Submitted to: Journal of Economic Entomology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 5, 1996
Publication Date: N/A
Interpretive Summary: The Russian wheat aphid (RWA), Diuraphis noxia (Kurdjumov), is a worldwide pest of wheat and barley. It was first found in the United States in the mid-1980s. Since then, it has caused a combined direct and indirect economic impact in excess of $1 billion. Cumulative yield losses exceeded 106 million bushels by 1993, and over 20% of small grain acreage in 16 western states was infested. Fungi have potential as safe and effective biological control agents for this aphid, but there is a lack of understanding of the factors that influence aphid susceptibility to infection. Two species of fungi were tested against adult Russian wheat aphids in laboratory assays. The objectives were to quantify the dose-mortality relationship and to screen a variety of fungal strains for their potential as aphid microbial control agents. A sensitive assay technique was developed in which adult aphids, 0-2 days of age, were sprayed with fungus spore suspensions on cut barley leaf sections. An average of 46 to 282 spores per square centimeter, depending on the fungal strain used, were required to kill 50% of test aphids. The time to death ranged from just over 3 days to more than 12 days, depending on strain and dose. The use of this assay technique will facilitate studies of biotic and abiotic factors that influence Russian wheat aphid susceptibility to fungal infection in the laboratory and in the field.
Technical Abstract: Two hyphomycetous fungi with potential as biocontrol agents were used in bioassays against the Russian wheat aphid, Diuraphis noxia (Kurdjumov). Dose-response assays were done using 2 isolates each of Beauveria bassiana (Balsamo) Vuillemin and Paecilomyces fumosoroseus (Wize) Brown and Smith. A sensitive assay technique was developed in which adult aphids, 0-2 days of age, were sprayed with fungus spore suspensions on cut barley leaf sections. LC50 estimates were 91 and 282 spores / cm2 for two B. bassiana isolates and 46 and 56 spores/cm2 for two P. fumosoroseus isolates. Slopes ranged from 1.43 to 1.76. Aphids were significantly less susceptible to B. bassiana than to P. fumosoroseus, indicated by the lack of overlap among fiducial limits. LT50s varied inversely with dose for 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 dose. In standardized screening assays, relative potencies ranged 0.2 to 2.1 among 23 isolates of B. bassiana, and from 0.3 to 3.7 among 14 isolates of P. fumosoroseus. Based on the relatively narrow range of potencies, only one isolate of P. fumosoroseus will be tested further. The use of this assay method will facilitate studies of biotic and abiotic factors that influence Russian wheat aphid susceptibility to fungal infection in the laboratory and in the field.