Title: DEVELOPMENT OF A NOVEL BIOASSAY FOR ESTIMATION OF MEDIAN LETHAL CONCENTRATIONS (LC50) AND DOSES (LD50) OF THE ENTOMOPATHOGENIC FUNGUS BEAUVERIA BASSIANA, AGAINST WESTERN FLOWER THRIPS, FRANKLINIELLA OCCIDENTALIS
Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: May 22, 2005
Publication Date: July 21, 2005
Citation: Ugine, T.A., Wraight, S.P., Brownbridge, M., Sanderson, J.P. 2005. Development of a novel bioassay for estimation of median lethal concentrations (lc50) and doses (ld50) of the entomopathogenic fungus beauveria bassiana, against western flower thrips, frankliniella occidentalis. Journal of Invertebrate Pathology. 89:210-218.
Interpretive Summary: The western flower thrips is a key pest of greenhouse floriculture. Current management systems rely almost exclusively on sprays of chemical-based pesticides and biopesticides; however, potential of microbial biocontrol agents is being explored. Among the most important of these are the insect pathogenic fungi. A great number of fungi with thrips control potential are available in culture collections maintained worldwide, however, evaluation of these agents has been hindered by lack of a simple, inexpensive bioassay protocol. Existing bioassay systems designed to contain these minute, cryptic insects are based on custom-fabricated units that are difficult to manipulate. This paper describes the successful development of a bioassay that utilizes inexpensive (disposable), easily manipulated plastic cups. The assay can serve as the basis for both large-scale screening of fungal pathogens and precise characterization of pathogen virulence. Initial assays revealed that thrips readily acquired (picked up) lethal doses of fungal spores from spray residues on plant foliage and verified that the strain of Beauveria bassiana (GHA) comprising the commercial product BotaniGard is highly virulent against western flower thrips under laboratory conditions. These findings will contribute to the assessment and development of fungal pathogens for microbial biocontrol of western flower thrips.
An efficient methodology for bioassay of fungal pathogens against western flower thrips (Frankliniella occidentalis) was developed, using Beauveria bassiana as the model fungus. It was determined that percent mortality did not vary with the method of fungus application (direct spray versus exposure to treated foliage). Peak mortality of second-instar thrips, under constant exposure to conidia, occurred five days post-inoculation. Age of second-instar thrips (relative to the time after molt to the second instar) had a highly significant effect on susceptibility to fungal infection. As insects aged from early second instar towards the pre-pupal stage, they became less susceptible to infection by B. bassiana. Conidia acquired > 24 h after the initiation of the assay produced only low levels of infection and mortality (mean < 20%). A test of the final bioassay system was conducted in a series of assays aimed at determining the LD50 of B. bassiana technical powder against second-instar western flower thrips. B. bassiana strain GHA was highly virulent against western flower thrips, exhibiting an LD50 of 33-66 conidia/insect.