- Sorghum

Sorghum Sorghum

Spotted Stemborer

Insect

Chilo partellus


In a Nutshell

  • Irregular feeding scars, holes and windows on leaves.
  • Affected stems have "dead heart" symptom.
  • Stunted growth.
  • Light brown moths with similar-colored forewings, hindwings are white.

Symptoms

Young caterpillars of spotted stem borer feed on tender tissues of the plants. They bore tunnels into the leaves and whorls, leaving irregular scars, holes and windows. The older larvae tunnel the stems and feed on the internal tissues, hindering the transport of water and nutrients. This feeding activity leads to the so-called 'dead heart' symptom where the stem is hollow and only caterpillars and their frass can be observed inside. The upper part of the plants dries up partly or entirely. Early attacked plants are stunted in growth and may lodge. Older caterpillars also tunnel extensively in cobs. Overall, the feeding activity increases the incidence and severity of fungal or bacterial diseases.

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Hosts

Trigger

Adult moths are light brown and have a wingspan of 20-25mm. The forewings are light brown with some darker patterns while the hindwings are white. Adults are active at night and rest on plants and plant debris during the day. Females lay creamy white eggs in batches of 10-80 eggs onto the surface of the leaf. Caterpillars have a reddish-brown head and a light brown body with darker stripes running longitudinally and dark spots along the back, hence the name. The range of host plants is wide and includes sorghum, millet and maize. Climate conditions significantly impact the moths' life cycle. Warm and relatively humid conditions are particularly favorable. The pest most commonly occurs in hot lowland areas, and seldom above altitudes of 1500 m.

Organic Control

Preparations based on Bacillus thuringiensis, neem oil extracts or Beauvaria bassiana have been used to control the pest. Plants such as molasses grass (Melinis minutiflora) or greenleaf desmodium (Desmodium intortum) produce volatile agents that repel the moths. Parasitoid wasps Cotesia sesamiae, C. flavipes and Trichogramma chilonis lay eggs into the larvae of spotted stemborer. Another wasp, Xanthopimpla stemmator, attacks the pest when it is in the pupal stage. Natural predators include earwigs and ants. They provide effective population control.

Chemical Control

Always consider an integrated approach with preventive measures together with biological treatments. Pesticides treatments should be weighted against the potential yield loss and the damage to the biodiversity of the area. Insecticides based on deltamethrin or chlorantriniliprole applied in granular form in the whorl can provide control against spotted maize stem borers.

Preventive Measures

  • Grow tolerant plants, if available locally.
  • Monitor the field regularly to spot feeding damage.
  • Intercrop with legumes, or cowpea and molasses grass (Melinis minutiflora) to repel the spotted stem borer caterpillars.
  • Maintain an optimum plant density to deter the insect.
  • Sow 2-3 rows of trap crops on all sides.
  • Use plant or pheromone traps around the field.
  • Remove plants with signs of infection early.
  • Check for volunteer plants and alternative hosts and remove them.
  • Plant either earlier or later to avoid peak populations of the insect and peak feeding activity.
  • Ensure good fertilization to get vigorous plants but avoid excessive use of nitrogen that increases pest attack.
  • Implement wide crop rotation with non-host plants (for example cassava).
  • Remove and destroy all crop residues after harvest.

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