The biocontrol potential of entomopathogenic nematodes (EPNs) against soil-dwelling life stages of the onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae) was evaluated under laboratoryand semi-field conditions. The pathogenicity of native isolate of Steinernema feltiae (T1) and threecommercial strains, S. feltiae, S. carpocapsae (Rhabditida: Steinernematidae) and Heterorhabditis

bacteriophora (Rhabditidae: Heterorhabditidae) were tested at concentrations of 400 and 1000infective juveniles (IJs)/cm2 against the second instar larvae (L2), prepupae and pupae of the thrips. This was followed with trials compared efficacy of S. carpocapsae on the thrips fed on different host plants, i.e. onion shoots, Allium cepa (L.), pods of green beans, Phaseolus vulgaris (L.) andcucumber fruits, Cucumis sativus (L.). In the semi-field experiments, effects of two EPNs, S. carpocapsae and H. bacteriophora, at concentrations of 104 or 2×104 IJs/ml against the thrips larval stage were tested. Our results showed that the highest onion thrips corrected mortality (CM) was recorded for prepupae (92%) and pupae (92.59%) at 1000 IJs/cm2 by S. feltiae (T1) and S. carpocapsae, respectively and the mortality caused by the commercial strain of S. feltiae against thrips larval stage at 400 IJs/cm2 was the lowest rate (CM= 3.7%). Among different host plants, green bean feeding induced the highest CM by S. carpocapsae at 1000 IJs/cm2 against the onionthrips larvae. In the semi-field experiments, application of two concentrations of EPNs had nosignificant effect on the larval mortality, compared to the control treatment.