Effect of Different Cultivars of Potato on Population Density of the Onion Thrips, Thrips tabaci Lindeman and its Natural Enemies under Field Condition in Ardabil Province

In Iran, potato Solanum tuberosum L. is an important crop, and its cultivation extends over more than 186,000 ha annually. The onion thrips, Thrips tabaci Lindeman, is a highly polyphagous species and a serious pest of a wide range of economically important crops. In Iran, T. tabaci causes damage on different agricultural crops including: potato, onion, cucumber and tobacco. Onion thrips feeds by piercing the surface of the tissues (leaves and leaf buds) and sucking up the exuded cellular contents. On infested leaves, the empty cells create silvery-white spots, silver damage, which cause yield loss. Use of insecticides for controlling T. tabaci has low efficacy because females lay eggs within leaf tissues, larvae hide in leaf domatia and between the inner leaves of plants, and pupae rest in the soil. Therefore, it is necessary to use alternative management approaches for control of T. tabaci such as resistant cultivars integrated with natural enemies. The interactions between natural enemies and resistant cultivars have a direct effect on the pest by effecting its developmental rate, fecundity and survival rate. They may also have an indirect effect on natural enemies by increasing generation time of the pest and, as a result, increasing exposure time of the pest to its natural enemies. In this study, we assumed that the different potato cultivars affect population density of T. tabaci and diversity of its natural enemies. Therefore, the present research was designed to evaluate the onion thrips density and species diversity of its natural enemies on four potato cultivars under field condition.

Tubers of four potato cultivars including Agria, Savalan, Arensa and Fantasia (commonly cultivated in Ardabil region) were selected for evaluation of population density of T. tabaci and diversity of its natural enemies under field condition during 2011-2012. The tubers were obtained from the Seed and Plant Improvement Institute of Karaj, Iran. Tubers of four tested cultivars of potato were planted in four experimental fields, each of 500 m2,at four different locations in the Ardabil plain. The fields were managed according to the local practice with flood irrigation and hand weeding once every ten days. No insecticides were applied in these fields. The onion thrips populations were monitored on the four cultivars of potato from late stem elongation to petal fall stages by examining four plants from each cultivar chosen randomly from each of four fields (16 plants per cultivar). The numbers of thripslarvae and adults and each of natural enemies per plant were counted using a 20X hand lens. Moreover, onion thrips and its natural enemies were collected from each of potato cultivars, transferred to the laboratory, and identified to species by morphological characteristics under a stereomicroscope or a microscope according to the valid identification keys. The Shannon diversity index (H), Shannon evanesces index (E), and Morisita-Horn index (CMH) were then calculated for each of four potato cultivars by use of EstimateS Win 8.20 software (6). Data on population densities of onion thrips and its dominant natural enemies were analyzed using one-way ANOVA and significant differences among cultivars were compared using Tukey's HSD test. 

In two years, the lowest densities of onion thrips larvae and adults were observed on Agria. In this study, 21 species of onion thrips predator were collected and identified. Amongst the predator species, Aeolothrips intermedius Bagnall and Orius niger (Wolff) had the highest relative abundance on each of four potato cultivar. The predator A. intermedius with small size and ability to penetrate into leaf domatia, faster development rate and higher fecundity is effective in control of herbivore thrips on host plants (19). Moreover, previous researches have reported that the species of Orius are efficient in control of onion thrips. The Shannon diversity index for the complex of predators was greatest on Agria. Species richness and their relative abundance are two main factors for measuring Shannon diversity index in each ecosystem. Moreover, the values of Morisita–Horn index for the complex of predators between the four cultivars of potato ranged from 0.948 to 0.989. The value of this index varies between 0 and 1. When the value of this index increases from 0 to 1, the species complex of predators between two cultivars becomes more identical. In two years, densities of two predator species A. intermedius and O. niger on Agria and Savalan were significantly higher than on Arensa and Fantasia. Furthermore, the greatest percentage of females of two predator species was observed on the thrips-infested plants of Agria. These results indicated that the preference and performance of two mentioned predators were higher on Agria and Savalan among the tested cultivars.

Based on the results of our study, it can be concluded that the cultivation of Agria could be useful in reduction of T. tabaci population and conservation of its predators in potato fields.