· Millet · Sorghum · Maize
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.
Adult moths are light brown and have a wingspan of 20 to 25 mm. 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, thereby 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.
Parasitoid wasps Cotesia sesamiae and Cotesia flavipes 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. Finally, plants such as molasses grass (Melinis minutiflora) or greenleaf desmodium (Desmodium intortum) produce volatile agents that repel the moths. Preparations based on Bacillus thuringiensis, neem oil extracts or Beauvaria bassiana have also been used to control the pest.
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 applied in granular form in the whorl can provide control against spotted sorghum stem borers.
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