| Summary: | 碩士 === 國立臺灣大學 === 植物醫學碩士學位學程 === 101 === Red imported fire ant (Solenopsis invicta) was introduced into Taiwan in 2003 and spread rapidly across the northern Taiwan since introduction. To minimize negative impacts by fire ants, large-scale eradication program with intense bait broadcast was immediately carried out by multiple government agencies. It seems, however, the infested area continues expanding with the presence of numerous sporadic sites of fire ants outside the core infestation. Such situation can be considered as an endless dilemma of fire ant control and attributes to re-colonization by fire ants into bait-treated area from surrounding populations. Along with reducing budget recently, the eradication program therefore is no longer a feasible option, and long-term containment then appears to be a new management perspective. It has been suggested that area-wide suppression of fire ants is even more promising if toxic baits are teamed up with self-sustaining biological control agents, thus indicating that employment of biocontrol agents as supplement for bait treatment may hold the key for successful containment of fire ants in Taiwan. Paucity of natural enemies therefore leads to the present study where Photorhabdus luminescens (Taiwan isolate), a symbiotic bacterium harbored by a soil-born entomopathogenic nematode Heterorhabditis bravicaudis, and its potential were evaluated as a microbial control agent. Failure to re-culture the bacterium from cadavers of workers fed with non-filtered bacteria broth suggests bacterium-derived secondary metabolites as possible causes for such mortality. The average mortality reaches 94.66% in 10 days as workers are fed with liquid bait with bacterially filtered broth, while 98.94% for treated small colony fragments. Larvae in small colony fragments fed with filtered bacterial broth possess shriveled body and were found dead in few days after treatment, suggesting that they may receive secondary metabolites from nestmate through trophalallaxis, which is critical for controlling social insects. Semi-thin section reveals that most of internal tissue structure of brood cadaver is tattered or smashed, which possibly results from apoptosis induced by bacterium toxin (e.g., Makes Caterpillars Floppy Toxins, Mcf). Overall speaking, the present study shows that the secondary metabolites form P. luminescens are effective in controlling fire ant and are favored over bacterium itself in term of pathogen transmission simply because the digestive tract of ant workers are too narrow to take bacterium in. Furthermore, the secondary metabolites form P. luminescens may represent a new class of pesticides, and future studies should focus on its flexibility and compatibility with other pathogens or with conventional management approaches of fire ants .
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