The western flower thrips, Frankliniella occidentalis (Pergande), is one of the most important agricultural pests worldwide; it invaded China in 2003. Increasing F. occidentalis damage to vegetables has been observed in recent years. Several species of the genus Orius have been evaluated experimentally and established throughout Europe as potential biological control agents of these thrips. Orius similis Zheng is the dominant species in southern China, and this species appears viable as a controlling agent of F. occidentalis.
The functional response describes the rate at which a predator kills its prey at different prey densities and can thus determine the efficiency of a predator in regulating prey populations. As an assessment of the effectiveness of O. similis in controlling F. occidentalis populations, the functional response and searching efficiency of O. similis to F. occidentalis were studied at five kinds of space heterogeneity, and the population control of O. similis to F. occidentalis on pepper in cages in the greenhouse at five levels of spatial heterogeneity with enemy: pest ratios of 3:120, 6:120, 9:120, and 15:120. Spatial heterogeneity significantly affected the functional response of O. similis. The number of thrips eaten by O. similis increased with increasing density of F. occidentalis at every examined level of spatial heterogeneity. Predation rate increased quickly at lower thrips density and increased slowly at higher thrips density. The highest predation rate was observed in small bottles. The functional responses of O. similis to F. occidentalis fit Holling's typeⅡ equation under all conditions. Instantaneous attack rate and searching efficiency were used to assess the predation efficiency of O. similis on F. occidentalis. Higher instantaneous attack rates and searching efficiencies in O. similis predicted more efficient regulation of F. occidentalis populations at 5 different levels of spatial heterogeneity. A maximum instantaneous attack rate of 0.9204 occurred in small bottles, where searching efficiency was optimal. The lowest instantaneous attack rate of 0.6303 occurred in large bottles, where searching efficiency was lowest. Abundance of both larval and adult thrips decreased quickly at higher enemy:pest ratios, and O. similis abundance decreased when thrips density decreased. The ability of O. similis to control F. occidentalis populations was weak at the enemy: pest ratio of 3:120. Population control was stable for F. occidentalis adults, and control reached 50% for F. occidentalis larvae at five weeks with an enemy:pest ratio of 6:120. Control of F. occidentalis adults and larvae was 74.70% and 81.01%, respectively, at the enemy: pest ratio of 9:120. Rates of biological control reached 95.71% for F. occidentalis larvae and 89.02% for adults at the enemy: pest ratio of 15:120. These results indicate that O. similis hunted F. occidentalis more efficiently when space was small and spatial heterogeneity was low. The most suitable enemy: pest ratio was 9:120, which may result in better biological control at a relatively low cost.