Assessment of potential barriers of Metarhizium anisopliae as a biocontrol against bed bugs

Bed bugs, Cimex lectularius, were exposed to conidia of the entomopathogenic fungus Metarhizium anisopliae by topical, spray, and contact treatments. One week post-exposure, inconsistent mortalities were observed, averaging 30% across all treatment groups and replicates. Microscopic examination of topically-treated bed bugs with a M. anisopliae strain that expresses green fluorescent protein (GFP) revealed that while numerous conidia attached to the cuticle, there was no evidence of cuticle penetration. The two major aldehydes secreted by bed bugs and considered defensive chemicals, (E)-2-hexenal and (E)-2-octenal, inhibited fungal growth in vitro by direct contact and via indirect concentrated exposure to these volatile compounds ("fumigation"). Exposure by fumigation of M. anisopliae conidia to (E)-2-octenal for intervals as short as 0.5h was sufficient to inhibit all fungal growth. The inhibition of fungal growth by bed bug aldehydes is discussed in the context of other biotic and abiotic barriers to infection.