Common names: Spotted sorghum stem borer, Spotted stem borer, Maize stalk borer, Spotted stalk borer, Durra stalk borer, Pink borer

Latin names: Chilo partellus, Chilo zonellus, Argyria lutulentalis, Crambus zonellus

Life cycle and description

• Adult stage: Adult moths are straw-colored and small (approx. 15 mm long). Hind wings are white, while forewings are straw-colored with darker scale patterns forming longitudinal stripes close to the wing margins. Adult moths rest with wings folded over the abdomen. The adult stage lasts from 2 to 12 days. Adult females can lay between 200 and 600 eggs during a lifetime, in separate batches of 10-80 eggs.
• Egg stage: White, scale-like and 1.5 mm wide. Typically they are deposited in overlapping clusters on the underside of leaves close to the midribs, with an incubation period of 4-5 days in the warm season.
• Larval stage: Immature larvae are yellowish, spotted, and 1-2 mm in length. Mature larvae are crystalline and spotted with black warts on each body segment. They have four purple/brown longitudinal stripes on the back of the body, 20-25 mm in length, and have a prominent reddish-brown head. Larvael stages last 14 to 28 days.
• Pupate stage: Pupae are light yellow/brown to dark red/brown. Pupae have belts of small spines on the dorsal anterior margins from the fifth to seventh abdominal segments, six dorsal spines, and two large flattened ventral spines on the last abdominal segment. Pupation occurs within the tunneled stem. A thin exterior wall window is left at the end of the tunnel to enable adults to emerge. Pupae are about 15 mm long.
• In Africa, the spotted stem borer is not considered as serious as the African maize stem borer, pink stem borer, and African sugarcane borer.
• The lifecycle can be completed in 30 to 35 days.
• Spotted sorghum stem borers are considered particularly destructive in India, where 5 to 6 overlapping generations can occur during the year without diapause.
• In regions with a winter or dry season, diapause occurs in the larval stage in the stems or crop stubble/residue. Diapause may continue for up to 6 months, following which pupation occurs.

Confirmation

Spotted sorghum stem borers can be distinguished from other species of Chilo by examination of the male and female genitalia.

Problems with similar symptoms

African maize stalk borer, Oriental corn borer

Where and why it occurs

Spotted sorghum stem borers are most prevalent in regions with mild growing conditions and widespread cultivation of graminaceous species. Continuous monoculture of cereals increases the likelihood of pest infestation.

Geographical distribution

Australia, central and eastern Africa, south and southeast Asia, and Japan. Closely related species have been documented in North America.

Host range

Primarily maize, sorghum, and pearl millet. Also foxtail millet, finger millet, bulrush millet, sugarcane, rice, and some wild grasses.

Damage

• Mechanism of damage: Feeding in the whorl and midribs of leaves can lead to 'dead heart', which can terminate plant growth and development or result in excessive tilers that are barren. Losses are also incurred due to loss of photosynthetic leaf area caused by larval feeding. Boring in the stem can result in lodging.
• When damage is important: Damage is most critical if infestation results in dead heart or excessive crop lodging.
• Economic damage: Plants with dead heart are unlikely to produce ears. Lodging will decrease harvest. Economic damage will depend on level of infestation and its timing.

Management principles

Monitoring

• Plant should be monitored for larval feeding damage including leaf perforations, damage in the whorl, dead heart, and stem tunneling. Larvae can be sampled and reared to adulthood to aid in identity confirmation.
• Moth traps and pheromone traps can be used to monitor adult populations.

Control

• Early planting will enable the crop to avoid periods of heavy pest infestation later in the season.
• Rotation with a non-host crop will reduce pest populations.
• Removal and destruction of plants showing early signs of infestation, such as pinhole perforations or dead heart, may reduce pest infestation.
• CIMMYT fully supports conservation agriculture with the emphasis on zero-tillage due to its numerous advantages. Disking or plowing under of stubble is suggested in some resources to decrease the survival rate of the larvae in diapause. However, it will only be effective if done to all fields in the district, as moths from non-treated fields will redistribute the pest species.
• Resistant maize varieties such as Ganga-5 offer the most cost-effective and practical means of pest management.
• Several natural enemies exist that help control spotted sorghum stem borers naturally. These include the parasitic wasps Trichogramma australicum and T. exigum which parasitize eggs.
• Pheromone traps can be used to trap adults prior to mating.
• Insecticides applied in granular form in the whorl can provide control against spotted sorghum stem borers.

Prepared by Gabrielle Turner, David Bergvinson, Biswanath Das 

References

CAB International. 2002. Crop Protection Compendium. Wallingford, UK: CAB International.

Granados, G. 2000. Maize Insects In Tropical Maize - Improvement and Production. Mexico, D.F.: CIMMYT.

Ortega, A. 1987. Insect pests of maize: A guide for field identification. Mexico, D.F.: CIMMYT.

Panwar, V.P. 1995. Agricultural Insect Pests of Crops and Their Control. New Delhi: Kalyani Publishers.