Spider mite suppression in herbs by Phytoseiulus persimilis and Neoseiulus californicus

Abstract: Since few years the Austrian consumers could choose among a great variety of fresh herbs in supermarkets. However, a short look in the plant protection product register reveals, that the possibilities for the producers to suppress pest species in herbs by beneficials are strongly limited. The prime reason for this indication gap is the lack of efficacy data on potential beneficials against thrips, spider mites or white flies in herbs. Here, we evaluated the suppression potential of Phytoseiulus persimilis and Neoseiulus californicus against the two-spotted spider mite Tetranychus urticae on lemon balm plants. Three treatments were conducted in separate greenhouse compartments: (1) T. urticae, (2) T. urticae + P. persimilis, (3) T. urticae + N. californicus. Spider mite population development and biomass of the plants after finishing of the experiments were compared among treatments. Although both predator species were able to suppress the spider mites within 9 weeks, spider mite densities decreased earlier in the treatment with P. persimilis. Additionally, the biomass of the plants treated with P. persimilis was higher than that in the treatments without P. persimilis. Two conclusions can be deduced from these results for practical aspects of biological spider mite control in herbs: first, both phytoseiid mite species can be used for spider mite control in herbs, however, P. persimilis, highly specialized preying on spider mites was more efficient than the generalist N. californicus. Second, a suppression of spider mites after 9 weeks is not acceptable in practice because of high crop loss. Consequently, the ratio spider mites: beneficials was too high in our experiments. The optimal release rates of the beneficials in herbs will be determined in experiments under practical conditions.