Effect of timing of application of Nosema marucae on the control of the spotted stalk borer Chilo partellus infesting sorghum

A local variety of sorghum (Serena) was planted in a screenhouse and infested with first‐instar Chilo partellus larvae. Sorghum rows were then inoculated at random with an aqueous suspension of Nosema marucae containing 1.6 ×10⁸ spores ml^‐1 at 2‐day intervals from 1 to 16 days after infestation (DAI). Foliar damage was low in plants inoculated with the pathogen at 1 and 3 DAI, where the highest score was 0.2 on a scale of 0–9. However, leaf damage increased rapidly as the interval between infestation and spraying of N. marucae became longer. At 19 DAI the foliar damage score on plants that had been inoculated at day 16 was 8.4. Similarly, the proportion of plants with either fully or partially formed heads decreased from 97.9% in plants sprayed at 3 DAI, to 10.8% at 11 DAI and 0% at 16 DAI. The timing of application of N. marucae is therefore important in the utilization of the microsporidian for the biological control of stem‐boring lepidopterans. This study suggests that a single, well timed application of Nosema spores can effectively control C. partellus for up to 19 days following oviposition.