Interactions between infestations of onion thrips,Thrips tabaciLindeman, the three growth stages of onion (prebulbing, bulbing, and sizing), and onion yield were investigated in field and greenhouse experiments. Significant negative correlations existed between thrips numbers and yield only during bulbing, although percentage of yield reduction varied considerably between studies. During bulbing stage, a 2–3% yield reduction per 10 thrips was obtained for two field studies. Under greenhouse conditions, 10 thrips per plant resulted in a 7% yield reduction. In an additional study, plants were mechanically defoliated at 0, 30, and 60% levels during the three onion growth stages. This also resulted in yield reduction following damage during bulbing stage.