Comparison of banks grass mite and twospotted spider mite (Acari: Tetranychidae): Responses to insecticides alone and in synergistic combinations

This research was designed to evaluate responses by Banks grass mite, Oligonychus pratensis (Banks), and twospotted spider mite, Tetranychus urticae Koch, to insecticides alone and in different combinations using 24-h vial bioassays. The twospotted spider mite was more tolerant to many insecticides than the Banks grass mite. Amitraz and S,S,S-tributyl phosphorotrithioate (DEF) synergized 2 organophosphorous insecticides, diazinon and dimethoate, and the pyrethroid bifenthrin against the Banks grass mite. Piperonyl butoxide (PB) antagonized the miticidal activity of esfenvalerate against the Banks grass mite. In contrast to twospotted spider mites, amitraz did not synergize the organophosphorous insecticides, and piperonyl butoxide was a good synergist of the pyrethroid esfenvalerate (98.3-fold increase). DEF synergized bifenthrin and dimethoate against the twospotted spider mite. Mixtures of organophosphorous + pyrethroid mixtures were generally better for twospotted spider mites than Banks grass mites. Insecticide toxicity of the organophosphorous + pyrethroid mixtures was increased from 2.3 to 18.2 times. The synergistic and antagonistic activity of the insecticide + synergist mixtures and organophosphorous i pyrethroid mixtures resulted in twospotted spider mites being less tolerant to several mixtures than were the Banks grass mites. A 3-chemical mixture of dimethoate and bifenthrin with amitraz or piperonyl butoxide caused 52.7- and 94.7-fold increases in toxicity against the twospotted spider mite. The synergistic activity by the chemical mixtures suggest that metabolic degradation and target site insensitivity may be involved in twospotted spider mite resistance. Further studies are needed to improve our understanding of the specific mechanisms in each spider mite species for each organophosphorous and pyrethroid insecticide. The proper insecticide mixture may be an effective tool for managing Banks grass mite and twospotted spider mite infestations.