Maize (Zea mays L.) is the third most important cereal in the world after wheat and rice. In Nigeria, maize is popular and widely grown essentially because it matures during the “hunger period” and can be prepared in a variety of ways. In southern Nigeria, maize is a major component of the cropping system serving as hunger breaker while other crops are yet to mature.
In the rain forest zone of southern Nigeria, two crops of maize are possible per year due to the bi-modal rainfall pattern of the zone. The first season crop can be planted from mid March to first week of April while the second season planting is from mid August to early September. The maize produced in the early season is quickly consumed to avoid damage due to high humidity related diseases and pests. Storage is best with late maize during the onset of the dry season. Unfortunately, late season maize production is seriously limited by the activities of stem borers (Obi, 1991). The pink stem borer (S. calamistis (Hampson)) and the sugarcane stem borer (E. saccharina (Walker)) are the two stem borer species of economic importance in Southeastern Nigeria (Harris 1962; Appert, 1970; Bowden, 1976). The activities of the larva on the maize plants result in leaf feeding and stem tunneling, which in turn lead to reduced translocation of nutrients and assimilates, death of young plants (dead heart), lodging of older plants and direct damage to maize ears (Usua, 1968; Ezueh, 1978; Bosque-Perez and Mereck, 1990). All these damage activities tend to cause yield reduction and crop failure. Yield loss of between 10 to 100 % have been reported for stem borer attack in this region (Usua, 1968)
Control measures advocated for stem borers include direct use of insecticides, cultural control practices especially inter-cropping, early planting and good sanitation including burning of crop residue and the use of host plant resistance (HPR) (Lawani, 1982). Host plant resistance when strategically deployed in appropriate cropping system is both cost effective and environmentally safe. Therefore, it is often regarded as the hub in any integrated pest management (IPM) intervention for stem borer control (Teetes, 1985; Kogan, 1982; Belloti, 1990).
Whenever good sources of resistance for desirable traits are identified, appropriate breeding methods, such as recurrent selection, can be employed to increase the frequency of such desirable genes in order to further increase productivity of such crop. Crop improvements depend mainly on the availability of genetic variability. Such variability can be obtained through introduction, selection from available variation, generated through mutation or through the use of biotechnological tools to obtain desired genes for desirable traits. Conventional method of developing resistant varieties involves the identification and use of resistant germplasm in breeding programmes. In looking for resistant sources, one approach is to search for germplasm in areas where stresses are prevalent. This approach can identify genotypes with resistance to local stresses including diseases and insect pests that are also adapted to local ecological problems such as low soil pH, low soil nutrient and root and stalk lodging (Fajemisin et al., 1985; Kim et al., 1985; Eberhart et al., 1991).
Maize is not native to Southern Nigeria therefore, all the maize varieties grown in this region must have been improved varieties introduced in not too distant past and maintained by the farmers over the years. Usually, farmers’ selections of seeds for the next crop represent a form of mass selection for tolerance to environmental stresses such as insect pests, plant diseases, drought etc. Evidence of exploitable genes for resistance to maize stem borers is available in literature (Ajala et al., 1995 and Ngwuta et al., 2001). At IITA, some sources of resistance to S. calamistis and E. saccharina have been identified and used to form TZBR populations (Bosque-Perez et al., 1989, Kling and Bosque-Perez, 1995). In the course of developing resistant populations, efforts were aimed at breeding for resistance to these borers separately (Kling and Bosque-Perez, 1995). Some workers (Williams and Davis, 1984; Smith et al., 1989; Wiseman and Davis, 1990; and Mihm, 1995) have noted that the best strategy to a successful host plant resistance programme is the development of multiple insect resistant varieties. This approach is currently being used at IITA to develop genotypes with resistance to both Sesamia calamistis and Eldana saccharina (Schultess and Ajala, 1997; Ajala et al., 2002). The aim of this study was therefore to identify potential sources of multiple resistances to stem borers of interest and to generate genetically broad based reciprocal populations for further improvement efforts. Reciprocal populations have the advantages of complimenting each other for maximizing heterosis either as varietal crosses or in inbreds extracted from them and for continuous improvement of the two populations.
The objectives of this study were to:
- evaluate local and a few improved populations from Southeastern Nigeria for agronomic traits and stem borer damage parameters
- investigate the major characters responsible for the variations among the maize genotypes assembled, and group them into homogenous subsets so that representative genotypes can be selected for further studies
- investigate the combining ability and heterosis for agronomic attributes and stem borer damage parameters in the selected genotypes, and
- identify the heteroic groups that can be used in inter-population improvement schemes for the development of high yielding varieties or hybrids.