Comparison of pathogenicity of two entomopathogenic nematodes, Steinernema abbasi and S. carpocapsae, to Orgyia postica (Walker) (Lepidoptera : Lymantriidae)

Abstract

蟲生線蟲為微生物防治方法上具發展潛力之因子之一，本試驗藉由兩種蟲生線蟲對小白紋毒蛾(Orgyia postica (Walker))之致病力測試，評估於田間應用之潛用性。利用不同濃度之兩種蟲生線蟲(Steinernema abbasi ; Steinernema carpocapsae)懸浮液接種二齡至五齡小白紋毒蛾幼蟲，結果發現除五齡外，S. carpocapsae對二至四齡幼蟲所造成之半致死時間(LT50)均較S. abbasi為短。二齡幼蟲對於S. abbasi之感受性較低，死亡率為50-74.1%之間，其他齡期均可達到78.3-100%之死亡率。於四種溫度20、25、30及35°C下，以20 IJs/0.5 ml之線蟲懸浮液接種四齡幼蟲，則S. abbasi之LT50分別為61.2、42.8、29.9及21.6 h；而S. carpocapsae之LT50則為50.6、25.1、21.5及18.3 h，溫度升高時兩種線蟲之LT50也隨之縮短。模擬夏季與冬季之溫度與光週期下，分別接種20及30 IJs/0.5 ml兩種線蟲懸浮液於四齡小白紋毒蛾，夏季 (L : D = 13 h 20 min : 10 h 40 min ; 31 : 27°C) S. abbasi之LT50為43.1及38.8 h，S. carpocapsae之LT50為37.9及36.2 h；而冬季 (L : D = 11 : 13 h ; 21 : 16°C)時，S. carpocapsae之LT50為53.5及57.1 h，S. abbasi於冬季之死亡率僅達20及26.7%，此結果顯示兩種線蟲於冬季之效果較差，且S. abbasi不適用於該季節。於開放空間模擬田間試驗中，在早上及傍晚將300及500 IJs/ml之兩種懸浮液接種於茶樹上三齡幼蟲，發現施用時間所造成之死亡率無顯著差異，介於73.3-93.3%，S. abbasi在早上施用之LT50分別為46.6、48.1 h，而傍晚施用分別為57.8、45.1 h，而S. carpocapsae之LT50分別為38.4、34.9 h及37.3、41.2 h。上述試驗結果，提供蟲生線蟲對於小白紋毒蛾致病力之初步探討，且顯示其在田間防治上具有潛用性。

Entomopathogenic nematodes (EPNs) have been regarded as potential biological control agents against insect pests. In this study, two EPNs, Steinernema abbasi and Steinernema carpocapsae, were assayed to determine their pathogenicity against the small tussock moth, Orgyia postica (Walker) in the laboratory. When inoculated with different concentrations of nematodes to 2nd-5th instar larvae of O. postica, the LT50 values treated with S. carpocapsae were shorter than those with S. abbasi in most instars tested except the 5th instar. The cumulative mortalities of 2nd instar larvae treated with S. abbasi were 50-74.1%, while those of others were 78.3-100%. The LT50 values of S. abbasi against 4th instar larvae inoculated with 20 IJs/0.5 ml/larva at 20, 25, 30 and 35C were 61.2, 42.8, 29.9 and 21.6 h, respectively, while those of S. carpocapsae were 50.6, 25.1, 21.5 and 18.3 h, respectively. These results showed that the LT50 values declined as elevated the incubating temperatures inoculated with both nematodes. Inoculations of 4th instar larvae of O. postica with S. abbasi and S. carpocapsae at 20 and 30 IJs/0.5 ml/larva were conducted in artificial simulated seasons where summer was L : D = 13 h 20 min : 10 h 40 min ; 31 : 27C and winter L : D = 13 : 11 h ; 21 : 16C. The LT50 values infected with S. abbasi at 20 and 30 IJs/0.5 ml/larva under summer were 43.1 and 38.8 h, respectively, but the cumulative mortality under winter was only 20-26.7%. The LT50 values infected with S. carpocapsae at 20 and 30 IJs/0.5 ml/larva under summer were 37.9 and 36.2 h, respectively, whereas those under winter were 53.5 and 57.1 h, respectively. The pathogenicity of both nematodes in winter was lower than in summer. In opening space trials, when inoculated 3rd instar larvae with S. abbasi and S. carpocapsae at 300 and 500 IJs/ml, the cumulative mortalities resulted from applications in the morning and evening were not significantly different between two treatments. The LT50 values infected with S. abbasi at the same concentrations in the morning were 46.6 and 48.1 h, respectively, while those in the evening were 57.8 and 45.1 h, respectively. In S. carpocapsae, the LT50 values in the morning were 38.4 and 34.9 h, respectively, while those in the evening were 37.3 and 41.2 h, respectively. Therefore, it is suggested that both EPNs are potential to be applied as biocontrol agents against O. postica in field conditions.