## 4 Discussion  
The purpose of our study was to investigate the efficiency of the indigenous predatory mite _N. californicus_[10; 11] in a hop plantation, as allowed by the rules on biological plant protection [16]. Based on these rules, the use of native organisms is permitted for biological protection in Slovenia. _N. californicus_ is also efficient in cases when two-spotted spider mites do not exceed the damage threshold [5], which influences the choice of a biological control agent. At the time of the experiment, the weather conditions were favourable for the development of _N. californicus_ populations since the development of this predatory mite is most intense in the temperature range of 15-35 \({}^{\circ}\)C [5]. Because the temperatures in the experimental period did not exceed 30 \({}^{\circ}\)C, we also recorded fewer specimens of the two-spotted spider mite, whose development is known to last 8-12 days under optimal conditions (30-32 \({}^{\circ}\)C) [17]. On the day of the release of the predator, the average daily temperature reached almost 24 \({}^{\circ}\)C, and 46% R.h. was recorded. The first counting of two-spotted spider mites was performed at a daily temperature of 18.3 \({}^{\circ}\)C and after two rainy days. We assume that the population of two-spotted spiders could be higher in no-rain conditions since previous studies [18] reported that the _T. urticae_ population is restrained mainly by rainfall in the open field conditions. A third counting of the two-spotted spider mite population was also performed after five rainy days. On the day of release of the predator (4 July) and collecting cones (21 August), we recorded 20 days with R.h. values below 50%. It is known that the tested predatory mite is not sensitive to low R.h. [19]. The economic threshold depends upon the growth phases of hop [20]. At the beginning of June, we found 1-2 adult mites/leaf, while 5-10 adults per leaf can cause severe damage  
Figure 4: Average daily precipitation, average daily temperature and average daily R.h. (from 4 July 2015–31 August 2015). if they are found in mid-July. In the treatment that involved abamectin, we established ovicidal effects on two-spotted spider mites a week after spraying, while mobile specimens exhibited significantly fewer effects. The results indicate the emergence of resistance of harmful mites, which has been reported by some other studies [21,22]. Previous studies did not confirm the ovicidal effects of abamectin; rather, these studies mainly discussed the acaricidal mode of action of abamectin [23,24]. Ovicidal effects were also confirmed from the treatment in which we applied abamectin and hexythiazox a few days apart. It has been confirmed that hexythiazox acts as a growth inhibitor [25]. In our research, a hexythiazox-based acaricide was used in July, which was also demonstrated previously [26]. The selection of insecticides and fungicides in acaricides in our experiment was based on data demonstrating the influence of individual active substances on useful organisms obtained from the IOBC website [27], as our research primarily involved chemical substances that do not have effects or display little effect on predatory mites. The exception was the systemic insecticide imidalcoprid, which we used in the control treatment against hop aphids, and it also displayed effects on two-spotted spider mites [28] and the predatory mite _N. californicus_[29]. The second application of the acaricide Nissorun in the Acaricide 2 treatment also did not prove to be an efficient measure for suppression of two-spotted spider mites, which additionally confirms the hypothesis that natural enemies of two-spotted spider mites have a role in suppression strategies targeting this harmful pest on hop plants. The optimal effects of the predatory mite were recorded 14 days after release, when we detected the lowest number of eggs and mobile forms of the two-spotted spider mite. This observation can be attributed to the usual lifespan of this predator, which is 20 days [30].