To understand the effect of plant availability/structure on the population size and dynamics of insects, a specialist herbivore in the presence of two of its parasitoids was studied in four replicated time-series experiments with high and low plant availabilities; under the latter condition, the herbivore suffered from some periods of resource limitation (starvation) and little plant-related structural refuges. Population dynamics of the parasitoid Cotesia vestalis was governed mainly by the delayed density-dependent process under both plant setups. The parasitoid, Diadegma semiclausum, under different plant availabilities and different coexistence situations (either +competitor or –competitor) showed dynamics patterns that were governed mainly by the delayed density process (significant lags at weeks 2–4). Both the competing parasitoids did not experience beneficial or costly interferences from each other in terms of their own population size when the plant resource was limited. Variation in the Plutella xylostella population under limited plant availability is higher than that under the other plant setup. For both parasitoids, under limited plant setup, the extinction risk was lower when parasitoids were engaged in competition, while under the unlimited plant setup, the mentioned risk was higher when parasitoids competed. In this situation, parasitoids suffered from two forces, competition and higher escaped hosts.

This paper addresses the competitive interaction between Cotesia sesamiae, a gregarious larval endoparasitoid that is native to Africa, and Cotesia flavipes, an introduced parasitoid from southern Pakistan which has recently been established in East Africa for biological control of the exotic stemborer, Chilo partellus. The two parasitoids are morphologically similar, and they both attack medium-sized to large larval stages of gramineous lepidopteran stemborers. Based on previous work which suggested that Cotesia flavipes was competitively superior to Co. sesamiae, we speculated that Co. sesamiae may be locally extirpated in some areas of Africa. However, evidence from the field does not support this prediction. Recent field surveys in areas where both parasitoids occur, revealed that Co. sesamiae was still present, and densities were not declining. The interaction of the two parasitoids, as well as the influence of this interaction on host regulation, are discussed.