The invasion of entomopathogenic nematodes, Steinernema abbasi and Steinernema carpocapsae into Aedes albopictus larva, and defense reaction of the mosquito

Abstract

蟲生線蟲Steinernema abbasi與Steinernema carpocapsae感染白線斑蚊 (Aedes albopictus) 幼蟲與蛹，結果顯示S. abbasi與S. carpocapsae僅對三、四齡幼蟲有致病力，而一、二齡幼蟲與蛹均無致死效果；以S. carpocapsae感染單隻幼蟲之致病力較S. abbasi高，而感染多隻幼蟲則以S. abbasi之致病力高於S. carpocapsae。相同濃度之蟲生線蟲感染白線斑蚊三、四齡幼蟲，無論S. abbasi或S. carpocapsae均以感染單隻幼蟲之致病力高於感染多隻幼蟲之致病力。 探討蟲生線蟲之入侵途徑，以S. abbasi (104 IJs/ml) 感染白線斑蚊四齡幼蟲，發現將S. abbasi在水中感染白線斑蚊四齡幼蟲之途徑主要是經由口進入消化道後由胃盲囊侵入斑蚊幼蟲體腔內。利用蟲生線蟲S. abbasi感染白線斑蚊蛹體之模式，發現S. abbasi除了可經由喇叭狀的呼吸管進入蛹體外，亦可直接穿刺蛹表皮，此試驗顯示，雖然S. abbasi不具口鉤或齒等構造，然亦可侵入白線斑蚊蛹體。 蟲生線蟲S. abbasi與S. carpocapsae侵入白線斑蚊幼蟲體內皆可引發幼蟲之包囊反應，試驗結果顯示，受S. abbasi感染之白線斑蚊四齡幼蟲引發之包囊反應最多可形成31個完整的囊體，而以S. carpocapsae感染白線斑蚊四齡幼蟲則體內最多可發現有15個完整之囊體。然試驗亦發現，無論以S. abbasi或S. carpocapsae感染，只要單隻線蟲入侵白線斑蚊幼蟲體內，即有可能致死，在S. abbasi之試驗結果顯示，體內僅單隻線蟲所造成的死亡率高達81%，而S. carpocapsae所造成的死亡率則可達94%。 利用穿透式電子顯微鏡觀察蟲生線蟲S. abbasi感染白線斑蚊幼蟲，在其體腔內所引發之包囊反應。幼蟲受S. abbasi感染十分鐘後，可發現一層具有高電子密度之非細胞性均質物質，在線蟲的體壁聚集形成體液性囊體。感染一小時後，漿血球開始貼附於體液性囊體上，形成囊體外層之細胞性防禦反應。感染後24-48小時，細胞性囊體最外圍形成一圈類似基底膜之雙膜構造，包囊反應即終止。

The pathogenicity of entomopathogenic nematodes, Steinernema abbasi and Steinernema carpocapsae, to Aedes albopictus larvae and pupae showed that both S. abbasi and S. carpocapsae were pathogenic to 3rd and 4th instar larvae, there was no any pathogenic to 1st, 2nd instar larvae and pupae. The pathogenicity of S. carpocapsae was higher than that of pathogenicity in the experiment which there was only a larva in the container. However, the pathogenicity of S. abbasi was higher than that of S. carpocapsae in the experiment which there were thirty larvae in the container. The different combinations of nematodes and mosquitoes in inoculation caused the different mosquito mortalities; the more larvae inoculated in one container, the lower mortality was caused. The invasion route of S. abbasi into the hemocoel of 4th instar larvae of Ae. albopictus was found via the mouth and gastric caecum mainly. In pupa of Ae. albopictus, S. abbasi was able to penetrate into it through the spiracle or intersegmental membrane, although this nematode lacks mouth or penetrative hook. Encapsulation was induced by both S. abbasi and S. carpocapsae in the hemocoel of Ae. albopictus larva. The encapsulation capacity of Ae. albopictus larva was maximal 31 menalized-capsules per larva in S. abbasi and 15 in S. carpocapsae. Both S. abbasi and S. carpocapsae were able to kill the mosquito larvae by only one nematode invaded, in this context, the percentage of dead mosquito larvae was 81% as invaded by S. abbasi and 94% by S. carpocapsae. Ultrastructural examination of encapsulation of S. abbasi by Ae. albopictus larva was observed using transmission electron microscopy (TEM). The humoral encapsulation was primarily formed by electron dense homogenous structure without cellular material, which formed an inner layer of capsule enclosing S. abbasi 10 min after inoculation. About 1 hr after inoculation, the humoral encapsulaton was formed completely when plasmatocyte started to attach onto the surface of humoral capsule, which formed the outer layer of cellular capsule. Encapsulation process was terminated at 24-48 hr after IJs inoculation by formation of the basement membrane-like structure around the whole capsule.