ENTOMOPATHOGENICITY OF SEVERAL FUNGI TOWARD THE ENGLISH GRAIN APHID (HOMOPTERA, APHIDIDAE) AND ENHANCEMENT OF VIRULENCE WITH HOST PASSAGE OF PAECILOMYCES-FARINOSUS

Virulence of the entomopathogenic fungus Paecilomyces farinosus was tested against the English grain aphid, Sitobion avenae (F.), before and after selective subculturing of the fungus. When the stock culture of P. farinosus was tested against S. avenae, an LT50 of 11.1 d was observed. P. farinosus was subcultured by two methods. In the first method, P. farinosus was subjected repeated passages through the host. In the second method, the fungus was repeatedly subcultured on an agar medium that contained sterilized S. avenae cuticle as the only nutrient source. In both cases, after three selective subcultures of P. farinosus, a decrease in the LT50 to almost-equal-to 5.0 d was observed. To our knowledge, the effect of serial in-vitro passage of an entomopathogenic fungus on cuticle of the host insect and the associated LT50 has not been previously studied. Repeated subculturing of the more virulent strain of P. farinosus on nonselective media did not result in an increase in LT50. Five other entomopathogenic fungi were assayed for their virulence towards S. avenae. These fungi were Verticillium lecanii (Zimmerman) Viegas, Erynia neoaphidis Remaudiere & Keller, Beauveria bassiana (Bals.) Vuillemin, Beauveria brongniartii (Saccardo) Petch, and Conidiobolus obscurus (Hall & Dunn) Remaudiere & Keller. V. lecanii showed the greatest virulence, with an LT50 of 2.4 d. E. neoaphidis had an LT50 of 8.0 d, whereas treatment with B. bassiana resulted in an LT50 of 9.5 d. B. brongniartii had the lowest virulence of the strains tested against S. avenae with an LT50 of 11.7 d.