Abstract The efficacy of Epiblema strenuanaWalk., a stem-galling moth, as a biological control agent for Parthenium hysterophorusL. (ragweed parthenium) was examined in a glasshouse pot experiment. The importance of the timing of insect attack and the presence of competition from Cenchrus ciliarisL. (buffelgrass) to the level of control obtained was tested. This was achieved by applying E. strenuanaeggs at two stages of the weed's development: prior to stem elongation (35 days after emergence) and after stem elongation (53 days after emergence), in the presence or absence of moderate competition from C. ciliarisseedlings. Application of a moderate number of E. strenuanalarvae, at 53 days, reduced the number of immature capitula (36%), mature capitula (41%), and viable seeds (39%) produced by P. hysterophorusplants. Timing of the application of E. strenuanawas important, with earlier application (at 35 days) causing a significant reduction in plant height (34%) and a more significant reduction in the number of mature capitula (74%) and viable seed (74%) produced. Competition from C. ciliarishad a significant, and usually greater, effect on all of these characters, as well as significantly reducing the aboveground biomass of the weed. Plant height was the only measured character which was less affected by competition from C. ciliaristhan by E. strenuanaattack. Reduction in weed seed production was greatest when E. strenuanawas applied prior to stem elongation and when the weed was also experiencing competition from C. ciliaris. In fact, a synergistic interaction was detected between plant competition and insect attack, and seed production was reduced by more than expected when these factors were combined. With both factors simultaneously present, seed production was reduced to 2% of that of the control plants. E. strenuanaattack did not affect the quality of P. hysterophorusseeds produced. These levels of control do not often occur in the field and a number of possible explanations for this are discussed.