Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 1, 2003
Publication Date: April 1, 2003
Citation: Caesar, A.J. 2003. Synergistic interactions of soilborne plant pathogens and root-attacking insects in classical biological control of an exotic rangeland weed. Biological Control. 28:144-153. Interpretive Summary: Combinations of insects and plant pathogens cause a more rapid rate of host plant damage and death than either factor alone. This has been noted in many instances, some of them rather dramatic, such as the Dutch elm disease epidemic in North America. An apparent combined effect of insects and plant pathogens has been observed to cause rapid reductions in populations of the highly invasive exotic rangeland weed leafy spurge. At least two species of soilborne plant pathogen were consistently isolated from leafy spurge damaged by larvae of Aphthona spp. and other root-attacking insects. To investigate this interaction, studies were set up to confirm whether this observed correlation between insects and soilborne pathogens could be reproduced in controlled conditions. Companion studies were conducted to determine the relative contribution of insects and plant pathogens to leafy spurge mortality. Another question asked was somewhat theoretical: whether there were respective intermediate "dosages" or levels of the two biological agents that in combination were of similar effectiveness to the highest level (within the range tested in the study) of each player singly or in combination. That is, was there a level of each agent at which the combined effect approached the magnitude of the effect achieved by the maximum dosage of each agent alone or in combination? Results indicated that combinations of fungi were better than single fungi in causing plant damage. Combinations of either of two fungi, Rhizoctonia solani and Fusarium oxysporum with insects caused a more rapid rate of damage than insects alone. The relative contribution of the plant pathogen Rhizoctonia solani to death of leafy spurge plants in controlled studies was 2.5 to 4 times greater than the Aphthona flea beetle within the ranges tested. It is recommended that procedures for selecting new agents against perennial weeds incorporate the chief statistical method used in the study: relative risk-based survival analysis.
Technical Abstract: Over several years in a program of biocontrol 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 disease occurrence 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. Kaplan-Meier survival curves were used to examine the effects on time to mortality of combinations of various inoculum densities of Rhizoctonia solani per gram of air-dried soil 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; the effect was significant at 5 and 15 beetles per plant using log rank tests. Additionally, at five flea beetles per plant, the rates of weed mortality in association with the second highest fungal inoculum concentration were similar to the highest level, indicating a minimum effective concentration for effective synergism exists. Cox regression analysis of competing risks was used to examine the relative contribution of plant pathogens and insects to weed mortality in the microcosms. Supplementing flea beetle establishments with plant pathogens is proposed as a means of causing greater degrees of impact and higher proportions of release sites successfully impacted. Based on these findings, including a propensity for insect-plant pathogen synergisms as a selection factor for candidate agents 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.