Enhancing Hoverfly Activity with Flowering Buckwheat for Effective Control of Onion Thrips (thrips Tabaci) in Onion–barley Intercropped Fields

Onion thrips (Thrips tabaci) are a major pest of onion crops, but they can be controlled using syrphid larvae, which are omnivorous, as biological control agents. The introduction of secondary plants may enhance syrphid activity and contribute to the suppression of onion thrips population in onion–barley intercropped fields. Therefore, two experiments were conducted to evaluate the effect of introducing secondary plants on the population of onion thrips and the occurrence of syrphids in onion–barley intercropped fields. In the first 2-year experiment, buckwheat, lacy phacelia, and coriander planted around barley-intercropped onion fields did not result in a significant reduction in the number of onion thrips. However, in the second experiment, which employed mixed intercropping of barley and buckwheat, significantly greater suppression of onion thrips population was observed in the mixed intercropping plots than in plots containing only barley intercropping. In addition, the number of syrphid eggs on intercropped barley was significantly higher in the mixed intercropping plots than in plots containing barley alone, demonstrating that planting flowering plants near barley can attract hoverflies and increase oviposition on barley. Furthermore, three-year experiments revealed more syrphid larvae on onion plants than on barley, with eggs found only on barley. These findings indicate that hoverflies oviposit on intercropped barley; then, the hatched larvae move to onion plants to prey on onion thrips. Overall, this study offers great insights into the potential use of intercropping with flowering plants to boost natural biological control of onion thrips, providing implications for sustainable pest management in onion production.