Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 5, 2005
Publication Date: September 13, 2005
Citation: Ugine, T.A., Wraight, S.P., Sanderson, J.P. 2005. Acquisition of lethal doses of beauveria bassiana by western flower thrips exposed to foliar spray residues of formulated and unformulated conidia. Journal of Invertebrate Pathology. 90:10-23.
Interpretive Summary: Western flower thrips, a key pest of greenhouse ornamental crops, are minute insects that inhabit cryptic habitats (especially flower heads). This makes them difficult spray targets, and efficacy of microbial biopesticides will depend, in large part, on the capacity of thrips to acquire lethal doses of the active agent from spray-treated plant surfaces. Oil formulants have been shown in many cases to enhance efficacy of insect pathogenic fungi, an effect usually attributed to their action as spray stickers, and this study was conducted to investigate effects of formulation on secondary pickup of fungal spores by western flower thrips. Results were unexpected in that thrips acquired equivalent doses of Beauveria bassiana spores from spray residues of three very differently formulated materials: spores formulated in emulsifiable oil or clay (wettable powder) versus essentially unformulated spores (suspended in water). Lack of a formulation effect was noted on all thrips body parts. Also unexpected was a finding that though the number of spores acquired by thrips increased with increasing application rate, the rate of spore pickup (spores acquired per total applied) actually decreased. Result suggested either that the spray residues possessed repellent properties, or that spores became less amenable to pickup as residues increased. These results improve our understanding of dose acquisition by insect pests and raise numerous researchable questions about fungal formulation and its application to improve biopesticide efficacy.
Secondary acquisition of Beauveria bassiana conidia was recorded on the whole bodies and selected body parts of second-instar nymphs and adult females of western flower thrips exposed 24 h to foliar spray residues of three differently formulated conidial preparations: conidia formulated in emulsifiable oil or clay (wettable powder) versus essentially unformulated conidia (technical grade powder suspended in water with a surfactant). Formulation had no effect on dose acquisition and no effect on virulence of acquired conidia. The mean nymphal LC50/LD50 was 116 conidia/mm2 and 52 conidia/insect, respectively; the values for adults were 19 conidia/mm2 and 5 conidia/insect. Greatest numbers of conidia were recorded on the legs and abdomens of nymphs and on the legs, wings and thoraxes of adults. Highest conidial concentrations (per unit surface area) were on the mouthparts and heads of nymphs and on the legs and wings of adults. As would be expected, numbers of conidia acquired increased with residue concentration (application rate). However, an inverse relationship was noted between acquisition rate (conidia acquired/total conidia applied) and residue concentration. The mechanism underlying this response was not determined. However, there was no indication that any body parts (e.g., tarsi) became saturated with spores, which suggests that either the thrips were repelled by the conidial residues or that as the concentration of conidia on the substrate increased, conidia somehow became more difficult to acquire. Slopes of the LC probit regressions were lower than those of the LD regressions (mean 1.14 vs 1.78), suggesting that the low slopes often obtained in fungal assays could be partly an artifact of unequal rates of dose acquisition at low versus high application rates.