The ManicaPost
Samuel Kadungure Senior Reporter —
IF a farmer detects little green and brown caterpillars in the field, then their crop is under attack from the fall armyworm – a pet notorious for ravaging the maize crop and if left to multiply – can wipe the entire crop before marching to the next available source of food.
Fall armyworms are a caterpillar species that are uncommon in Zimbabwe, and their emergence this season took the nation by surprise, as unsuspecting farmers mistook them for the famous stalk borer.
The fall armyworm is a chronic pest that feeds on a variety of well-fertilised crops such as maize, pearl millet, sorghum and pastures and initially farmers applied the wrong chemicals, which failed to combat the worms that continued to multiply and defoliate crops.
Agronomist, Mr Evan William Craig, said: “Farmers used stalk borer chemicals to contain these worms without success and it was until a research was done that it emerged where we were missing it. The fall armyworm and the stalk borer have similar eating, damaging and destruction habits, though the fall armyworm can cause extensive destruction than the stalk borer.”
The fall armyworm eats from the ground and cause severe leaf damage. Their droppings (nhoko) are muddy.
“When they attack crops, the leaves are reduced to a lace, and this can lead to deterioration in yields as the leaf surface area of the crop is reduced,” said Mr Craig.
Leaves are the primary way plants interact with the atmosphere and take care of their basic needs. They enable photosynthesis – the process by which leaves absorb light and carbon dioxide to produce carbohydrate (food) for plants to grow – to occur.
The leaf is to absorb light and carbon dioxide to produce carbohydrates and the rate of photosynthesis is higher in plants with larger surface area as it allows for more plant pigments to absorb light energy, enabling a more productive photo-system stage in photosynthesis. If the leaf surface area is reduced, the plant’s capacity to manufacture food is also reduced, leading to a decline in yield.
“The surface area of the leaf determines the final potential yield of a plant and the more the fall armyworm damages the leaves, the more the potential yield is reduced. The other danger posed by the fall armyworm is that if it is not controlled on time, it can also destroy tussles, leaving no pollen grains to pollinate the plant,” said Mr Craig.
The fall armyworm is different from the gregarious African armyworm and is recognisable through its feeding waste and white laces on the crop leaves.
There are five species of armyworms – the fall armyworm, true armyworm, beet armyworm, southern armyworm and yellow-striped armyworm.
Armyworms get their name because they often move in large numbers and in waves, as they search for food.
Of these, the beet armyworm is the most devastating, attacking a wide range of crops and pastures. Fall armyworms consume crops and pastures and cause extensive damage through widespread stripping of leaf blades.
They eat everything in an area and once the food supply is exhausted, the entire “army” will move to the next available food source.
Acting Agritex Provincial Officer, Mrs Phillipa Rwambiwa, said control measures had so far failed to tame the pest as farmers are discovering it at a growth stage when it could no longer be controlled.
“Keeping a close eye on one’s crop is the key to early detection. Scouting is very important. If detected early, spot treatment is the management measure recommended to prevent a widespread infestation and reduce injury to the crop,” said Mrs Rwambiwa.
Fall armyworms are green, brown or black, with a distinct white line between the eyes that forms an inverted a Y pattern on the face. They are very small at first, causing little crop damage and as a result infestations often go unnoticed.
“When small, it is greenish with a brown head and dots along the body. When mature, it’s about one and half centimetres long and the quantity of leaves they eat increases dramatically. The tricky about controlling it is applying the right pesticide in right mixture as per instructions. Farmers should not add a lot of water to the chemical as this can cause resistance,” said Mr Craig.
Given their immense appetite, great numbers and marching ability, the worms can damage entire fields or pastures in a few days.
Research shows that the fall armyworms go through five to six larval stages within 14 to 21 days. Each larval stage is called an “instar” and their ability to cause damage increases with each instar stage (as they become larger). They soon develop the ability to chew large holes in the leaves and eventually can strip the entire plant of foliage. After the final instar, the armyworm larvae will move down to the soil, bury themselves and then pupate.
Within 14 days, they will emerge as adult moths and their lifecycle is normally 36 days.
Larvae will feed primarily during the cooler hours of the day or longer on cloudy days. During the daylight hours it is best to look under leaves or in shaded areas.
Small larvae begin feeding on the bottom blades of grass.
“The other important thing in controlling the fall armyworm is the chemical application method. Farmers should not just spray in a huff or randomly because they will miss the growing point or funnel in which the pest takes refuge in. The chemical must be sprayed slowly and the nozzle should be precisely targeted at the growing funnel. The knap sack nozzle should drop the chemical directly into the funnel. Some farmers are actually using plastic bottles to drip the chemical into the funnel,” said Mr Craig.
Mr Craig said there were adequate pesticides that can combat the pest that farmers can access from reputable suppliers countrywide.
Boom sprayers are effective when spraying chemicals which are absorbed in the plant. When the pest feeds on the leaves, it eats the poison and gets controlled.
Farmers with overhead irrigation can mix chemicals in irrigating water. As they irrigate for long hours with treated water drenches the plant.
