Evaluation of efficacy and application method for control of onion thrips (Thrips tabaci) in dry bulb onions in Washington state

Onion thrips (Thrips tabaci) are a persistent pest of onions worldwide and are the key pest of onions in Washington State. The objectives of our project were to evaluate insecticide efficacy and application method for thrips control in onions. During 2013, 2014 and 2015 onions (var. Tamara) were established at the WSU Pasco Research Farm and grown using standard grower practices. Plots (7.5 feet wide and 30 feet long) were established in a random complete block design with four replications. Efficacy was evaluated by counting the number immature and adult thrips per plant on 10 individual plants per plot in the field. Additionally, bulbs from each plot were sized and yield determined for each onion size class to determine the impact of the treatments on bulb size and yield. All data for each sample date was analyzed by ANOVA and treatments compared to non-treated controls in pairwise t-tests. Thrips populations were significantly reduced with applications of products including methomyl, spinetoram, spirotetremat, abamectin, and cyazypyr. Bulb size profile and yield was also significantly impacted by effective insecticide applications. Application of insecticides by chemigation was effective with methomyl, spinetoram, and cyazypyr.