The importance of methodology and strain selection when determining efficacy of insecticides against bed bugs

Selection of an appropriate bioassay technique and insect strain(s) are known to be important factors when attempting to accurately detect and monitor for insecticide resistance or define the efficacy of an insecticide.  Recent studies with both susceptible and resistant strains of the common bed bug, Cimex lectularius, have indicated these principles similarly apply to bed bugs and must be considered prior to undertaking diagnostic bioassays. Age, access to a blood meal, and the period since repletion may all influence the outcomes of bioassays with bed bugs. Dry residual deposits of insecticides, in particular those of neonicotinoids, also have the potential to overestimate resistance ratios or provide a false negative indication of efficacy when viewed in comparison to more applicable topical or wet residual exposure methods. Resistance monitoring of Australian field strains has also revealed that a wide spectrum in the magnitude of resistance can exist between strains that express identical resistance mechanisms, and that laboratory strains held in culture for long periods of time may lose resistance or change resistant genotypic frequencies. When factored in to the proliferation of field strains with various combinations of multiple and/or cross resistance mechanisms, this clearly presents a challenge to product manufacturers, registration bodies, and pest managers as to how they can ensure the experimental methodology and strain selected is most appropriate for the desired purpose or outcome. The results of laboratory investigations to provide informed guidance on recommended ‘best practise’ bioassays with bed bugs will be presented.