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First-instar larvae feed in the young terminal leaf whorls producing characteristic patterns of small holes and 'window-panes' where tissues have been eaten away. Later they eat into the growing points, which may be killed so that the dead central leaves form characteristic dry, withered 'dead-hearts'. Older larvae tunnel extensively in stems, eating out long frass-filled galleries which may weaken stems and cause breakages. Larvae also tunnel into maize cobs and into the peduncles of sorghum and millet inflorescences and may seriously affect grain production.
Most stem borer attacks on cereal crops result from infestation by more than one species and, since there are important differences in biology and ecology that limit the effectiveness of some techniques, integrated pest management programmes must be devised to meet local conditions. Many different chemical and non-chemical control measures have been developed and applied since 1920, when Mally reviewed early work in South Africa, and these have been summarized, with bibliographies, by Harris and Nwanze (1992). The main elements available for inclusion in modern IPM programmes are noted.
Crop residues. Appropriate disposal of crop residues after harvest can reduce carry-over populations of diapause larvae and so limit initial establishment of the pest on the following season's crops. Burning or burying by deep ploughing are effective but in areas where stems of cereals are used as building and fencing materials it may be better to devise means of destroying diapausing larvae without destroying the stems. Adesiyun and Ajayi (1980) showed that partial burning of sorghum stalks kills 95% of B. fusca larvae within them and also improves the quality of the stems as building material. Alternatively, simply leaving stems lying on the ground exposed to the full heat of the sun for a month or so after harvest has been shown to reduce populations of diapause larvae (Harris, 1962; Gebre-Amlak, 1988). Using crop residues for fodder and silage has also been recommended (Wahl, 1926).
Cultivation by discing and ploughing may also be effective, and, when preceded by slashing, can reduce larval populations by almost 100% (Kfir, 1990).
Intercropping. Experiments in Kenya suggest that intercropping maize and/or sorghum with cowpeas may reduce damage caused by B. fusca (Amoako-Atta and Omolo, 1983; Reddy and Masyanga, 1988; Regine and Coderre, 2000). Early planting is effective in reducing stem borer damage on sorghum in the highlands of Eritrea (Haile and Hofsvang, 2001).
Diversionary maize stem borer management system. In this system, ovipositing moths are repelled by an intercrop and subsequently attracted to a discard perimeter crop. Studies were conducted in fields of maize intercropped with desmodium and a Napier grass perimeter ('push-pull' system) and maize monocrops. Maize stem borer colonization, oviposition preference and incidence of stem borer larvae and pupae were significantly lower in 'push-pull' plots than maize monocrop plots at all sites. Similarly, the various crop damage levels were mostly significantly higher in maize monocrop than in 'push-pull' plots. Maize yield per plant and per plot were mostly significantly higher in 'push-pull' than in maize monocrop plots (Midega et al., 2005). Similar tests were conducted in Cameroon and Benin where different grasses were used as trap plants for ovipositing females (Ndemah et al., 2002). Gohole (2003) studied the effect of intercropping maize and sorghum with molasses grass on the foraging behaviour of stem borer parasitoids but concluded that there was no enhancement and that the molasses grass seemed to be more important to the herbivore than to the parasitoids.
Field trials were carried out in the humid forest zone of Cameroon to investigate the effects of crop rotation, cover crops and bush fallow on infestations by B. fusca and on yield of maize. A continuous maize production system was compared with crop sequence systems, in which maize followed a grain legume (cowpea, soyabean), cover crop (mucuna, pigeon pea) or a bush fallow. It was concluded that an increased nutritional status of the plants led to an increase in borer attacks at the early stage of plant growth, but also to improved plant vigour, resulting finally in a net benefit for the plant and grain yield (Chabi Olaye et al., 2005a). A similar study in the humid forest zone in Cameroon investigated the effects of intercropping on infestation levels and parasitism of B. fusca. Two trials were planted per year, one during the long season and one during the short rainy season. Maize monocrops were compared with maize/legume or maize/cassava intercrops. The results showed that the larval egg batches were regularly dispersed in the maize monocrop and aggregated in the intercrops, consequently, larval densities were much lower in intercrops compared to monocrops. The intercropping of miaze with non-host plants did not affect larval parasitism (Chabi Olaye et al., 2005b).
Adetiloye et al. (2002) evaluated cropping systems for control of maize stemborer and the effects on maize growth and grain yield in Nigeria. The rotation of early season cowpea monocrop with late season maize monocrop gave the highest maize plant survival, plant height and grain yield, whereas late season maize monocrop planted after an early maize monocrop gave the worst performance. The stemborer population in early maize monocrop plant residues was almost double that of intercropped maize plots.
Screening maize and sorghum genotypes for resistance to B. fusca has until recently been limited by the lack of effective techniques, especially the inability to rear B. fusca on artificial diets. Much screening has therefore been against field infestations, often against complexes of different borer species. In Ethiopia, barely 1% of nearly 6000 indigenous sorghum genotypes showed promising levels of tolerance (Gebrekidan, 1985) but in South Africa several lines of maize have intermediate levels of tolerance to whorl feeding by first-instar larvae (Barrow, 1989). Mechanisms of resistance are not well understood but effects of preferential oviposition have been reported by van Rensburg et al. (1987, 1989) and Barrow (1985, 1989) suggested that three factors related to resistance are present in maize, the first killing early instar larvae, the second repelling larvae and the third retarding larval development.
Potentials for biological control are currently being investigated, especially in East and South Africa. Classical biological control by introduction of parasitoids from Asia and/or the Americas has been attempted on a number of occasions but with little success so far. The general situation in Africa has been reviewed by a number of authors over the past 30 years (see Harris and Nwanze, 1992) and implementation programmes are now in progress in East and South Africa. Geographic differences in host acceptance and suitability do exist and were studied in Zimbabwe (Chinwada et al., 2003). Kfir (1995) reported on 18 species of parasitoid developing on B. fusca in South Africa, of which the indigenous species Cotesia sesamiae and Bracon sesamiae were most abundant, and discussed proposals for further introductions of exotic species into South Africa, which could involve transfers within Africa of B. fusca parasitoids not known to occur in South Africa and/or introductions from outside Africa of stemborer parasitoids from other parts of the world. Getu et al. (2003) reported the widespread distribution of Cotesia flavipes in Ethiopia although it had never been released in the country. Opportunities for exploiting the former approach within Africa were summarized by Mohyuddin and Greathead (1970) but will now require reassessment following the completion of more recent collaborative studies on stemborer parasitoids in Africa (Polaszek, 1998). Stemborer parasitoids have been released and their populations evaluated in different countries (Mozambique (Cugala et al., 2001), Uganda (Matama Kauma et al., 2001), Zambia (Sohati et al., 2001), Zimbabwe (Chinwada et al., 2001)) of southern and eastern Africa. C. flavipes was observed in Tanzania, where it may have spread from Kenya (Nsami et al., 2001). The extent of the differences within Africa that might be exploited is further emphasised by recent studies on maize in eastern Ethiopia (Yitaferu and Walker, 1997) where a previously unknown and recently described braconid, Dolichogenidea fuscivora, was found to be the major larval parasitoid of B. fusca. It was active throughout the year and achieved levels of parasitism as high as 71% in the dry season and ca 18% in the wet season.
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
Although few rigorous experiments have been reported, it is generally accepted that B. fusca is a major pest of maize in much of tropical Africa. Maize plants are less able to tolerate stem borer attack than sorghum and pearl millet plants and the effect on grain yields is therefore greater. In Tanzania and Kenya, yield losses of ca 12% for every 10% plants infested were reported (Walker, 1960; Walker and Hodson, 1976). In Côte d'Ivoire a statistical model of crop losses, based on six experiments on maize, indicated that plant destruction was mainly caused by B. fusca and that crop losses could be accurately estimated on the basis of samples taken 40 and 80 days after crop emergence (Moyal, 1996). Crop losses decreased when stand density was reduced and increased when the crop was suffering from water stress (Moyal, 1995).
Crop loss experiments on sorghum in Nigeria indicated a complex situation where selective oviposition by females on larger plants masked yield reduction (Harris, 1962) but in Ethiopia, Megenasa (1982) reported a 15% grain loss in sorghum after late attack by B. fusca larvae on developing seed-heads.