| Interspecific competition between parasitoids is a key factor to impact the community stability of the system of plant, insect and natural enemy. Meanwhile, the interspecific competition is also an important aspect to select and release effective natural enemies in the agro ecosystem. Encarsia sophia and E. formosa are important parasitic insects against whiteflies. Previous studies indicated that the host-feeding parasitoid E. sophia(Girault and Dodd), a heteronomous hyperparasitoid, reproduces by arrhenotoky with males developing as secondary ectoparasitoids on females of their own or of other Encarsia and Eretmocerus species, particularly selects E. formosa used as secondary hosts. We have also found that E. sophia adults are destructive host feeders like E. formosa, and they exhibit stronger host-feeding capacity than other whitefly parasitoids. The number of whitefly nymphs killed through host feeding by E. sophia is similar to that by parasitism. With the objectives of evaluating the ecological safety of releasing E. sophia and providing theoretical evidence for applying them against whiteflies, life tables for E. sophia and E. formosa were established, and interspecific competition between them and the impacts on suppression of T. vaporariorum in the laboratory and field were investigated. Main results as followed.1. The experimental population life tables for E. sophia and E. formosa on T. vaporariorum were established. Based on the parameters of their life tables, the values of R0, T, rm and λ for E. formosa and E. sophia were 171.53, 18.03, 0.2854, 1.3303, and 61.6, 16.1991, 0.2544, 1.2897, respectively. Generally, various characteristic parameters of E. formosa were higher than those of E. sophia, and there was a higher increasing tendency for E. formosa population. When adequate whitefly nymphs were offered, total number of hosts parasitized by a female adult of E. formosa was 2.7 times than that by E. sophia. Although total number of whitefly nymphs killed through host feeding by E. sophia(60.57) was significantly higher than that by E. formosa(42.71), total whitefly nymphs killed though parasitism and host feeding by E. formosa(220.80) was significantly higher than that by E. sophia(127.93).2. After Encarsia sophia and Encarsia formosa were released at ratios of 10:0, 8:2, 5:5, 2:8 and0:10 for controlling the greenhouse whitefly in the laboratory, Trialeurodes vaporariorum on tomato, their interspecific competition interactions and the impacts on suppression of whiteflies were investigatged. The percentage of E. formosa in total parasitoids was always kept about 50% after 60 days of both parasitoids were released at different ratios. Releases of any one parasitoid specie only or both of the parasitoids at different ratios provided low levels of whitefly population. Especially, the population of T. vaporariorum was always effectively suppressed with more than 83% whiteflies killed by parasitism and host feeding after E. sophia and E. formosa were released at a ratio of 5:5. The results indicated that the heteronomous hyperparasitoid E. sophia can’t replace the primary parasitoid E. formosa on T. vaporariorum. The competitive interactions between the two parasitoid species do not reduce the efficiency of whitefly suppression.3. In the field, we investigated the suppression effectiveness of E. sophia in combination with E. formosa on T. vaporariorum. The results showed that releases of any one parasitoid specie only or both of the parasitoids provided low levels of whitefly population. Generally, whitefly population always kept a stable and low level for the treatments of releasing E. formosa only and E. sophia in combination with E. formosa, in which whitefly density always kept 0.51-0.95 nymphs/cm2 on tomato leaf. At various investigation date, there was no significant difference in percentage of whitefly nymphs killed through parasitism and host feeding between releasing E. formosa only and E. sophia in combination with E. formosa. However, there was an unstable change of whitefly density for releasing E. sophia only. On 20 and 60 d after releasing parasitoids, the whitefly density for releasing E. sophia only was significantly higher than those for releasing E. formosa only and E. sophia in combination with E. formosa. |
