Pyrethroid resistance in Phytoseiulus macropilis (Acari: Phytoseiidae): cross-resistance, stability and effect of synergists.

Phytoseiulus macropilis Banks (Acari: Phytoseiidae) is an effective predator of Tetranychus urticae Koch (Acari: Tetranychidae). The objectives of this research were to study the stability of fenpropathrin resistance and the cross-resistance relationships with different pyrethroids, and also to evaluate the effect of synergists [piperonyl butoxide (PBO), diethyl maleate (DEM) and S,S,S-tributyl phosphorotrithioate (DEF)] on fenpropathrin resistant and susceptible strains of this predaceous mite. The stability of fenpropathrin resistance was studied under laboratory conditions, using P. macropilis populations with initial frequencies of 75 and 50% of resistant mites. The percentages of fenpropathrin resistant mites were evaluated monthly for a period of up to 12 months. A trend toward decreased resistance frequencies was observed only during the first 3-4 months. After this initial decrease, the fenpropathrin resistance was shown to be stable, maintaining constant resistance frequencies (around 30%) until the end of the evaluation period. Toxicity tests carried out using fenpropathrin resistant and susceptible strains of P. macropilis indicated strong positive cross-resistance between fenpropathrin and the pyrethroids bifenthrin and deltamethrin. Bioassays with the synergists DEM, DEF and PBO were also performed. The maximum synergism ratio (SR = LC50 without synergist/LC50 with synergist) detected for the three evaluated synergists (PBO, DEM, DEF) was 5.86 (for DEF), indicating low influence of enzyme detoxification processes in fenpropathrin resistance.