A Tritrophic Effect of Host Plant on Susceptibility of the Western Flower Thrips to the Entomopathogenic Fungus Beauveria bassiana

Authors: UGINE, TODD - CORNELL UNIVERSITY; Wraight, Stephen; SANDERSON, JOHN - CORNELL UNIVERSITY

Submitted to: Journal of Invertebrate Pathology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/7/2007
Publication Date: 10/1/2007
Citation: Ugine, T.A., Wraight, S.P., Sanderson, J.P. 2007. A tritrophic effect of host plant on susceptibility of the western flower thrips to the entomopathogenic fungus beauveria bassiana. Journal of Invertebrate Pathology. 96:162-172.

Interpretive Summary: Western flower thrips are important pests of many greenhouse crops. Growers generally rely on chemical insecticides for control, but there is interest in developing safer, sustainable, alternatives. Use of insect pathogenic fungi is one alternative; however effectiveness of these biocontrol agents is limited by a number of factors. Results from our previous studies suggested that host plant type was one factor determining thrips' susceptibility (we observed poor thrips control on impatiens). Host plant effects have been reported for other pest-pathogen systems; these have usually been attributed to plant chemicals that inhibit the fungi. In this study, our objective was to determine if thrips' susceptibility was affected by impatiens and to determine the cause of any observed effect. Thrips were exposed to Beauveria bassiana-treated bean vs. impatiens leaves and then transferred to clean bean or impatiens leaves and monitored for mortality. Treated thrips were examined microscopically to determine numbers of fungal spores attached to their bodies. Spores were exposed to intact and macerated impatiens leaves and checked for viability. Greater levels of infection were observed among thrips exposed to treated bean compared to impatiens; however, there was no evidence of fungal inhibition by plant chemicals, and the differences in thrips mortality were accounted for by the observation that thrips picked up 75% more spores from bean leaves than from impatiens leaves. This unexpected result indicates that host plant is an important factor in assessing effectiveness of fungi, and suggests that taking steps to improve delivery of spores to the target pest could overcome negative host plant effects in some cases.

Technical Abstract: Adult female western flower thrips (Frankliniella occidentalis) were exposed 12–24 h to bean (Phaseolus vulgaris) and impatiens (Impatiens wallerana) leaf disks treated with Beauveria bassiana conidia and then transferred to clean bean or impatiens at various times post-treatment. Significantly greater levels of fungal infection were observed when thrips were treated on bean versus impatiens, but exposure to impatiens following treatment had no effect on fungal infection (percent mortality). This result, combined with observations of no inhibition of germination of conidia exposed to intact or macerated impatiens foliage, indicated that the negative effect of the impatiens host plant was not due to plant chemical compounds (antibiosis). Further observations revealed that insects acquired (picked-up) 75% more conidia from treated bean disks than from treated impatiens disks. This difference in dose acquisition was determined to account for the observed difference in percent mortality (15%) following treatment on the two host plants. Median lethal doses (LD50) of B. bassiana were not significantly different on the two host plants, but median lethal concentrations were nearly 7-fold greater on impatiens. This difference was explained by disproportionate rates of conidial acquisition at measured rates of conidial deposition (an inverse relationship was observed between application rate expressed as conidia/mm2 and the number of conidia acquired. The mechanism underlying the differential rates of conidial acquisition from bean versus impatiens was not determined.