Ravenous armyworms wreak havoc

Armyworms attack field crops, especially maize, however, they can also attack pearl millet, sorghum, legumes and other crops.

12 February 2020 | Agriculture


Agronomic pests are among the most detrimental hindrances to crop farmers and can result in significant losses.

“Although pest damage differs in severity, the bottom line is that they reduce crop yield, exaggerate production costs and ultimately make the crop enterprise unrewarding,” said Emilie Abraham, a technical officer within Agribank’s Agri Advisory Services Division.

According to Abraham, it should be noted that armyworms are among the ‘hungriest’ pests that destroy crop fields and can drastically reduce yield per unit area, particularly if mitigation measures are not put in place in a timely manner.

Abraham said the name armyworm is derived from their invasive behaviour of attacking crop fields in groups. The Armyworm species belongs to the Noctuidae family along with other agronomic worm pests such as cutworms, bollworms and budworms. Common species include true armyworms (Pseudaletia unipuncta) that attack crops in the cold season (winter) and fall armyworms (Spodoptera frugiperda) that attack crops in autumn/summer.

According to Abraham, the Oshikoto, Oshana, Ohangwena and Zambezi regions are mostly affected by fall armyworms, as indicated by the agriculture ministry.

Armyworms attack field crops, especially maize, however, they can also attack pearl millet, sorghum, legumes and other crops.

“It should be noted that fall armyworm outbreaks frequently occur during periods of drought or after prolonged dry spells as most of their natural enemies (eg birds, ground beetles and rodents) which could eliminate or reduce their numbers are suppressed, leaving armyworms to populate,” said Abraham.

According to her, moths are responsible for laying eggs that hatch into larva/worms, thus, most people may observe the presence of numerous moth populations during December and January, particularly at night.

Female armyworm moths are ash-grey in colour, and several research efforts revealed that one female armyworm can lay egg masses ranging from about 1 500 to 2 000 at night, and usually place their eggs on plant foliage. Hatching takes place within two to four days, allowing them to populate in a very short period. Researchers have revealed that heavy rains can disrupt the life cycle of fall armyworms by washing eggs off leaves and onto the ground, said Abraham.

If the production cycle goes undisrupted, eggs can hatch into larvae, which can be described as light green to cream in colour with a dark head capsule. As the larvae continue to feed, they becomes darker with light-coloured lines down the sides of the body.

Abraham said farmers are advised to be vigilant and scout their fields frequently as small armyworms feed less. As they grow, their feed intake increases significantly to such an extent that the feeding patterns can easily be noticed.

“Farmers should not wait for extreme damage to occur. Close crop inspection is required to detect the pest on time.”

Abraham said this is to ensure that appropriate management decisions are embarked on to inform integrated pest management (IPM).

Moreover, it should be noted that the effective control of armyworms can be achieved when pesticide application is done promptly.

“It is imperative to scout in the morning and afternoon while pests are active and can be easily detected.”

Signs to be noted for during scouting include leaves that are chewed on the underside and larvae excrement that resembles dark grass seeds as well as dead, wilted and defoliated crops.

In most cases, insecticide control is warranted when an average of three or more worms per square foot occurs. Furthermore, the scouting date and the number of pests identified should be recorded, said Abraham.

According to her, IPM is key, however, to ensure its effectiveness, scouting and field assessment is a pre-requisite.

Physical methods include handpicking, crushing egg masses/caterpillars, the application of ash/sand and the digging of trenches.

However, it is observed that these control methods are uneconomical on a large scale and thus synthetic pesticides are required to control fall armyworms.

Abraham said pesticide application can be done systematically or on contact.

“When selecting synthetic remedies, farmers should take cognisance of factors such as local availability, cost, correct dosage and residual activity, taking into consideration the withdrawal period.”

In addition, agro-chemicals chosen should be among the chemicals approved by the agriculture ministry.

Pesticide choices can include lambda, cyperfos, impligo, belt and others and these can be solicited from local agriculture stores. It is also important to note that research has revealed that armyworms retaliate by becoming resistant to pesticide applications. Therefore, farmers should use and alternate at least two different pesticides to avoid resistance. “The application should be done after every seven days and spraying should be done early in the morning and late in the evening because the pest mostly feeds at night.”

Abraham said the use of pheromone mass traps can also be a good way to control the pests as it is proven to be environmentally friendly. “Irrespective of the method chosen, the efficacy heavily depends on proper timing.”

She added that the government continues to make efforts to assist particularly small-scale farmers to control armyworm pests when resources allow. “However, farmers who can afford to purchase the pesticides are urged to meet the government halfway.”

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