Pests and Disease: How to Control Armyworms in Maize

Tuesday, January 17, 2017


There are widespread media reports that the armyworm is threatening this season's maize production in many maize growing regions of Zimbabwe. The government has disbursed insecticides to some areas to prevent widespread crop failure. The same pest has already caused a national crisis in Zambia. We have received requests from emerging farmers asking for information on how to handle the armyworms in their maize. Instead of responding individually we thought it would be helpful to share information broadly here to help reduce major crop losses from our country's most important food crop.

Let's begin.

What is the Fall Armyworm (FAW)?

The armyworm is a destructive crop pest. The moth hatches a caterpillar which feeds on cereals such as maize. The caterpillars are about 3-4 cm long and have a velvety black shiny head with an inverted V-shaped mark on the head and white stripes along their sides and the back. They are called armyworms because they march in large numbers from the grass onto crops and can destroy entire fields of cereal crops such as maize, sorghum, and millet.

The other English name for the armyworm is the black worm.The local language names for the armyworm are imhogoyi or mhundururu.


Symptoms
  • Holes in the maize leaves and sometimes only the midribs of the maize plant are left in the more severe cases.
  • Outbreaks mostly occur after the first rains when grasses have started growing which armyworms need to multiply.

Prevention

" An ounce of prevention is worth a pound of a cure."

  • Use resistant seeds: choose and plant disease resistant maize cultivars.
  • Planting time: plant early, late planted maize suffers from severe damage from second generation armyworms.
  • Weed early and often: but don't dig around the roots. Grassy fields are food for young caterpillars.
  • Companion planting: keep flowering broad-leaf plants around the fields as they host natural enemies of pests such as armyworms.
  • Sanitise: tools with bleach after use; remove and wash clothing to avoid spreading pests and disease.
  • Scout: your fields regularly especially in the early morning or late evenings when armyworm moths feed and are active. 
  • Prepare your land well: to expose and destroy armyworms.
  • Crop rotation: a 3-to 5-year rotation is best.
  • Don't burn grasslands: or you will destroy the natural habitats of armyworms.
  • Crop cleanup: at the end of the growing season remove plant debris. Keep your fields clean using cover crops.

Solutions:

When prevention fails and armyworms threaten to remove more than 20% of the leaves on your maize, you need to use biological or chemical controls to stop them.  Quick intervention is important to prevent widespread crop losses.

Biological and Physical Controls
  • Use insect traps: such as pheromone traps for early detection, and to attract insects. Check your traps weekly to monitor insect levels and begin to control them. 
  • Hand pick or gather the caterpillars in a container such as a jar. Fill the jar with very hot water or alcohol and set in the sun until the bugs die. This is feasible for small farms or small-scale production. 
  • Dig a 30cm deep trench around the maize field to trap marching caterpillars and destroy them by crushing or burying them.
  • Sanitise: break of affected plants, and remove and destroy diseased leaves. Do not compost them, dispose of them in the garbage. Practice cleanliness in your fields such as using clean tools and washing your hands. 
  • Use floating row covers. Drape the lightweight, white fabric over plants to prevent moths laying eggs and to ward off insects. 
  • Use light horticultural oil spray during the growing season to control armyworms. You can add it to BT for improved effectiveness.
  • Attract or buy beneficial insects (predatory and parasitic insects). Some natural predators of armyworms are praying mantis, lacewings, parasitic wasps, flies, spiders, bats and birds. Grow plants in the carrot family ( herbs, dill, coriander) to attract beneficial bugs. 
  • Apply biopesticides (plant-derived) such as pyrethrin, neem or BT. Biopesticides are less toxic to humans and birds. 
    • Pyrethrin is a natural pesticide made from dried flowers that are ground into a powder, mixed with water and sprayed onto plants. Apply it directly to the armyworms, spraying the undersides of the maize plant leaves as well as the tops. Pyrethrin paralyses the armyworm on contact.
    • B.T. (Bacillus thuringiensis) gives excellent control of leaf-eating caterpillars like armyworms. It paralyses the armyworms' digestive system. Spray it to cover all plant surfaces entirely especially the undersides. Apply it to the infested plants every 10 to 14 days until pest is removed. Ask for locally available commercial formulations of Bt strains.
    • Use neem leaves. Neem is the extract of a tree native to Africa and Asia that controls most pests.  Crush 5 litres of mature neem leaves and soak them overnight, apply neem liquid as a spray or drench it on to young caterpillars. Neem restricts the appetite and growth of the armyworms.  

Chemical Controls

Chemicals can be effective when applied correctly, but always try to use least toxic methods first.

ZFC has a list of recommended chemicals for armyworms registered in Zimbabwe on their Facebook page. You can see it here. It is however not currently updated. Consult with the Ministry of Agriculture/Agritex for an updated list of registered pesticides.

Regionally recommended chemical controls:
  • Spray chlorpyrifos based products (organophosphates) such as Dursban, Chlorban and  Chlorpyrifos 500EC onto the caterpillars.
  • Spray deltamethrin-based products ( pyrethroid) such as Deltanex EC, Decis 50 EC and Decis Forte.

5 Rules for Spraying Chemicals:

  • Use the least toxic product first
  • Keep chemicals out of the reach of children
  • Read the product label and use product only on the plant, or plant type for which it is labelled
  • Follow instructions on the label
  • Wear protective gear: goggles respirator mask, gloves and long-sleeved shirts
Further reading

Keep track of armyworm forecasts on the Armyworm network page here.

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