Synergy between plant essential oils and Pseudomonas isolate secondary metabolites against the two-spotted spider mite Tetranychus urticae Koch

Abstract

In the pursuit of developing effective bio-acaricidal agents, the toxicity by residual contact and repellency of the essential oils (EOs) of Origanum compactum, Mentha piperita and Artemisia herba-alba and their mixtures against adults of Tetranychus urticae was investigated. The oils demonstrated repellency and toxicity to mites based on dose. Combinations of the EOs in binary and ternary forms had antagonistic and additive effects on the mite's toxicity and repellency, with the exception of combined O. compactum and M. piperita EOs that displayed a synergistic acaricidal relationship. The chemical composition of the oils was identified via GC/MS analysis; major constituents were thymol (49%) in O. compactum, linalool (45.81%) in M. piperita and camphor (25.08%) in A. herba-alba. Further, we evaluated the acaricidal activity of secondary metabolites produced by Pseudomonas species using cell-free supernatants, and reported on the mode of entry of Pseudomonas supernatants into adult mites. The bioactive acaricidal compounds were most effective (57.6 to 96.1% mortality) when the entire integument of T. urticae was contacted compared to the ventral side only (15.3 to 80.7%). The toxicity of the most active bacterial supernatant was investigated at 1/2, 1/4 and 1/8 dilutions in combination with the EOs. The combinations synergistically killed the mite. Lethal effects were seen 72h after exposure. Bio-acaricidal formulations may take use of synergistic combinations to manage the two-spotted spider mite.