STANDARDIZED BIOASSAY AND SCREENING OF BEAUVERIA-BASSIANA AND PAECILOMYCES-FUMOSOROSEUS AGAINST THE RUSSIAN WHEAT APHID (HOMOPTERA, APHIDIDAE)

Two hyphomycetous fungi with potential as biocontrol agents were used in bioassays against the Russian wheat aphid, Diuraphis noxia (Kurdjum ov). Dose-response assays were done using 5-7 concentrations of 2 isol ates each of Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces f umosoroeus (Wize) Brown & Smith. A sensitive assay technique was devel oped in which adult aphids. 0-2 d of age, were sprayed with fungal spo re suspensions on cut barley leaf sections. LC(50) estimates were 91 a nd 282 spores per square centimeter for two B. bassiana isolates and 4 6 and 56 spores per square centimeter for two P. fumosoroseus isolates . Slopes ranged from 1.43 to 1.76. Aphids were significantly less susc eptible to B. bassiana than to P. fumosoroseus, indicated by the lack of overlap among fiducial limits. LT(50)s varied inversely with dose f or all 4 isolates and ranged from 4.6 d to 12.2 d for B. bassiana, and from 3.3 d to 12.2 d for P. fumosoroseus, depending on isolate and do se. In standardized screening assays, relative potencies ranged from 0 .2 to 2.1 among 23 isolates of B. bassiana, and from 0.3 to 3.7 among 14 isolates of P. fumosoroseus. Use of this assay method will facilita te studies of biotic and abiotic factors that influence Russian wheat aphid susceptibility to fungal infection in the laboratory and in the field.