Submitted to: Society for Invertebrate Pathology Annual Meeting Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: May 17, 2004
Publication Date: August 1, 2004
Citation: Ugine, T.A., Wraight, S.P., Sanderson, J.P. 2004. Dose-dependent acquisition of beauveria bassiana conidia by western flower thrips, frankliniella occidentalis (pergande). Society for Invertebrate Pathology Annual Meeting Proceedings. 37:76.
Bioassays evaluated the efficacy of B. bassiana strain GHA against 2nd-instar and adult female thrips. Preparations included an unformulated technical powder, a clay-based wettable powder, and an emulsifiable oil formulation. Insects were exposed to treated bean leaf disks and mortality was assessed after 5 days. Conidia were counted on thrips' bodies 24 h after exposure and on the leaf disks (conidia/mm2), and LD50 and LC50 estimates were generated. For all fungal preparations, the acquisition rate, defined as the number of conidia observed on the whole thrips body divided by the total number of conidia to which the thrips were exposed, unexpectedly decreased as the application rate increased. In addition, the LD regression slopes were higher than the LC slopes (LD 1.9 and 1.5 versus LC 1.3 and 0.7 for 2nd-instars and adult females, respectively). In the regression analyses, it is assumed that conidial concentrations on the leaf disks are a reliable measure (at least a reliable relative measure) of the doses to which the insects are exposed. This may not be the case, however, if conidia repel the insects or somehow become more difficult to pick up as concentration on the leaf increases. At the higher concentrations, doses would be overestimated, resulting in under-estimation of the regression coefficient (slope). This result suggests that the low slopes often obtained in fungal assays may arise, at least in part, as an artifact of unequal rates of dose acquisition at low versus high application rates. Greenhouse tests investigating the effect of increasing application rate on efficacy and the number of conidia acquired 24 h post application also unexpectedly showed no relationship between the number of conidia on the bodies of adult thrips and application rate.