INFECTION OF THE MIGRATORY GRASSHOPPER (ORTHOPTERA, ACRIDIDAE) BY INGESTION OF THE ENTOMOPATHOGENIC FUNGUS BEAUVERIA-BASSIANA

Infection of the migratory grasshopper, Melanoplus sanguinipes (F.), b y ingestion of Beauveria bassiana (Balsamo) Vuillemin conidiospores wa s investigated. Laboratory bioassays were performed where grasshoppers were fed a wheat-bran formulation containing B. bassiana spores. Gras shopper mortality was directly related to the ingested spore dosages. Probit analysis estimated LD50 at 3.4 x 10(5) conidia per host, with 9 5% FL from 2.1 x 10(5) to 5.5 x 10(5). The LT(50)s for the top 3 dosag e 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 th e 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 br ought 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 int ernal mouthpart surfaces of the grasshopper were the predominant sites of spore attachment and germination. The presence of numerous hairs a nd cavities on these structures greatly facilitates fungal spore attac hment and subsequent germination. It would appear that fungal invasion by way of the gut is an uncommon occurrence.