Submitted to: Biocontrol Symposium Proceedings
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2003
Publication Date: April 1, 2003
Citation: Caesar, A.J. 2003. Insect-plant pathogen synergisms for the biological control of rangeland weeds (duplicated-see log 159421). Biocontrol Symposium Proceedings. Interpretive Summary: The practical application of the author's earlier scientific findings that plant pathogens are two to four times more likely to cause mortality to an invasive weed than insects and that there is an achievable level of plant pathogens that can be combined with insects to acieve successful biocontrol is discussed. Combining plant pathogens found successful biocontrol is discussed. Combining plant pathogens found in connection with weed tissue damage by insects which are candidates for eventual release against a target weed could be easily isolated grown in the lab and tested in combination with insects in caged-plant setups currently used for testing insects themselves on the target weed and non-target plant species. Statistical survival analysis procedures readily available in most computer statistical software packages can be used to determine whether there is true interaction between a candidate insect and one or more plant pathogens or plant-harmful microbes. Against perennial invasive weeds, the reduction of unnecessary and ineffective releases, and the hazards to nontarget native species would be major advantages. At the same time, a result could be fewer, more-likely-to-be-effective insects. Cost savings could be in the millions of dollars per biocontrol programs.
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 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.