Preliminary Studies On Trichogramma Species Suitable For Biological Control On The Rice Striped Stem Borer, Chilo Suppressalis (Lepidoptera:Pyralidae)

The striped stem borer, Chilo suppressalis (Walker), is the most economically important rice pest in Jilin Province, China. Currently the only control measures for this pest are the application of chemical insecticides, and the dosage of chemicals annually sprayed against them is the most in all pest species. The relative simple pest species in rice plantation in this region is suitable for applying biological control measures with natural enemies. With an the objective of screening suitable candidate species for controlling the striped stem borer, the effectiveness of four Trichogramma species (T. japonicum, T. chilonis, T. dendrolimi and T. ostriniae) collected from the local paddy fields were evaluated from the activity under different temperature and humidity regimes, parasitism on different age eggs of the rice striped stem borer and adaptability on alternative breeding hosts. Results as follows:1. The parasitism by four indigenous Trichogramma species on the rice striped stem borer under different temperature and humidity regimes.With the objective of screening suitable candidate species for controlling the striped stem borers, the parasitism of these four Trichogramma species on their native host eggs, C. suppressalis, was evaluated at five temperatures (18,22,26,30and34℃) and four relative humidity regimes (30,50,70and90%). The results indicated temperature and humidity significantly affected the parasitism of all species. Parasitism differed significantly among Trichogramma species at all temperatures tested with an exception of30℃. T. dendrolimi significantly parasitized more eggs of C. suppressalis than the other three species at18-26℃. At26℃, there was no significant difference in number of host eggs parasitized by T. japonicum, T. chilonis and T. dendrolimi. At30-34℃, however, T. japonicum parasitized most host eggs, followed by T. dendrolimi. T. japonicum parasitized most eggs of C. suppressalis at relative humidity regimes of30%-50%. However, there was no significant difference in number of host eggs parasitized by T. japonicum, T. chilonis and T. dendrolimi at relative humidity regimes of50%-70%. T. ostriniae parasitized least eggs at relative humidity regimes of30-70%. At90%RH, T. chilonis parasitized most host eggs, followed by T. ostriniae and T. japonicum, then T. dendrolimi. In contrast, T. dendrolimi as well as T. japonicum performed better within a wider-adapted temperature range. T. chilonis apparently performed better than the other species at70-90%RH, whereas, the relative narrow range of temperature suitable for performance (only at26℃) will be a limitation of candidates for T. chilonis.2. The parasitism by four Trichogramma species on different age eggs of the rice striped stem borer.In this experiment, we preliminarily investigated the effect of host-egg age (0,2,4day) on the parasitism by four Trichogramma species on their native host, C. suppressalis. The results indicated different age eggs of C. suppressali were all accepted by the four Trichogramma species with a clear tendency to parasitize less egg of C. suppressalis with increase of host age. The four Trichogramma species tested in the current study all exhibited a clear tendency to parasitize less eggs of C. suppressalis with increase of host age. When0-day old eggs were offered, T. dendrolimi parasitized the largest number of hosts, followed by T.japonicum and T. ostriniae, then T. ostriniae. Although the most hosts were also parasitized by T. dendrolimi on2-day old eggs, not significantly differ with other three parasitoid species. When4-day old eggs were offered,all four Trichogramma species parasitized low number of hosts. Overall, the parasitism of T. dendrolimi and T. japonicum on0-4day old of rice stem borer eggs was better than the other species.3. The adaptability by four Trichogramma species on alternative breeding hosts.When the eggs of Corcyra cephalonica were offered for intermediate hosts, all the four Trichogramma species could use them. The number of host eggs parasitized by T. ostriniae, T. chilonis and T. dendrolimi (56.0-59.7eggs/24h) is significantly higher than that parasitized by T. japonicum(37.5eggs). The development time of T. chilonis on the eggs of C. cephalonica was the shortest (8.7d), followed by T. japonicum (9.1d), then T. dendrolimi and T. ostriniae (9.5d).When the eggs of Antherea pernyi were offered for intermediate hosts, T. dendrolimi, T. ostriniae and T. chilonis could successfully parasitize them. However, only T. dendrolimi and T. chilonis in them completed development and successfully emerged from eggs of A. pernyi. The development time of both Trichogramma species did not significantly differ (approximately12d). The numbers of emergence holes and wasps of T. dendrolimi on the eggs of A. pernyi were significantly higher than those of T. chilonis. On the whole, the eggs of A. pernyi were the best breeding host for T. dendrolimi, followed by T. chilonis.Generally, T. japonicum and T. dendrolimi appear to be the candidate species for biological control on C. suppressalis under the microclimate of fluctuant temperatures combined with high humidity in paddy field.