Infestations of stem borers are detected by walking through young crops looking for characteristic feeding marks on funnel leaves, the presence of dead hearts and holes in tunnelled stems. Samples of affected stems are then dissected to retrieve caterpillars and pupae.
As other stem borers cause similar symptoms, retrieval of caterpillar or pupae and confirmation of their identity by rearing adults for identification by a taxonomic specialist is essential to ensure a correct diagnosis.
The presence of this species in older crops and in crop residues may be detected by taking random samples of stems or stools for dissection.
Practise good crop hygiene, this includes the destruction of crop residues (stems and stubbles). Remove volunteer crop plants and/or alternative hosts. This reduces carryover of stem borers from one growing season to the next, and will help to limit the most damaging attacks on young crops early in the growing season.
Manipulation of sowing dates may also be used to avoid periods of peak adult activity. However, this is not practical in situations where lack of water is a major constraint as farmers often plant after first rains.
Manipulation of sowing dates may also be used to avoid periods of peak adult activity.
Improvement of soil fertility
Studies on several stem borers in Africa showed that soil nutrient levels, such as nitrogen, greatly influenced nutritional status of the plant, and the plant's tolerance to stem borer attack. Although an increase in nitrogen is related to higher pest loads and tunnel damage, there is also an increase in plant vigour with a net benefit to the plant reflected in lower yield losses (Setamu et al., 1995).
Trials in Tanzania to evaluate the effect of nitrogen fertilisation (0,50,70,100 kg N/ha) on pest abundance, plant damage and yield loss of maize due to stem borers showed the beneficial effect of nitrogen on the maize plant's abilities to compensate for damage by the spotted stem borer. Yield loss decreased with an increase in nitrogen application and the effect was stronger under high than low borer infestation levels (ICIPE, 2005; Mgoo et al., 2006).
Intercropping and habitat management
The importance of plant biodiversity in maize agroecosystems for reducing borer's infestation on maize has been recognised in sub-Sahara Africa. Studies have shown that intercropping maize with cowpea is an effective way of reducing damage by the spotted stem borer caterpillars migrating from neighbouring plants. The effect is variable, if the crop to be protected is not planted after the companion crops.
Intercropping maize with molasses grass (Melinis minutiflora), which is a non-host for stem borers, significantly reduced stem borer infestation on maize. A significant increase of parasitism of stem borers by the wasp Cotesia sesamiae was also observed. Molasses grass produces volatile agents, which repel stem borers but attract the parasitic wasp. In addition, the molasses grass is an effective cover crop and provides good fodder for livestock. Greenleaf desmodium (Desmodium intortum) repels egg-laying stem borer moths, and in addition, when intercropped with maize, suppresses and eliminates Striga.
Planting an outer encircling row of some highly preferred hosts as trap plants is another useful diversionary tactic for management of stem borers. Examples of trap plants are Napier grass (Pennisetum purpureum) and Sudan grass (Sorghum vulgare sudanense), common fodder plants in Africa. Napier grass is highly attractive to egg laying moths, but only few caterpillars complete their lifecycles, since when they enter the stem the plant produces a gummy substance that kills the caterpillars. Sudan grass provides natural control of stemborers by acting as a trap crop (attracting moths) and as a reservoir for its natural enemies.
A simple habitat management strategy has been developed combining use of intercropping and trap crop systems. The strategy is known as "Push-Pull", whereby farmers use Napier grass and Desmodium legume (silverleaf and greenleaf desmodium) as intercrops. For more information on push-pull click here or refer to www.push-pull.net (click to follow link).