of Florida. |
 Head capsule of fall armyworm, showing light-coloured inverted “Y” on front of head. Photo: Lyle J Buss, University of Florida. |
Fall army worms?
They are green to nearly black worms with stripes and can grow to be 3,5 cm long. They have a distinct Y on their heads and are most active in the early morning and late afternoon when it is cooler. In the heat of the day, they tend to hide in the cold soil.
Their lifecycle is about 30 days in the summer, 60 days in spring and autumn, and 90 days during the winter. Their eggs are dome-shaped and measure 0,4 mm in diameter and 0,3 mm in height. Although the number of eggs they lay vary, they typically lay 100 to 200 eggs at a time and the total egg production per female averages about 1 500, but can be as many as 2 000. The female also deposits a layer of greyish scales over the egg mass and between the eggs. This gives it a furry or mouldy appearance. The eggs hatch after only one to two days during summer.
Fall army worm larvae start off as green and will turn a brownish colour with lateral white lines, while the head is a red brownish colour. Dark spots on the body also form over time, and the head of the adult fall army worm has a white inverted Y. This stage of their life cycle lasts about 14 days in the summer and 30 days in the winter. Pupation normally takes place in the soil at about two to eight centimetres deep.
The duration of the pupae stage is about eight to nine days in the summer, but can reach 20 to 30 days in the winter. The fall army worm moths have a wingspan of about 40 mm. The adults are nocturnal and are most active during warm humid evenings. The female normally deposits her eggs within four to five days of her life, and the duration of the adult’s lifespan is about 10 to 21 days.
 Typical adult male fall armyworm. Photograph by Lyle J Buss, University of Florida. |
 Typical adult female fall armyworm. Photograph by Lyle J Buss, University of Florida. |
Plants and destruction
This species most frequently consumes maize, sorghum, Bermuda grass and grass weeds. The larvae can defoliate plants and spread quickly, thus the name army worm. Young larvae consume leaf tissue from one side leaving the epidermal layer intact. As they grow, larvae begin to make holes in the leaf and eat it from the edges inwards.
Older larvae can cause extensive defoliation, often leaving only the ribs and stalks of maize plants, or a ragged and torn appearance. Larvae also will burrow into the bud destroying the growth potential of plants, or clipping the leaves. In maize, they sometimes burrow into the ear, feeding on kernels in the same manner as maize earworm, Helicoverpa zea.
Maize leaf damage caused by the fall armyworm. Photograph by Paul Choate, University of Florida.
Eradicating fall army worms
The first step to eradicating these insects is to keep a close eye for any sign of damage such as brown patches on your lawn or chewed leaves on your plants. Also look for the presence of caterpillars that resemble their appearance. As soon as you spot any of these signs or pests, you must act quickly.
Take immediate action to prevent further damage. Apply insecticides containing carbaryl or bifenthrin, as these are most effective against fall army worms. To maximise the effectiveness, spray insecticide in late afternoon or early evening when these worms are most active. Crop rotation also disrupts their life cycle and intercropping can reduce the likelihood of severe infestation. Encourage natural enemies like wasps, birds and beetles, or plant genetically modified crops that are resistant to fall army worms. Combining multiple strategies will provide you with more sustainable and effective management.
Reference: https://entnemdept.ufl.edu/creatures/field/fall_armyworm.htm
