future harv/est - Search CIMMYT repository

Breeding Mai-;.e Germplasm Tolerant to Major Insect Pests 327Grain yield losses:The percentage of avoidable loss primarily due to C. partellus during kharif (rainy) seasonvaried from 24.3 to 36.3 in different agroclimatic regions of India (Chatterji :el al. 1969). In Kenya,average annual losses due to Chilo sp. stem borer are about 18% (\Valker, 1967; Warui and Kuria,1983). The grain yield losses in Pakistan were estimated 44% (Mohyuddin and Attique, 1978). Allthese losses have been reported from experimental fields and no authentic estimates have been madefor losses suffered due to SSB on maize in the farmer's fields.Economic Injury Level ElL):In India, Sarup et al., (J 977) work OUl the economic threshold of SSB and reported themaximum reduction in yield and relatively higher damage in between 1O-20-day old crop, regardI ssof number of eggs released. The tolerance of plants began to increase when the crop was 17-day oldand identified 1O-15-day old crop, the most critical stage of the crop growth to initiate controlmeasures. Sharma and Sharma (I 87) reported increase in per cent yield losses with increase in pestdensity and observed maximum yield reduction up to 95%. In Kenya. Reddy and Sum (1991) reportedthat maximum yield reduction occurred at 20-DAE and minimum yield loss occurred at 60-DAE. Theeconomic injury level (ElL) was 3.2 and 3.9 larvae/ plant for 20 and 40-day old plants, respectively.The reason for reduction in yield loss with advancement in crop age could be due to increasedtoughness of leaves and stem with age, which adversely affect the establishment of neonate larvae.Basis of resistance:Genetic basis of resistance:No information is available on major gene(s) conditioning resistance to SSE (c. partelllls).However, considerable data are available on the genetic basis of resistance to European corn borer(ECB), Ostrinia nubilalis Hubner. Mathur (l99l) reviewed the work done in relation to genetics ofresistance to SSB and closely related insect ECB. In case of SSB, Singh (1967) obser ed that therewas a predominance of dominant alleles in the eight inbred lines which were tested for geneticanalysis. He concluded that for every gene or gene groups wherein positive and negative genesappeared in equal proportions. The inheritance seems to be multigenic or polygenic in nature(Sharma and Chatterji, 1972; Pathak, 1990). The studies based on diallel crosses showed additive andnon-additive effects of genes (Singh, 1967; Warsi and Agarwal, 1985,1986; Pathak, 1990 and Ajala,1992). Pathak (1990) reported that resistance was dominant over susceptibility. Additive effects werehighly significant for leaf feeding, dead-heart and stem tunnelling. However, a high magnitude ofnon-additive (dominance and epistatic) gene effects were detected for dead-hearts. He also concludedthat resistance to SSB is conferred only by nuclear genes.Guthrie and Russell (1987) found that the resistance in maize to the leaf feeding by firstgeneration ECB is conditioned by at least 8 genes and the resistance to sheath-collar feeding by thesecond generation is conditioned by a minimum of 7 genes. The reciprocal translocation studiesrevealed that at least 12 of the possible 20 chromosome arms contributed a minimum of J3 genesinvolved in resistance. Only 2 or 3 of the 12 chromosomes are in common in gene resistance to the twoECB generations. Thus, resistance to ECB is conditioned by the two different mechanisms. Thisnumber of genes ruled out the possibility of backcross procedure to transfer resistance to susceptiblemaize genotypes.A gap exists in genetic basis of resistance to SSB. Efforts should be made to isolate resistantgenotypes and detect the gene(s) on related chromosomes which may be responsible for suchresistance.