In 2022, we conducted an onion experiment at the Laboratory Field of the Biotechnical Faculty in Ljubljana, where we studied the insecticidal activity of 4 inert dusts (quartz sand, zeolite, diatomaceous earth and wood ash) and a plant powder of the tree of heaven on the important onion pests, the onion thrips (Thrips tabaci Lindeman) and the onion fly (Delia antiqua Meigen). The field was divided into 3 blocks within which 7 treatments were randomly assigned. We also had a positive control, where we used a registered synthetic insecticide, and a negative control, which was not treated with the above mentioned agents. The onion spraying was carried out over 6 dates. During the growing season, damage to the plants caused by tobacco thrips and onion fly was assessed using the standard EPPO method. At harvest, in each treatment, the crop was separated into rotten and healthy bulbs, counted and weighed. They were dried separately for 3 weeks and the process was repeated after drying. The extent of damage caused by tobacco thrips was similar in all treatments and the most effective method was the use of a synthetic insecticide. On average, 1 to 2 rotten bulbs due to onion fly infestation were found in all treatments at each inspection. The overall lowest number of rotten bulbs were when we used wood ash. In the field crop, there were more rotten than healthy bulbs in each treatment, with the positive control crop being the healthiest on average. After drying, the average yield of the wood ash treatment had the most healthy bulbs, while the yields in the other treatments started to rot more severely during drying. The synthetic insecticide inhibited to a certain extent the attack of the studied pests, but rotting increased sharply during the drying period. The use of wood ash also proved to be relatively effective against the onion fly. The synthetic insecticide inhibits the attack of the studied pests to a certain extent, but during the drying period, rotting increased significantly. The use of wood ash also proved to be relatively effective against the onion fly.
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