本土產蟲生線蟲(Steinernema abbasi)跳躍行為及寄主搜尋能力篩選之研究

Abstract

Effects of the nematode numbers tested and jumping distance on the jumping behavior of Steinernema abbasi (Sa), and selection of its searching ability strains through generations were investigated in this study. When tested different numbers of Sa with 45 mm distance from the lid of an experimental tray, their jumping percentages were significantly different between the groups with 1 x 105 or 5 x 104 infective juveniles (IJs) (56.4 and 40.6%) and 1 x 104 or 5 x 103 IJs (2.9 and 0.1%). With different distance (21, 33, 45, 57 mm) from the lid, the percentages of jumping IJs were 25-33.1 % in 21 to 45 mm, but only 15.5% in the 57 mm. The LT50 values of Spodoptera litura 5th instar larvae inoculated with various IJs having different jumping abilities at 15 IJs/0.5 ml/insect were 22.1 to 23.7 h, whereas those of IJs without jumping behavior were significantly longer at 26.5 h. However, all treatments could cause 100% host mortality. At 30 IJs/ml/insect, the LT50 values tested with IJs showing different jumping abilities were 22.1 to 23.7 h, but prolonged to 26.5 h in those without jumping behavior. When compared with Sa0, the IJs subjected to five rounds of selection produced a 2-fold increase in successful host-finding against Galleria mellonella, while those of 8 or 12 round selection increased to 3 and 6 fold, respectively. Distribution of nematodes with positive moving ratios were 40.6 in Sa0 and 52.7% in Sa5 at 6 cm from the host insect, 25.7% in Sa0 and 54.8% in Sa8 at 9 cm, and 26.4% in Sa0 and 61.1% in Sa12 at 12 cm. The percentages of Sa0 and Sa12 (12 cm) succeeded in host-finding against G. mellonella were significantly different at 4.6 and 15.4%, respectively, while the ratios against S. litura were not significantly different at 0.8 and 3.2%, respectively. When inoculated with 200 IJs/ml/insect, the shortest LT50 of S. litura 5th instar larvae was 20.9 h in Sa5 at 6 cm vertical distance in the soil, all treatments could cause 95% mortality. When increased the distance from the host from 6 cm to 9 cm, only Sa8 could reach 70% mortality, its LT50 being 41.6 h. However, the mortalities of other treatments were less than 50%. Comparing different searching strategies, the percentages of positive movement of ambusher, Steinernema carpocapsae (Sc); intermediate, Sa0, and cruiser, Steinernema arenarium (Sare), were 40.7, 26.4 and 0.7%, respectively, but the negative movement of Sare at 95% was much higher than others. If the nematodes were provided with S. litura as their host, the positive movement proportion in Sare increased to 10.9%. Percentage of positive movement in Sa12 (12 cm) was 61.1%, being the best strain showing searching ability in all treatments. By observing the jumping behavior of IJs in the non-selected parental strain (Sa0) and those after 5, 8, 12 rounds of selection, the highest percentage of jumping was 40.8% in Sa5 (6 cm), while the lowest was 10% in Sa12 (12 cm), indicating that selection of horizontal host-finding could not reflect the vertical jumping behavior of the same strain. A well host-finding strain of S. abbasi was discovered in this study, and its jumping and host-finding behaviors were observed. These results could possibly be useful for the pest control using entomopathogenic nematodes.