Studies were carried out in 1983 to determine the relative abundance and species composition of three lepidopterous stem borers in upland and irrigated rice at Ibadan, southwestern Nigeria. The relationship between irrigated rice crop phenology and the population of the borers and stalk-eyed fly was also studied.

White stem borer, Maliarpha separatella was the predominant species in upland and irrigated rice, followed in upland rice by pink stem borer, Sesamia calamistis, and striped stem borer, Chilo zacconius and in irrigated rice by C. zacconius and S. calamistis. The proportion of M. separatella was higher in irrigated rice than upland. Three population peaks were observed annually in both upland and irrigated rice.

Studies on the relationship between irrigated rice phenology and field population of lepidopterous borers and a dipteran borer showed that the stalk-eyed fly, Diopsis spp., occurred before 20 days after transplanting (DAT) in ITA 212 and continued up to 90 DAT: the peak was observed at 40 DAT. The lepidopterous borers did not appear until 40 DAT and populations peaked at 80 DAT.

Ten rice varieties were studied for their relative susceptibility to the stalk-eyed fly, Diopsis thoracica West. Preference for oviposition was observed and some of the factors which appeared to pre-dispose the plants for oviposition were number of tillers per hill, leaf angle, plant height, stem diameter, leaf colour, plant age and plant spacing. The relative developments of Diopsis thoracica in the different varieties are given. Varietal susceptibility to infestation by the insect seemed to be different from varietal suitability for its growth. Insect damage affected the yield components and total yields, but the extent was varietal. The level of infestation, the production and time of production of compensatory tillers seem to be important factors in yield reductions. Implications of some of these results in a pest management programme are highlighted.