Development of biological control method using natural enemy Scolothrips takahashii Priesner against Tetranychus urticae Koch on strawberry in nursery period

Strawberry (Fragaria × ananassa Duchesne) is a major crop in Japan. One of the most serious pests hindering strawberry production is the two-spotted spider mite Tetranychs urticae Koch. T. urticae causes serious damage to strawberries, such as stunting of plant growth and reduction in quality and yield. In Fukuoka, chemical method can't control T. urticae effectively. First, I monitored the effects of several acaricides on T. urticae. Then, to establish IPM program with natural enemy in strawberry, I studied about dominant species composition of natural enemies during nursery period of strawberry and evaluated about control effect of the predominant natural enemy as a control of agent of T. uriticae. Finally, I investigated about the spider mite control effect of the conservation of natural enemy by using selective pesticide (treatment) in the agricultural fields. 1. I investigated the effects of seven acaricides on the two-spotted spider mite, Tetranychus urticae Koch (green form) collected from nine strawberry greenhouses in Fukuoka Prefecture, with the leaf disc dipping method. All the tested females were barely susceptible to many acaricides. These results showed acaricide resistant populations have become established in all fields. The present results suggest that T. urticae Koch on strawberry are resistant to these acaricides and it would be difficult to control this mite with only agrochemicals in Fukuoka Prefecture. 2. To validate the possibility of IPM utilizing natural enemies against T. urticae Koch which have the resistant to these acaricides during a nursery period of strawberry, I investigated the species composition of their natural enemies both on kidney bean traps infested with T. urticae Koch and on strawberry seedlings in four nursery fields. While Scolothrips takahashii and phytoseiid mites were predominantly captured on the traps, the predator was the only natural enemy observed on strawberry seedlings in all fields. Even when the densities of spider mites on strawberry seedlings were relatively low, the predatory thrips were observed frequently. These results suggest that S. takahashii could be useful to control spider mites in IPM for a seedling-rearing period of strawberry. 3. To determine whether the predatory thrips S. takahashii can be used as a control agent of the two-spotted spider mite T. urticae Koch at strawberry nurseries in summer, I examined the effects of releasing S. takahashii on populations of T. urticae. In S. takahashii release treatments, T. urticae numbers decreased to almost zero within 24 days and were significantly lower than those in the control treatment at 14 days; the decrease pattern of T. urticae populations in these experimental treatments was similar to that seen with P. persimilis release. Then we also examined the spider mite control effect of the conservation of S. takahashii by using selective pesticide (treatment) in comparison with the non-selective pesticides control method which excludes the predator (control) in an experimental field. The number of spider mites in the S. takahashii conservation treatment was significantly lower than that in control by the conservation of S. takahashii. These results suggest that S. takahashii can be an effective control agent against T. urticae in integrated pest management programs to protect strawberry plug plants in summer. 4. Although S. takahashii Priesner is an important predator of spider mites in strawberry, little is known about their susceptibility to pesticides. I examined the influence of 26 pesticides commonly used in strawberry production on the survival of S. takahashii larvae by using a leaf-dip bioassay method in a laboratory. Acetamiprid, nitenpyram, thiacloprid, pyridalyl, spinosad and lufenuron were harmful to the predators, because their mortality rates 48 h after the applications were 100%. In contrast, other 20 pesticides were harmless because of the low mortality rates (0-6. 3 %). I also examined the residual effectiveness of the six harmful pesticides and found that pyridalyl and lufenuron remained extremely effective up to 10 day after the treatments. 5. To evaluate integrated pest management (IPM) utilizing natural enemies S. takahashii against T. urticae Koch on strawberry during the seedling-rearing period, we investigated the seasonal occurrence of spider mites and their natural enemies on strawberry plug plants in the three agricultural fields to which selective insecticides had been sprayed during 2012-2014. In the agricultural field studies, S. takahashii was the only natural enemy observed on strawberry plug plants while the spider mite numbers was low density in all fields in each year. These results suggest that S. takahashii was an effective control agent against the spider mite, and could contribute to the development of IPM for strawberry during the nursery period. This is the first report about utilizing natural enemy S. takahashii in strawberry in Japan. These research results will contribute to control T. urticae effectively and reducing acaricides applying time, avoidance lowering acaricides sensitively.