DEVELOPING MAIZE (ZEA MAYS) POPULATIONS RESISTANT TO STEM BORERS FOR SOUTHEASTERN NIGERIA

ABSTRACT

Development  of  maize  populations  resistant  to  stem  borers  depends  largely  on  the existence of useful genes or alleles, which can combine to confer resistance to progenies. Such genes are often available in areas of stress, having been responsible for the survival of such crops over the years. Pink stem borer, Sesamia calamistis (Hampson, Noctuidae) and sugarcane borer, Eldana saccharina (Walker, Pyralidae) are endemic in southeastern Nigeria. Damages  caused by the larvae of these moths  are more prevalent during the second planting season (August-November).  Genetic diversity for a range of agronomic and resistance attributes within 209 local maize collections from southeastern Nigeria and

3 improved check varieties were investigated in field trials in randomised complete block design (RCBD) with two replications across three environments. Data collected from the evaluations were subjected to both uni- and multivariate statistics. Furthermore, four traits namely, leaf feeding, ear damage, shoot breakage and yield were used from across three environments  to  construct  a  selection  index.  The  multivariate  analysis  on  the  plant attributes,  using  canonical  discriminant  analysis,  revealed  the  agronomic  and  borer damage  parameters  that  contributed  significantly  to  the  total  variation  observed  in different environments.  Out of the  four canonical discriminant  functions obtained, two had significant (P=0.05) eigenvalues accounting for over 98 % of the total variation. The first canonical function was mainly associated with yield while the second was associated with the borer damage attributes. Rank summation index (RSI) used to rank the entries for resistance to stem borers identified 11 genotypes representing top 5 % of the total as resistant. In the second experiment the 11 genotypes and their hybrids, made in a diallel fashion were evaluated for agronomic and borer damage attributes in seven environments in RCBD with three replications. Data collected were subjected to analysis of variance and  those  found  significant  (P=0.05)  were  further  subjected  to  diallel  analysis  using

Griffing’s method 2 model 1 for fixed effects. Significant GCA and SCA effects  were obtained  for most of the traits studied in the various environments  and in the  pooled environment thus indicating that additive and non-additive gene effects were involved in the expressions of the traits studied. However, in a few cases, only GCA  or SCA was important  thus  indicating  the  relative  importance  of  the  genetic  component  of  the variance. The assessment of the agronomic and borer damage attributes of the parents and the crosses indicate that the variety crosses were not superior to the parents in most of the traits. The significant differences observed between the parents and the crosses for dead heart and leaf feeding damage parameters is suggestive of the occurrence of exploitable heterosis  for  the  development  of  genotypes  that  are  resistant  to  stem  borer  attack. Genotypes SE NG-33, SE NG-65 and TZBR Syn W had high negative GCA values for dead heart while SE NG-62, SE NG-148, TZBR Syn W and TZBR ELD 3 C2  had the

high negative GCA values for leaf feeding damage. For ear damage, SE NG-65, SE NG-

67, SE NG-119, SE NG-148 and AMA TZBR-W-C1  had high negative GCA estimates. Genotypes SE NG-33, SE NG-62, SE NG-65, SE NG-77, SE NG-106 and SE NG-119 had the highest positive GCA effects for grain yield. The nine genotypes selected formed two heterotic pools: Group A comprised SE NG-33, SE NG-77, SE NG-106, SE NG-148 and TZBR Syn W while Group B included SE NG-62, SE NG-119, AMA TZBR-W-C1 and TZBR ELD 3 C2. Average yield of the  grouped genotypes crossed in all possible combinations  was 1.06 t ha-1  showing  5 % yield increase. Furthermore,  the best  five

yielding crosses namely; SE NG-33 x TZBR ELD 3 C2, SE NG-62 x SE NG-77, SE NG- 62 x SE NG-106, SE NG-106 x TZBR ELD 3 C2 and TZBR Syn W x TZBR ELD 3 C2, selected may be used as population crosses or in the formation of composite varieties.

INTRODUCTION

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:

i.         evaluate local and a few improved populations from Southeastern Nigeria for agronomic traits and stem borer damage parameters

ii.        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

iii.       investigate the combining ability and heterosis for agronomic attributes  and stem borer damage parameters in the selected genotypes, and

iv.       identify the heteroic groups that can be used in inter-population improvement schemes for the development of high yielding varieties or hybrids.

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