Resistance in maize to the maize stalk borer, Busseola fusca (Fuller) (Lepidoptera: Noctuidae)
An in depth review is given of Host Plant Resistance research on borers in general, and on B. fusca in particular. The general biology and economic importance of B. fusca are also discussed. Several aspects of the general methodology of breeding for resistance to B. fusca are discussed. These topics include collection of overwintering larvae, termination of diapause, field infestation methodology and damage assessment. Approximately 20 first instar larvae were applied to the plant whorl when plants were about 30 - 40 cm tall. The primary method of damage assessment was to evaluate leaf damage on a 1 to 5 scale. Further criteria for selection of resistant germplasm are assessment of stunting due to stem boring and yield at harvest. The effect of plant resistance on B. fusca was investigated. There were clearly defined differences in leaf damage evident between different maize genotypes. Heritability of this resistance was demonstrated, and presumed to be an additive mechanism that reduced insect feeding, indicating antibiosis. There were significant differences between the number of larvae recovered from whorl tissue of different cultivars. This was ascribed to two resistance mechanisms exerting their effects within the first few days' feeding by larvae. One mechanism was short lived, but effective, antibiosis resulting in larval death, while the other, also short lived, was repellence, resulting in larval migration. Both mechanisms resulted in fewer larva feeding in the plants. Another longer lasting resistant mechanism affected larval growth and mass gain, resulting in smaller larvae. These mechanisms were found to be heritable traits. Differences in levels of resistance affecting larval mass gain were also determined for various parts of the tassel. For all inbreds, the peduncles were more susceptible than the tassel Cultivars also differed in the levels of resistance in the stern tissue. Resistance in leaf tissue did not necessarily mean that resistance occurred in the stern of that genotype. Some cultivars had resistance mechanisms present in both leaves and sterns, some had only one resistance mechanism in either part, and some were totally susceptible. The effect of the borer on the plant was investigated. Leaf damage was found to not be of any consequence, but severe stern damage caused extensive yield losses. There was good correlation between leaf damage and stern damage. Yield loss was most pronounced in longer season hybrids than in quick maturing hybrids. Methodologies utilized in the development of inbreds, populations and hybrids are discussed. It was concluded that borer resistant hybrids do have a place in the commercial market. However their performance under conditions of low or no infestation must be similar to that of other susceptible hybrids because control measures for B. fusca are not excessively expensive.