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United States Department of Agriculture

Agricultural Research Service

Title: INSECT-PLANT PATHOGEN SYNERGISMS FOR THE BIOLOGICAL CONTROL OF RANGELAND WEEDS

Author
item Caesar, Anthony

Submitted to: XI Symposium on Biological Control of Weeds
Publication Type: Proceedings
Publication Acceptance Date: December 10, 2003
Publication Date: September 15, 2004
Citation: Caesar, A. J. 2004. Insect-plant pathogen synergisms for the biological control of rangeland weeds In: Cullen, J. M. (Ed.), Proceedings of the XI International Symposium on Biological Control of Weeds, CSIRO, Canberra. pp. 493-495.

Interpretive Summary: Over several years of a biocontrol program of an exotic invasive weed species, an apparent association in the field between damage to roots of Euphorbia esula/virgata caused by root-attacking insects and soilborne disease resulting in mortality, led to investigations of pathogen-insect interactions. In studies using microcosms consisting of potted, caged plants of E. esula/virgata, combinations of Fusarium oxysporum, Rhizoctonia solani or both fungi with adults and larvae of the flea beetle Aphthona caused significantly greater rates of injury to E. esula/virgata than any single agent in greenhouse studies. Survival analysis was used to examine the effects on mortality of various inoculum densities of Rhizoctonia solani combined with 0, 5, and 15 Aphthona per plant. At each insect level per plant, increasing inoculum density increased the rate of mortality of E. esula/virgata; results indicated that a minimum effective concentration for synergism exists. Cox regression analysis of competing risks showed the relative contribution of plant pathogens was more than two times greater than insects in causing weed mortality. Based on these findings, including a propensity for insect-plant pathogen synergisms as a selection factor for candidate insects is recommended. Additionally, survival analysis applied to the target weed upon exposure to appropriate combinations of insects and pathogens is also recommended to help assess the potential effectiveness of candidate agents. Application of one or both of these recommendations could increase success in classical biocontrol of weeds and reduce associated costs and environmental risks.

Technical Abstract: Over several years of a biocontrol program of an exotic invasive weed species, an apparent association in the field between damage to roots of Euphorbia esula/virgata caused by root-attacking insects and soilborne disease resulting in mortality, led to investigations of pathogen-insect interactions. In studies using microcosms consisting of potted, caged plants of E. esula/virgata, combinations of Fusarium oxysporum, Rhizoctonia solani or both fungi with adults and larvae of the flea beetle Aphthona caused significantly greater rates of injury to E. esula/virgata than any single agent in greenhouse studies. Survival analysis was used to examine the effects on mortality of various inoculum densities of Rhizoctonia solani combined with 0, 5, and 15 Aphthona per plant. At each insect level per plant, increasing inoculum density increased the rate of mortality of E. esula/virgata; results indicated that a minimum effective concentration for synergism exists. Cox regression analysis of competing risks showed the relative contribution of plant pathogens was more than two times greater than insects in causing weed mortality. Based on these findings, including a propensity for insect-plant pathogen synergisms as a selection factor for candidate insects is recommended. Additionally, survival analysis applied to the target weed upon exposure to appropriate combinations of insects and pathogens is also recommended to help assess the potential effectiveness of candidate agents. Application of one or both of these recommendations could increase success in classical biocontrol of weeds and reduce associated costs and environmental risks.

Last Modified: 9/1/2014
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