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

Abstract

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.