| Summary: | 碩士 === 國立嘉義大學 === 生物資源學系研究所 === 101 === The impacts of abiotic factors on the pathogenicity of the infective juveniles (IJs) of Heterorhabditis brevicaudis DL49 (Nematoda: Heterorhabditidae) to beet armyworm, Spodoptera exigua Hübner, were studied. When IJs of H. brevicaudis DL49 were cultured at 15~35oC, seven days after treatment the survival rates around 90%, at 35oC, 14 days after inoculation the survival rates were the lowest at 52.5% and had significant difference among those cultured. The survival rates were 2%, 28 days after treatment at 30 oC while at 35oC, no live nematode was found, and the survival rates ca. 90% at 15oC. These data suggested that the optimal survival temperatures were between 15 and 25oC. At 25oC all the survival rates of H. brevicaudis DL49 at pH 3~11 were around 80% and no significant difference was found among different pH values; when temperature is raised to 30oC the fit pH dropped to pH 5~9. The survival rates were decreased when IJs of H. brevicaudis DL49 were exposed to 352 nm UV irradiation for 20 hours; the survival rate was ca. 9% after exposing 40 hours. The survival rate of H. brevicaudis DL49 was dropped to zero after rapid dehydration for 100 min. In contrast, after slowly dehydrating for 120 min, the survival rate was zero. Dehydration time and nematode survival rates were negatively correlated. The numbers of IJs emerged from host larvae were positive related with the body weight of infected host larvae (y = 117.93X2 +11.009X－0.304, R2=0.931) (X = S. exigua larvae weight (g), y = number of IJs). The pathogenicity of H. brevicaudis DL49 to S. exigua 3rd-5th instar larvae with different nematode concentrations (10, 30, 50, 80, 100 IJs/0.2ml) at temperatures 25, 30, and 35oC. The LC50 values of 3rd-5th instar larvae were 40.3, 15.8 and 12.8 IJs/0.2 ml at 25oC; 23.9, 11.7 and 8.5 IJs/0.2 ml at 30oC and 86.1,3.6 and 2.1 IJs/0.2 ml at 35oC, respectively. The older instar larvae were more susceptible to nematode infection than the younger instar larvae. The LC50 of pupae to H. brevicaudis DL49 were 163.3, 71.2 and 187.1 IJs/0.2 ml.
Inoculations of 3rd-5th instar larvae of S. exigua with 100 IJs/0.2 ml were tested at different temperatures. At 15oC, the mortality of S. exigua was zero; the LT50 values of H. brevicaudis DL49 against 3rd-5th instar larvae of S. exigua were 45.1、30.0 and 47.3 h；29.8、21.1 and 20.1 h；29.9、19.3 and 19.8 h, at 25, 30, and 35oC respectively. LT50 Values of various instars were decreased with increasing temperatures, only 3rd instar larvae decreased at 35oC. The mortality of S. exigua larvae was 90% when exposed to nematode for 8 hr. The symbiotic bacteria of H. brevicaudis DL49 was not pathogenic at 15oC, however the pathogenic was best at 30 oC.
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