Investigation on the synergism between environmentally friendly plant protection methods for suppression of onion thrips (Thrips tabaci Lindeman) and onion fly (Delia antiqua [Meigen]) on onion

Abstract: In 2023, we have conducted field experiment, where we have tested synergism
between different environmentally friendly plant protection methods against onion thrips
(Thrips tabaci) and onion fly (Delia antiqua) on onion. Field experiment took place at the
Laboratory Field of Biotechnical Faculty in Ljubljana. We have tested efficacy of seven
different treatments. Treatments include white clover (Trifolium repens) as intercrop, 4
different combinations of environmentally friendly plant protection methods, a negative
(untreated) and a positive control (use of insecticides). We have evaluated level of injuries
caused by onion thrips on onion leaves during growing season with 6-grade EPPO visual scale. The lowest level of injuries was recorded in treatments with white clover as intercrop.
Weakened plants caused by onion fly feeding were only detected on 21st June among four
evaluation dates. We have evaluated yield of healthy and rotten bulbs (infection with soil-borne pathogens has taken place through the boreholes of the pest’s larvae) in the field. After
harvesting, healthy bulbs were placed for 14 days into shadowed place for drying and for second evaluation of yield. To continue, the lowest yield of healthy bulbs was recorded in treatments with clover as intercrop (less than 3 t/ha). The highest yield of healthy bulbs was recorded in treatments, where we have tested synergism of spraying products containing entomopathogenic fungi and entomopathogenic nematodes in combination with sticky boards with lures for onion thrips and onion fly (more than 7 t/ha) and positive control (more than 6 t/ha). To conclude, we can say that alternative plant protection methods can offer alternative to classical insecticidal treatment against onion thrips and onion fly, but we have to focus on choosing the most suitable one or combination of several different plant protection methods as one treatment.