Five field and greenhouse populations of the twospotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae), were collected from five different districts across the island of Cyprus, both in field and greenhouse crops, and tested to determine levels of resistance. Standard leaf-disk spray application bioassay procedures were used to determine the LC50s for five chemicals: abamectin, acrinathrin, fenazaquin, pirimiphos methyl, and bifenazate. Selection of these compounds was based on the widespread use by farmers as well as on the frequent control failures against T urticae reported in the past. Resistance of T. urlicae was detected to abamectin, acrinathrin, fenazaquin, and pirimiphos methyl. The resistance ratios were calculated relative to the German susceptible reference strain. The highest resistance ratios at LC50 value were recorded for abamectin in a greenhouse rose population (RR = 3822), followed by a field bean (RR = 1356) and field tomato population (RR = 1320). Significantly high resistance levels were also found for acrinathrin where the highest resistance ratios at LC50 were recorded in a field bean T. urticae population (RR = 903). For fenazaquin, the highest resistance levels were recorded in a field tomato population (RR = 310). Lower resistance levels were found for pirimiphos methyl (13.3 < RR < 77.4) in all populations. Low susceptibility of T. urticae was observed for bifenazate (2.7 < RR < 24.4) in all populations. These results suggest that at least the use of abamectin and acrinathrin should be avoided or minimized for the control of T. urticae populations in indoor and outdoor environments.
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Vol. 106 • No. 4