干旱区生态环境知识资源中心

Infection of the migratory grasshopper, Melanoplus sanguinipes (F.), by ingestion of Beauveria bassiana (Balsamo) Vuillemin conidiospores was investigated. Laboratory bioassays were performed where grasshoppers were fed a wheat-bran formulation containing B. bassiana spores. Grasshopper mortality was directly related to the ingested spore dosages. Probit analysis estimated LD50 at 3.4 x 10(5) conidia per host, with 95% FL from 2.1 x 10(5) to 5.5 x 10(5). The LT(50)s for the top 3 dosage treatments where final mortality exceeded 50% were 9.6, 12.5, and 18.4 d for the respective dosages of 1.1 x 10(6), 5.4 x 10(6), and 1.0 x 10(7) conidia per host. The LT50 for the highest dosage (9.6 d) in the feeding bioassay was greater than that seen for insects immersed in a liquid inoculum but lower than for insects that had only their heads dipped. These data suggest that as a result of feeding, spores are brought to favorable infection sites within the head and mouthparts. The nature and sites of fungal infection by feeding were elucidated using epifluorescence and scanning electron microscopy. The exposed and internal mouthpart surfaces of the grasshopper were the predominant sites of spore attachment and germination. The presence of numerous hairs and cavities on these structures greatly facilitates fungal spore attachment and subsequent germination. It would appear that fungal invasion by way of the gut is an uncommon occurrence.