Effect of sorghum phenology on the control of Chilo partellus with Nosema marucae (microspora: Nosematidae)

An aqueous suspension of Nosema marucae spores was sprayed on foliage of sorghum plants that had been infested with neonate Chilo partellus larvae at 3, 5, 7 and 9 weeks after (plant) emergence (WAE). These periods corresponded to plants at the tillering, early booting, soft dough and late maturation phenological stages, respectively. The extent of borer infestation and plant damage was monitored until crop harvest, and compared with plants which had been similarly infested but not sprayed with the pathogen, and with plants in insect‐free control plots. When applied early, infestation with neonate borer larvae caused most damage to the plants and the greatest reduction in yield in the untreated plots. Early treatment with N. marucae spores at 3 and 5 WAE resulted in the greatest reduction in damage to plants and the highest improvement in yield of seeds. It is apparent, therefore, that for maximum crop protection the microsporidian needs to be applied when sorghum is in an early phenological stage.