The damage to crops is principally through the feeding activity of the caterpillars. They bore into the stems or the base of the panicle and feed on the internal substance, blocking the transport of nutrients and water. Exit holes of the caterpillar can also be observed on stems and panicles. The lack of supplies causes wilt of the affected plant parts. When opened longitudinally, the stems show ‘dead hearts’ symptoms, with the presence of larvae and frass inside.
Symptoms are caused by the purple stem borer, Sesamia inferens. Larvae overwinter as pupae inside stems or plant debris in the soils and emerge as adults during the spring, when weather conditions are benign. The moths are small, stout and light brown with a hairy head and body. Forewings are straw colored with a marked golden margin. Hindwings are translucent white with yellowish veins. Females lay clusters of rounded, pale and yellowish green eggs in several rows behind the leaf sheaths to protect them from predators. Caterpillars are about 20 to 25 mm long, pinkish in color, with red-brown heads and no stripes. They bore into the stem and feed on the internal tissues.
Several parasitoid wasps of the groups Telenomeus and Trichogramma are depositing their eggs into the eggs of Sesamia inferens and help to control the population. For example, the release of egg parasitoid, Trichogramma chilonis (8 cards per hectare) at 12 and 22 days after germination. The larvae are also parasitized by the wasps Apanteles flavipes, Bracon chinensis and Sturmiopsis inferens. Finally, the pupae are attacked by species of Xanthopinpla and Tetrastichus. Bio-insecticides based on extracts of the fungus Beauveria bassiana and the bacteria Bacillus thuringiensis are also effective against purple stem borer.
Always consider an integrated approach with preventive measures together with biological control treatments if available. A prophylactic treatment with insecticides in the form of granules or sprays (with chlorantriniliprole, for example) can be applied to the foliage to control the pest population.