Weeds and natural enemy regulation of insect pests in upland rice; a case study from West Africa

Effects of five weed management regimes on abundance of weeds, insect pests , generalist predators and on pest damage and rice yield were investigated in upland rice in Cote d'Ivoire over two years. In both years there was a h ighly significant negative correlation between weed biomass and grain yield across all treatments. Only two pest insect groups, Nephotettix spp. (Cica dellidae) and seed sucking Heteroptera, were consistently more abundant in unweeded plots and had a consistent significant positive correlation betwee n abundance and weed biomass across all seven treatments. These polyphagous groups may have been more dependent on resources provided by weeds than th e other pests studied. However, pest damage was not affected by presence or absence of weeds, suggesting that populations were below a damage threshol d. Ants were the most abundant predators in the rice canopy and were most a bundant in unweeded treatments. Abundance of both ants and spiders was sign ificantly positively correlated with weed biomass across treatments. Abunda nce of reduviid bugs was positively correlated with weed biomass only in 19 95. Any benefits due to presence of weeds in the crop were completely outwe ighed by loss of crop yield due to weed competition. However, if crop losse s due to weeds were sufficiently reduced, it is possible that significant l osses due to insect pests might emerge.