Chemical control and resistance of the onion thrips (Thrips tabaci) in Israel

The onion thrips (OT) is a major pest in Israel and inflicts serious damage and significant annual losses in many Allium crops, especially by direct feeding and reducing leaf quality. Chemical insecticides are the main control method used for reducing OT populations. The insecticides that are commonly used are spinetoram, spinosad, emamectin benzoate and abamectin., In chives, less selective insecticides, such as carbosulfan and dichlorvos, are used between growing cycles. The susceptibility of OT populations to these insecticides has been monitored during the last four years across Israel. Many OT populations were found resistant to one or more of these insecticides particularly very high levels of resistance to spinosad. These resistances often resulted in control failures. The levels of resistance to some insecticides changed during and between seasons. Possible factors affecting resistance are the number of sprays, surrounding farms, climatic conditions and OT immigration. The levels of resistance to spinosad increased significantly under laboratory selection. The fecundity of the selected populations was positively correlated with the resistance to spinosad. Using cytochrome P450 monooxegenase, esterase and glotathion S-transferase inhibitors revealed the involvement of esterases in the resistance to spinosad. The response of susceptible and resistant OT laboratory populations to spinosad was tested under different temperature regimes. The results showed strong positive synergism between temperatures and the insecticide efficacy. To improve the resistance monitoring, we developed a fast bioassay for testing the susceptibility of collected OT populations to several major insecticides within 48 hours. This bioassay will be integrated into an IPM program for improving OT control in Israel.