Synergistic interaction of soilborne plant pathogens and root-attacking insects in classical biological control of an exotic rangeland weed

Abstract

Abstract This investigation of the possible role of pathogen–insect interactions on the mortality of Euphorbia esula/virgata plants was prompted by repeated observations of an apparent association in the field between damage to the roots of this plant caused by root-attacking insects and disease occurrence. In studies using microcosms in a greenhouse 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 spp. caused significantly greater rates of injury to E. esula/virgata than any single agent. Kaplan–Meier survival curves were used to examine the effects on time to mortality of combinations of various inoculum densities of R. 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 5 flea beetles per plant, the rates of weed mortality in association with the second highest fungal inoculum concentration were similar to the mortality at the highest inoculum level, indicating that a minimum effective concentration is needed for effective synergism to exist. Cox regression analysis of proportional hazards was used to examine the relative contribution of plant pathogens and insects to weed mortality in the microcosms. The results indicated that plant pathogens are more than twice as likely to cause mortality of the target weed than insects under typical conditions and, under optimum conditions, are over four times more likely to do so. The results support the idea that supplementing flea beetle establishments with plant pathogens can be an effective means of both causing higher rates of successfully impacted release sites and greater biocontrol impact at individual release sites. Based on these findings it is recommended that a test of propensity for insect–plant pathogen synergisms should be a selection criterion for candidate agents. Additionally, it is recommended that survival analysis be applied to the target weed exposed to appropriate combinations of insects and pathogens as a means to 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.