Suppression effect of intercropping with barley on Thrips tabaci (Thysanoptera: Thripidae) in onion fields

Onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), is a global pest of onion plants and is difficult to control using chemicals. Therefore, we investigated whether intercropping with barley in open onion fields can suppress T. tabaci population more effectively as compared to a conventional monoculture. We found that barley intercropping in onion cultivation reduced the population of T. tabaci; this effect became stronger at a later stage of cultivation. We also found that insecticide application in the early onion cultivation period in intercropped barley helped to suppress T. tabaci population throughout the cultivation period. However, the intercropping with barley influenced the growth of onion, causing reduction of bulb size. Trimming barley to approximately half the height of the onion plants mitigated the adverse effect on bulb size, while the thrips population on onion plants was maintained low as same as when intercropped barley was not trimmed. This implies that the barrier effect when intercropped with barley is not a major cause to reduce T. tabaci populations on onion plants. Meanwhile, carabid beetles (Coleoptera: Carabidae) were significantly more abundant where onion was intercropped with barley, irrespective of trimmed or not, than those where onion was cultivated alone. Thus, the enhanced effects of indigenous natural enemies of T. tabaci could be anticipated when onion is intercropped with barley.