Selection for resistance and susceptibility to methidathion and cross resistance in Amblyseius womersleyi Schicha (Acari: Phytoseiidae)

Artificial laboratory selections for resistance and susceptibility to methidathion were performed in Amblyseius womersleyi Schicha. After four selections for resistance and three selections for susceptibility, the resistance ratio (R/S) at the LC50 increased from 16 to 342. The toxicity of eight pesticides was evaluated in the R and S strains of this phytoseiid mite. The highest resistance ratios were observed for the insecticides methidathion, acephate, and malathion, for which the R strain was respectively 311, 20.4, and 13.1 times more resistant than the S strain. In the case of the pyrethroid bifenthrin, no cross resistance was observed for this chemical.