The effect of manure application and host plant spacing on the infestation level and damage of Thrips tabaci in onion were investigated under field conditions and in cage experiments. Manure applications before and at transplanting were used in a factorial combination with three levels of plant spacing as treatments. Plots with no manure application were included as controls. In field experiments, significantly fewer number of adults and nymphs of T. tabaci per plant were recorded in plants grown in soil with manure applied before transplanting at a rate of 20 t/ha. Plants grown without manure application harboured significantly higher numbers of T. tabaci at 45 days after transplanting and late growth stages. Plots that received manure application before transplanting (20 t/ha) with 20 × 30 cm and 20 × 25 cm spacing showed markedly reduced mean thrips counts per plant, and had significantly higher leaf size compared to those transplanted with manure application (20 t/ha) and at 20 × 30 cm plant spacing. The extent of damage incurred on onion plants by natural infestation of T. tabaci was mostly mild at 15 and 30 days after transplanting and intermediate damage was recorded on plants at 45 days and late growth stages. On the other hand, plants in plots without manure application and 20 × 20 cm and 20 × 25 cm spacing showed markedly reduced leaf size and had intermediate damage at 15 and 30 days after transplanting and sustained severe attack as the crop matures. In cage experiments, significantly lower mean number of larvae and adults were recorded from pots with manure application before transplanting followed by at transplanting manure application as compared to plots with no manure application. This study illustrated that optimum organic manure application at the appropriate time had an effect on the infestation level of onion thrips. Therefore, manure application three months before transplanting was found to be the critical period for optimum plant growth and had the lowest onion thrips infestation.