| Summary: | 碩士 === 國立中興大學 === 昆蟲學系 === 93 === The virulence of a symbiotic bacterium, Xenorhabdus sp., of entomopathogenic nematode, Steinernema abbasi, was compared with Xenorhabdus nematophilus of Steinernema carpocapsae, to Spodoptera litura larvae was conducted in this study. The larvae were killed after injection with five doses of two symbionts, and the mortality rate was increased depending on post injection times. The LD50 values at 24 h and 48 h post incubation time were 99 and 146.5 cells/larva with Xenorhabdus sp., and 10 or lower and 6,037 cells/larva with X. nematophilus, respectively. When injected with 105 cells of Xenorhabdus sp., the haemocytes count of S. litura larvae was reduced with the incubation time. In the time sequences of invading test S. carpocapsae invaded first followed by S. abbasi, and S. carpocapsae symbiont was recovered from S. litura dead larvae. In contrast, when S. abbasi invaded earlier than S. carpocapsae, Xenorhabdus sp. would recover from dead insects while Xenorhabdus sp. could inhibit the growth of X. nematophilus in a co-culture test. Some 6th instar larvae of S. litura were dead when injected with Xenorhabdus sp. cultured for 72 h. Even though the culture was treated with heat, few injected larvae were dead. In addition to insecticidal activity, the culture of Xenorhabdus sp. could inhibit the growth of four human pathogenic bacteria including Staphylococcus aureus, Escherichia coli, Klebsiella pneumoniae, and Vibrio parahaemolyticus. Using Biolog and API 20E tests, two symbionts differed in 10 carbon source exhibitions and 1 Gelatin reaction, respectively. Xenorhabdus sp. showed brown colonies on YDC agar while X. nematophilus was off-white ones. All of these assays indicated that the symbiont in S. abbasi varied with X. nematophilus.
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