Stemborers are the most important pests of sugarcane. Several species of stemborers attack sugarcane. The most important is the African stemborer.
1) African sugarcane stalkborer (Eldana saccharina)
Adult moth has a wingspan of 30 to 40 mm. It has elongated, pale brown forewings, each with two small spots in the centre, and whitish hind wings with a short fringe. Adult moths congregate on the cane canopy at night, they are attracted to light and so populations can be sampled using light traps. Female moths lay batches of yellow, oval eggs behind the leaf sheath or in folds of dead leaves. Eggs become pink before hatching. Young caterpillars feed on the leaves eating away the upper layers of the tissue; this damage is known as windowing. Later, they penetrate into the stalks. Caterpillars usually burrow into single internodes. When burrowing in the stem, caterpillars push their excrement outside.
Large amounts of frass are often hanging from the exit hole made by caterpillars prior to pupation. Caterpillars are light brown to dark grey coloured with brown with very small dark coloured spots. Pupation takes place in the plants, in the stem or on the leaf sheath. At emergence, the moth leaves a large emergence hole in the stalk. Attacked plants are stunted; in severe attacks they may dry up and die. When very young plants are attacked "dead heart" results, followed by tillering of the plant.
Older plants or ratoon cane have internodes bored. Crop losses are hard to assess because they vary with the age of the cane. In unstressed, maturing cane, quite high numbers of borers may be present without serious crop loss, but where infested, stressed cane is left to stand from one season to the next, the loss may be total. This stemborer also attacks maize and sorghum.
2) Spotted cane borer (Chilo sacchariphagus)
It is a serious pest of sugarcane in the Indian Ocean Islands (Mauritius and Reunion), and also attacking sugarcane in Mozambique and South Africa.
- Adjustments to crop management are the most effective measures in the control of African sugarcane borer.
- Use borer-free planting material (setts).
- Check planting material for signs of stem boring, which may indicate the presence of caterpillar and/or pupae.
- Avoid plant stress, such as drought, since stressed crops are more prone to stemborer attack.
- Cut older canes out, as soon as possible after 12 months, because the numbers of borers accumulate as cane ages, especially after about nine months.
- Do not leave tops of plants in fields; in East Africa caterpillars of the African cane borer are largely found on the upper part of the plant, and crop residue will maintain carryover population for the next growing season.
- Proper fertilisation is important; in particular nitrogenous fertilisers have shown to influence stemborer attack. In South Africa, where the African sugarcane borer is a problem, a reduction in nitrogen fertilisation rate from 50 kg to 30 kg per hectare is recommended. However, the yield outcome should be considered when deciding in reducing nitrogen input, though nitrogen fertilisation enhances borer development it also enhances the plant's tolerance to borer attack, and the outcome in terms of yield may be positive.
- Trashing has been recommended for control of stemborers. De-trashing sugarcane crops during the 5th, 7th and 9th month of growth has been recommended in India for control of the spotted cane borer. Pre-trashing of mature cane reportedly reduces numbers of the African sugarcane borer by 30% or more (Carnegie, 1991).
- Burning attacked fields at harvest or burning of residual cane is sometimes suggested, as it is more likely to destroy the pests than trashing, However, there are concerns that it may do more harm than good by destroying natural enemies.
- Parasitic wasps and predators attack caterpillars of the African sugarcane borer. In particular Cotesia sesamiae is widespread. The introduced spotted cane borer is attacked by the parasitic wasp Cotesia flavipes introduced in the region for control of the spotted stemborer Chilo partellus. In Madagascar, parasitism of 60% of caterpillars of the spotted cane borer by this parasitic wasp has been reported (Kfir et al, 2002).