Studies on stalk borers of maize and sorghum in Lesotho

English: Investigations on various aspects of stalk borers of maize and sorghum were conducted in Lesotho during the 1995/96, 1996/97 and 1997/98 growing seasons. Aspects investigated were their distribution and relative abundance on these two crops; seasonal moth flight pattern of Busseola fusca; impact of natural stalk borer populations on maize and sorghum; response of some maize and sorghum varieties/hybrids to artificial infestations with B.fusca and Chilo partellus respectively; farmers' perceptions and farm management practices with regard to the management of stalk borers; relative efficacy of insecticides used for C. partellus control in Lesotho; effect of planting date of maize on B.fusca infestations; effect of intereropping with beans on B. fusca infestations in maize, and relative abundance of natural enemies of B. fusca and C. partellus. These investigations provided the following results: B. fusca was distributed throughout Lesotho and was important on both maize and sorghum. C. partellus occurred only in the central lowlands where it attacked only sorghum. Seasonal B. fusca moth flight activity was characterised by distinct periods of high and low activity, with three and two to three generations of moths per season in the lowlands and mountains respectively. Infestations of B. fusca in Lesotho caused substantial yield losses in susceptible maize varieties and hybrids. Lesotho's maize varieties and hybrids were observed to be susceptible, with yield losses of up to 39.4% observed under field conditions. Their susceptibility was further demonstrated through artificial infestations in glasshouse and field studies. The glasshouse study showed that the levels of antibiosis and antixenosis resistance to B. fusca in these varieties and hybrids were low, while the field studies showed only moderate resistance to foliar damage. Lesotho's sorghum varieties exhibited susceptibility to stalk borer damage, both under field conditions and artificial infestations with C. partellus. A glasshouse study also found antibiosis to be the main resistance mechanism against C. partellus larvae in whorls of resistant sorghum genotypes. The variety Tenant White, exhibited tolerance to damage, both under natural and artificial infestations.Lesotho's farmers regarded stalk borers as important pests. The commonly employed control measure was insecticide application. Although the use of routine farm management practices has potential for suppressing borer populations, they were not utilised in this regard. Comparison of the efficacy of six insecticides used for stalk borer control in Lesotho indicated granular formulations beta-cyfluthrin 0.05 % and carbaryl 2.5 % to be more effective against C. partellus larvae in sorghum whorls than spray formulations. Early planting of maize (in November) substantially reduced damage and yield loss caused by B.fusca. Intereropping maize with beans reduced damage and yield loss caused by B. fusca in maize. Furthermore, alternating rows of maize with rows of beans was more effective than alternating maize with beans within a row. The braconids Bracon sesamiae, Cotesia sesamiae, Euvipio sp. and Habrobracon brevicornis parasitized B. fusca larvae. Only B. sesamiae and C. sesamiae were recorded on C. partellus larvae. An unidentified ichneumonid was reared from C. partellus pupae, while a red ant species Dory/us sp. (Formicidae) was recorded as a predator of both B. fusca and C. partellus larvae. However, the activity of these natural enemies was generally low, increasing only towards the end of the growing season. These investigations have shown that stalk borer infestations are economically important in Lesotho. However, adopting such cultural practices as planting date adjustment, intereropping and destruction of crop residues can limit infestations. These practices, together with limited insecticide use, should form components of an integrated stalk borer management system. Future research efforts in Lesotho need to include the identification and development of maize and sorghum varieties and hybrids with high yields even under stalk borer infestations. Where such varieties are open-pollinated, they will be of particular benefit to the resource-poor farmers, who may not be able to afford hybrid seeds.