Influence of sulfur, powdery mildew, and the predatory mite Phytoseiulus persimilis on two-spotted spider mite in strawberry

Abstract

Strawberry plants frequently suffer from simultaneous or sequential attacks of powdery mildew (Podosphaera aphanis) and the two-spotted spider mite (Tetranychus urticae), and for many years control of these two plant-damaging organisms have been achieved by use of sulfur and the predatory mite Phytoseiulus persimilis, respectively. Sulfur, predatory mites, and powdery mildew have long been studied separately regarding their impacts on spider mites, but knowledge of their combined influence in that context is also needed to aid integrated pest and disease management. Therefore, we conducted controlled laboratory experiments to study the main and the interaction effects of sulfur, P. persimilis, P. aphanis, and T. urticae in strawberry leaf disc bioassays. The results showed that the predatory mite, powdery mildew, and sulfur had additive effects on reducing egg number and population growth of T. urticae. Compared to the control, populations of T. urticae on the leaf discs were decreased by 83%, 76%, and 61% after 5days of treatment with sulfur+powdery mildew+predatory mites, mildew+predatory mites, and predatory mites alone, respectively. The survival and egg production of T. urticae females was affected to the greatest extent by P. persimilis, followed by powdery mildew. Residual sulfur had a short-term suppressive effect on T. urticae egg production but did not influence survival and reproduction of P. persimilis. If these findings also apply under field conditions, controlling powdery mildew will not reduce the need for spider mite control in strawberry, as has been suggested for other fruit crops.