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Highlights: [•] We surveyed central Argentina for the establishment of Rhinocyllus conicus. [•] We recorded the presence and prevalence of the weevil. [•] The weevil is firmly established and spread in Argentina. [•] These results encourage the implementation of the IPM approach in South America. [Copyright &y& Elsevier]

Abstract: The release of non-native insects to control noxious weeds is commonly used to combat invasions without disturbing the environment through chemical or mechanical methods. However, introduced biological control agents can have unintended effects. This study was initiated to evaluate potential non-target effects of the flowerhead weevil, Rhinocyllus conicus Frölich, on Cirsium ownbeyi S.L. Welsh, a rare, native and short-lived perennial thistle in northwestern Colorado, northeastern Utah, and southwestern Wyoming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has naturalized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate (λ) from a count-based population viability analysis of the population was 1.03; however, large inter-year variation indicates this rare species is still vulnerable to local extirpation. The biocontrol weevil consistently damaged the developing seeds over the course of the study independent of changes in overall population size and variation in the number of flowering individuals. The target species, Carduus nutans L. (musk thistle) is generally absent near the study plots, which may limit the population levels of R. conicus that can be sustained in this area. Although R. conicus utilizes C. ownbeyi as a host plant, the late flowering period of this native thistle and the small size of the flower heads may limit the demographic impact of R. conicus on C. ownbeyi. [Copyright &y& Elsevier]

Abstract: Prediction of the outcomes of natural enemy introductions remains the most fundamental challenge in biological control. Quantitative retrospective analyses of ongoing biocontrol projects provide a systematic strategy to evaluate and further develop ecological risk assessment. In this review, we highlight a crucial assumption underlying a continued reliance on the host specificity paradigm as a quantitative prediction of ecological risk, summarize the status of our retrospective analyses of nontarget effects of two weevils used against exotic thistles in North America, and discuss our prospective assessment of risk to a federally listed, threatened species (Cirsium pitcheri) based on those studies. Our analyses quantify the fact that host range and preference from host specificity tests are not sufficient to predict ecological impact if the introduced natural enemy is not strictly monophagous. The implicit assumption when such use is made of the host specificity data in risk assessment is that population impacts are proportional to relative preference and performance, the key components of host specificity. However, in concert with shifting awareness in the field, our studies demonstrate that the environment influences and can alter host use and population growth, leading to higher than expected direct impacts on the less preferred native host species at several spatial scales. Further, we have found that straightforward, easily anticipated indirect effects, on intraguild foragers as well as on the less preferred native host plant species, can be both widespread and significant. We conclude that intensive retrospective ecological studies provide some guidance for the quantitative prospective studies needed to assess candidate biological control agent dynamics and impacts and, so, contribute to improved rigor in the evaluation of total ecological risk to native species. [Copyright &y& Elsevier]

Field experiments were conducted in Northeast Kansas under conditions of limited plant competition to evaluate the individual and combined impact of the imported weevils Rhinocyllus conicus Froelich and Trichosirocalus horridus (Panzer) on the introduced weed, musk thistle (Carduus nutans L.). No effects on seed production occurred when low larval densities of T. horridus (<20 per plant) fed in rosettes, with or without the later-arriving, head-feeding weevil, R. conicus, present. In contrast, under high T. horridus larval densities (66 per plant), production of flower heads was reduced compared to uninfested plants. This indirect effect resulted in a 30% loss in viable seed per plant (with R. conicus present). Plant survival was not affected by T. horridus within the range of larval densities tested. R. conicus, when present alone, reduced viable seed by approximately 45%, whereas high densities of both weevils reduced viable seed by 59%. Thus, under conditions of low plant competition, neither weevil substantially limited seed production by musk thistle. This suggests the need for a multi-faceted management program for musk thistle involving additional forms of stress, such as interspecific plant competition. [Copyright &y& Elsevier]

Field experiments were conducted to determine whether early vegetative feeding by Trichosirocalus horridus alters musk thistle (Carduus nutans) as a resource for the later-arriving Rhinocyllus conicus. Results showed that thistles with high larval densities of T. horridus (66 per plant) produced more and shorter flower stems, significantly fewer flower heads, and delayed flowering by 1 week when compared to uninfested thistles. Also, colonization and oviposition on flower heads by adult R. conicus were as much as three and five times lower, respectively, on thistles heavily infested with T. horridus than on uninfested thistles. As a result, 63% fewer R. conicus adults developed from T. horridus-infested thistles. Musk thistles that were infested with lower densities of T. horridus larvae (<20 per plant) also produced multiple stems that were usually shorter than uninfested thistles. However, no differences were observed in flower head production or in the behavioral responses of R. conicus to T. horridus-infested and uninfested plants. In a greenhouse experiment, the mortality rate of R. conicus was higher on musk thistles that were heavily infested by T. horridus, implying a change in musk thistle quality. However, this effect was apparent only at low R. conicus larval densities. T. horridus can indirectly and negatively affect R. conicus through a variety of mechanisms, but only when densities of T. horridus are high. This finding may have important implications for the continued redistribution of both weevils, which were imported for the classical biological control of musk thistle, a noxious weed in pastures and rangeland. [Copyright &y& Elsevier]

• Ecological context may affect oviposition by biocontrol agents in the field. • Rhinocyllus conicus did not respond to patch level attributes of Carduus nutans. • Capitulum size, plant height and number of capitula affected R. conicus oviposition. • Understanding of oviposition preferences will inform management decisions. Musk thistle, Carduus nutans , is a major noxious weed throughout its non-native range. The flower head weevil, Rhinocyllus conicus, deemed likely to be a strong candidate for biocontrol based on laboratory tests, has proven variable in its efficacy, suggesting a possible influence of ecological context. To improve our understanding of the dynamics of this system, we examined how R. conicus responds to the spatial distribution of musk thistle individuals. We manipulated the size and density of host plant patches to determine their effect on weevil oviposition. Neither patch size nor plant density significantly affected the number of eggs laid by R. conicus. However, plant characteristics such as flower head size, as well as plant height and number of flower heads per plant, significantly influenced oviposition. Although larger thistle flower heads provide more sites for oviposition, there is also more seed production in these heads. Thus, while R. conicus oviposition is highest in larger flower heads, the surplus of seed offsets the effectiveness of biological control. This observation may in part explain the variable levels of control by this biological control agent and ascertains circumstances when augmentative management procedures may be needed. [ABSTRACT FROM AUTHOR]