Evaluation of toxicity of Acmella oleracea extract against Tetranychus urticae in laboratory tests

Tetranychus urticae Koch is a polyphagous pest causing serious damage to crops. The uncontrolled use of synthetic acaricides have caused the development of resistant strains of the tetranychid making its control tricky. Research on botanical pesticides showed that these products could be valid alternatives to synthetic ones in IPM programs. Herein, the toxicity of Acmella oleracea (L) R.K. Jansen (Asteraceae) extract was assessed on T. urticae in laboratory tests. Acmella extract was prepared by Soxhlet extraction using n-hexane as solvent and it was characterized by N-alkylamides, being spilanthol the major constituent, as determined by HPLCDAD-MS analysis. The extract was tested on young adult females of T. urticae at concentrations of 312.5, 625, 1250, 2500, and 5000 μL/L. The extract showed high toxicity on T. urticae females at all concentrations. All females died within 4 days after the treatment with the two highest concentrations, whereas 22 and 20% of females remained alive at the end of the tests (8th day) with the two lowest concentrations. It is noteworthy that, in tests with the two lowest concentrations (312.5 and 625 μL/L), alive females oviposited at low levels within the first 4 days. Afterwards, their oviposition did not differ from that registered in the control. Acmella extract showed high toxicity against T. urticae females and a considerable rapidity of action at the highest concentrations. The short persistence of the extract on treated leaves should be considered an advantage within IPM strategy because of the short-time effects on natural enemies.