Selection for Abamectin Resistance in Tetranychus urticae and T. pacificus (Acari: Tetranychidae)

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Abstract
Three colonies of Tetranychus urticae Koch and two colonies of Tetranychus pacificus McGregor were selected in the laboratory and greenhouse with abamectin (avermectin B10). Two selection methods were used. One challenged ability of gravid females to survive 48 h on dried residues using concentrations that resulted in 40–60% mortality. The other technique selected against all life stages on bean plants. Concentration/response bioassays were used to determine whether a selection response was achieved. The three T. urticae colonies were selected 4, 6, or 15 times. LC50’s for two of these colonies before selection ranged from 0.06 to 0.15 ppm on different dates. After selection, the T. urticae colonies LC50’s were 0.12 and 0.24 ppm (values not significantly different from those of their respective base colonies). Likewise, for the two T. pacificus colonies (selected 2 and 15 times), no response to selection occurred. LC50’s of the unselected T. pacificus base colonies ranged from 0.04 to 0.12 ppm on different dates. Selected lines had LC50’s of 0.06 and 0.08 ppm, values not significantly different from LC50’s of their base colonies. These negative results do not preclude the possibility that resistance to abamectin will develop in other populations of T. urticae or T. pacificus. Because at least one of the T. urticae and T. pacificus colonies tested was resistant to propargite, cyhexatin, and fenbutatin-oxide, we suggest that cross resistances are unlikely.