Economic Injury Level of Thrips tabaci (Thysanoptera: Thripidae) on Welsh onion (Allium fistulosum L. var)

This study was conducted to develop economic injury level (EIL) and economic threshold (ET) of onion thrips, Thrips tabaci on welsh onion (Allium fistulosum L. var). The changes of welsh onion biomass and T. tabacci density were investigated after introductions of T. tabacci at different densities and days: By the results of the experiment in the early welsh onion cultivation periods (30d after transplanting), the biomass of welsh onion significantly decreased with increasing the initial inoculated density of T. tabacci. The relationship between cumulative insect days (CID) and yield loss (%) of welsh onion was well described by a nonlinear logistic equation. Using the estimated equation, EIL and ET of T. tabacci on welsh onion were estimated at 24 and 20 CID per plant, respectively. By the results of the experiment in the late cultivation periods of welsh onion (about 120d after transplanting), the welsh onion biomass was also significantly different inoculation thrips densities to welsh onion. The EIL and ET were calculated as 35 and 28 thrips per welsh onion plant. Until a more elaborate EIL-model is developed, the present result may be useful for T. tabacci management during the cultivations of welsh onion.