Submitted to: Biological Control
Publication Type: Peer reviewed journal
Publication Acceptance Date: 4/12/2010
Publication Date: 6/7/2010
Citation: Castrillo, L.A., Bauer, L.S., Liu, H., Griggs, M., Vandenberg, J.D. 2010. Characterization of Beauveria bassiana isolates associated with Agrilus planipennis populations in Michigan. Biological Control. 54:135-140. Interpretive Summary: Control of invasive insect pests costs municipalities, land managers and farmers billions of dollars every year. The emerald ash borer (EAB) is an invasive insect that does not yet have a natural suite of enemies, and it must be managed using existing tools until longer term control can be achieved. We compared the genetic relatedness and infectivity of several strains of the fungal pathogen Beauveria bassiana isolated from EAB in Michigan. We found that strains isolated from EAB appear to be idigenous to Michigan soils, indicating that beetles became infected here and that the soil can serve as a reservoir of fungal inoculum. We also found that indigenous fungal strains were similar to a commercial strain in their infectivity to beetles. The low prevalence of fungal infection in invasive EAB populations is probably due to the limited means of fungal dispersal from soil to above-ground beetle life stages. These findings improve our understanding of the possible impacts of fungal pathogens, both naturally occurring strains and those used as insecticides, and will help us design useful strategies for development of safe and effective biological control agents.
Technical Abstract: A survey of natural enemies of the emerald ash borer (EAB), a major invasive pest of ash trees, in Michigan resulted in a collection of Beauveria bassiana isolates from late instar larvae and pre-pupae. Since these isolates likely represent indigenous populations pathogenic to the exotic EAB, the isolates were characterized and compared to ash bark- and soil-derived isolates to possibly determine their sources and reveal the mode by which beetles pick up fungal inocula. Genetic characterization using 7 microsatellite markers showed that most of the EAB-derived strains clustered with bark- or soil-derived strains collected from the same site, indicating the indigenous nature of most of the strains found infecting EAB. Comparison of B. bassiana in the soil versus on bark showed higher numbers among soil-derived strains, and indicated that the soil serves a reservoir of fungal inocula. We hypothesize that fungal inocula came from indigenous B. bassiana in the soil that have been carried by rain splash to the lower trunk of ash trees. Additionally, fungal inocula could come from infected EAB or other insects infesting ash trees. These modes of fungus dispersal could limit the exposure of EAB to indigenous B. bassiana and may explain the low prevalence of infection observed in EAB populations. Strain virulence does not appear to be a limiting factor in infection levels since 3 of the 5 representative strains tested showed comparable virulence to the commercial strain GHA causing a > 60% mortality in EAB adults 5 d after exposure to 106 conidia/ml.