Hemipterochilus bembeciformis and Tropidodynerus interruptus nest in ground with rather similar soil characteristics (light to medium clay loam). Two nests of the former species and four of the latter were excavated. Both species used water to moisten the ground. Females of H. bembeciformis surmounted the nest entrance with a 1–4 mm high mud collar; the nest entrance of T. interruptus was simple but temporary closed with mud pellets by the wasps. One nest of H. bembeciformis was abandoned, while another contained two brood cells as ends of a branched burrow; all nests of T. interruptus were one-celled. A caterpillar of Acontia trabealis (Lepidoptera: Noctuidae) was recorded as a prey of H. bembeciformis, while females of T. interruptus hunted on larvae of Rhinocyllus conicus (Coleoptera: Curculionidae) extracting them from fading capitula of Carduus nutans subsp. leiophyllus. Cuckoo wasps Chrysis cylindrica and Ch. valesiana were observed at nests of both species, while only cocoons of the former species were recorded in the brood cells of both H. bembeciformis and T. interruptus. Chrysis cylindrica is recorded as a parasitoid, since its larva consumes the host larva and its cocoons were found inside cocoons of the host-wasp species. The cocoon of T. interruptus is described. Hemipterochilus bembeciformis and T. interruptus form a Müllerian mimicry ring with at least two other eumenine-wasp species co-occurring in the Tarkhankut Nature Park: Paragymnomerus signaticollis and Pseudepipona herrichii. The biology of all four species is discussed; all of them are confined to threatened steppe habitats and require our attention and conservation efforts.

Biological control programs provide simplified systems in which to investigate the influence of temperature on trophic interactions. We investigated the interaction between the invasive thistle Carduus nutans and its specialist herbivore, the biological control agent weevil Rhinocyllus conicus, along a temperature gradient spanning an urban-to-rural urbanization gradient in central Pennsylvania. As expected, more urbanized sites were warmer than rural sites. Nevertheless, C. nutans phenology was only slightly earlier at warmer sites, and the timing of observations of adult R. conicus on thistles did not shift earlier, leaving synchrony apparently unchanged. Despite unchanged phenological matching, seasonal patterns in weevil damage varied with temperature: early-appearing capitula at warmer sites were more heavily damaged than early capitula at cooler sites, yet overall a lower proportion of capitula was damaged at warmer sites. Rising temperatures may therefore reduce effectiveness of R. conicus for C. nutans control, as more capitula escape damage. Biological control systems and naturally occurring temperature gradients are useful systems in which to investigate climate change impacts, but a focus on the phenology of organisms’ presence or absence may not be sufficient; our study demonstrates that consideration of the interaction itself may be necessary.

ABSTRACT Silybum marianum (L.) Gaertner (variegated thistle, milk thistle) is one of the worst weeds of hill country pasture in the east coast regions of New Zealand. As part of an initial investigation into the potential for biocontrol, a survey of the phytophagous insects associated with S. marianum in New Zealand was carried out at 16 populations from 29 November to 16 December 2021. Phytophagous associations were determined by collecting insects on the plant and by seedhead dissections. The species collected were grouped into three categories: ‘specialists’, ‘generalists’, and ‘tourists’ that comprised 27.4%, 66.3%, and 6.3% of the total number of specimens collected, respectively. The specialist group was almost entirely comprised of the seedhead weevil, Rhinocyllus conicus (Frölich). While adults of R. conicus were commonly collected on variegated thistle, only 6.5% of seedheads contained larvae of the weevil, indicating that S. marianum is not a common developmental host. Generalist species commonly collected on S. marianum included: Nysius caledoniae Distant, Rhypodes clavicornis (F.), Closterotomus norwegicus (Gmelin) and Nezara virdula (L.). Overall, the field surveys document low levels of attack from generalist herbivores and minimal attack by the specialist weevil, R. conicus. Vacant niches are present on the weed in New Zealand, providing a strong case for introducing specialised biocontrol agents.

ABSTRACT Silybum marianum (L.) Gaertner (variegated thistle, milk thistle) is one of the worst weeds of hill country pasture in the east coast regions of New Zealand. A recent field survey in New Zealand found low levels of attack from generalist herbivores and minimal attack by the specialist weevil, Rhinocyllus conicus (Frölich). In the native range of Europe there are at least 41 insects and 12 fungi sufficiently specialised to be considered for biocontrol. The literature search revealed some potential biocontrol agents not known during the historical biocontrol surveys for S. marianum. These include the specialised biotypes of the tephritid fly, Terellia fuscicornis Loew, and the seedhead weevil, Larinus latus Herbst. The literature search also revealed the discovery of the stem-galling cynpid wasps, Aulacidea freesei Nieves-Aldrey and Phanacis zwoelferi Nieves-Aldrey, that are likely specific to S. marianum. The discovery of the smut fungus, Microbotryum silybum Vanky & Berner, host specific to Silybum species, is another promising biocontrol candidate. A shortlist of nine priority agents (eight insects and one fungal pathogen) were selected based on reported affinity for S. marianum, evidence for impact on the weed, and niches attacked. In the case of S. marianum, there are sufficiently specialised agents that attack several niches, including seeds, stems, and the root or root collar. Testing for complementary agents that attack different niches is likely to improve the chance of successful biocontrol. It is recommended that S. marianum be considered a priority target for classical biocontrol in New Zealand.

The management of biological invasions is a worldwide conservation priority. Unfortunately, decision‐making on optimal invasion management can be impeded by lack of information about the biological processes that determine invader success (i.e. biological uncertainty) or by uncertainty about the effectiveness of candidate interventions (i.e. operational uncertainty). Concurrent assessment of both sources of uncertainty within the same framework can help to optimize control decisions.Here, we present a Value of Information (VoI) framework to simultaneously analyse the effects of biological and operational uncertainties on management outcomes. We demonstrate this approach with a case study: minimizing the long‐term population growth of musk thistle Carduus nutans, a widespread invasive plant, using several insects as biological control agents, including Trichosirocalus horridus, Rhinocyllus conicus and Urophora solstitialis.The ranking of biocontrol agents was sensitive to differences in the target weed's demography and also to differences in the effectiveness of the different biocontrol agents. This finding suggests that accounting for both biological and operational uncertainties is valuable when making management recommendations for invasion control. Furthermore, our VoI analyses show that reduction of all uncertainties across all combinations of demographic model and biocontrol effectiveness explored in the current study would lead, on average, to a 15.6% reduction in musk thistle population growth rate. The specific growth reduction that would be observed in any instance would depend on how the uncertainties actually resolve. Resolving biological uncertainty (across demographic model combinations) or operational uncertainty (across biocontrol effectiveness combinations) alone would reduce expected population growth rate by 8.5% and 10.5% respectively.Synthesis and applications. Our study demonstrates that intervention rank is determined both by biological processes in the targeted invasive populations and by intervention effectiveness. Ignoring either biological uncertainty or operational uncertainty may result in a suboptimal recommendation. Therefore, it is important to simultaneously acknowledge both sources of uncertainty during the decision‐making process in invasion management. The framework presented here can accommodate diverse data sources and modelling approaches, and has wide applicability to guide invasive species management and conservation efforts.

The biotic resistance hypothesis asserts that native species may hinder the invasion of exotic species, which can occur either directly or indirectly by influencing interactions between exotic and local species. Aphid-tending ants may play a key role in the indirect biotic resistance to plant invasion. Ants may protect aphids, thus increasing their negative effect on exotic plants, but may also deter chewing herbivores, thus benefiting exotic plants. We studied native aphid-tending ants (Dorymyrmex tener, Camponotus distinguendus, and Dorymyrmex richteri) on exotic nodding thistles (Carduus thoermeri), which are attacked by thistle aphids (Brachycaudus cardui) and thistle-head weevils (Rhinocyllus conicus). We evaluated the impact of ants, aphids, and weevils on thistle seed set. We compared ant species aggressiveness towards aphid predators and weevils and performed ant-exclusion experiments to determine the effects of ants on aphid predators and weevils. We analysed whether ant species affected thistle seed set through their effects on aphids and/or weevils. The ant D. tener showed the most aggressive behaviour towards aphid predators and weevils. Further, D. tener successfully removed aphid predators from thistles but did not affect weevils. Excluding D. tener from thistles increased seed set. Analyses supported a negative indirect pathway between the aggressive D. tener and thistle seed set through aphid populations, while the other ant species showed no indirect effects on thistle reproduction. Therefore, aggressive aphid-tending ants may enhance biotic resistance by increasing aphid infestation on exotic invasive plants. This study highlights the importance of indirect biotic resistance in modulating the success of invasive species.

One of the main obstacles of classical biological control is that biological control organisms cannot be recalled once they are released in nature. It is particularly true for the flowerhead weevil, Rhinocyllus conicus Frolich, which was released as a biological control organism for the invasive musk thistle, Carduus nutans L. (MT). While weevils successfully suppressed introduced populations of musk thistles and other invasive thistle species, non-target attacks have been reported on multiple native thistles including federally listed threatened and endangered (T&E) thistle species. To investigate the foraging behavior of female weevils on invasive and native thistles, we examined volatile organic compounds (VOCs) emitted from MT and a T&E plant species, Sacramento Mountains thistle, Cirsium vinaceum Wooton & Standley (SMT) in the Lincoln National Forest, New Mexico. We used a dynamic headspace volatile collection system and gas chromatography-mass spectrometry to compare volatile profiles between MT and SMT. Female weevils reacted to 7 electrophysiologically active chemical compounds in the blends based on gas chromatography-electroantennography. The behavioral response of female weevils was indifferent when VOCs from both thistles were offered in y-tube olfactometry experiments. Yet, they preferred VOCs collected from MT to purified air. The searching time of female weevils was longer to VOCs collected from SMT over controls. Investigating signals during the initial host recognition of released biological control organisms may open new opportunities to reduce non-target attacks on T&E plant species.

The fear of unintended consequences is frequently used to argue against conservation interventions that range from climate engineering, to genetic editing of imperiled species, to actions as seemingly mundane as using seeds from non-local sources in restoration projects. There is no denying that unintended consequences are real and worthy of concern. Indeed, environmental textbooks are filled with descriptions of past interventions gone awry (such as the introduction of cane toads to Australia for biocontrol, the impacts of long-term wildfire suppression, or the use of DDT to control insect pests). However, there are also numerous counterexamples of interventions turning out as planned (for instance, barging salmon smolts around Snake River dams and human-assisted hybridization rescuing the depleted gene pool of Florida panthers). For actions under consideration, the question is how to weigh the possible unintended consequences versus the highly likely intended benefits. A June 2020 workshop (https://reviv​erest​ore.org/inten​ded-conse​quences), organized by Revive & Restore, assembled an international group of conservationists (including wildlife biologists, restoration scientists, geneticists, ethicists, and social scientists) to re-examine the precautionary principle and its associated focus on unintended and unanticipated consequences. Two observations make apparent the need for this reassessment. First, accelerating anthropogenic climate change and the expanding human footprint create ever-greater urgency for actions that could avert disasters or prevent human-driven extinctions. With most rivers dammed, a nitrogen cycle dominated by human alterations, and such severe global warming that within 50 years as many as one in three humans could be forced to migrate in search of a habitable environment (https://nyti.ms/2E5a0Wi), conservationists do not have the luxury of “doing nothing” out of fear of unintended consequences. Caution is prudent, but paralysis is unconscionable. Second, the science of risk assessment has advanced so that, although it is impossible to eliminate uncertainty, the likelihood of horrific ecological surprises is much less now than in previous decades. Ecologists today better understand ecosystems and indirect effects than they did in the 1960s and 1970s when several well-intended, but ill-fated, introductions were conducted (eg introducing the American red squirrel to Newfoundland to augment the diet of pine martens; introducing the seed weevil Rhinocyllus conicus to control exotic thistles in North America). Lessons learned with each intervention reduce the chance for future errors. Not only is risk assessment improving, but some of the tools for intervention are becoming refined. For example, the genomic and phenotypic changes that result from gene editing are much more precise than those wrought by more widely accepted techniques like hybridization and mutagenesis. Obviously, the answer is not to blithely ignore unintended consequences and adopt an “anything goes” attitude. However, it is time to recalibrate our traditional cautionary approach to environmental decision making with fine-tuning in four dimensions. First, the intended consequences of proposed interventions must carry more weight in analyses. Consider the public debate over transgenic Bt crops. The intended benefit of reducing the application of broadly toxic insecticides has largely been overshadowed by fears of environmental harms, for which there is no convincing evidence. Second, scientists should avoid being overly influenced by examples of disaster from generations ago and give more credit to recently accumulated empirical evidence. From reintroductions to genetic rescues, the vast majority of contemporary environmental interventions have produced their intended positive outcomes, yet a few historical cases of problematic outcomes continue to dominate public perception. Moreover, all actions, including inaction, entail the potential for unintended consequences. For instance, growing evidence shows that protected area creation – an intervention largely embraced by conservationists – typically displaces, rather than curtails, environmental harms and can prove counterproductive if local communities are alienated. Third, risk assessment relies on tools ranging from controlled experiments and practical experience to models and simulations. All of these tools help characterize risk, but empirical data should be far more reassuring than theoretical, but untested, models. Thirty years of globally widespread Bt crops with no ill-effects ought to inspire confidence, whereas speculative interventions such as solar geoengineering warrant greater precaution. Lastly, and most importantly, scientists need to recognize that the worst unintended consequences may not be environmental or ecological, but rather social. A carbon tax might be a great way to reduce greenhouse-gas emissions and a protected area may secure a remnant population of a declining species, but such actions disproportionately impose costs on marginalized human communities. A wider variety of voices must be invited to weigh in on what intended outcomes are desired, and how best to achieve them. While risk reduction efforts now better recognize and minimize unintended environmental harms, much work remains to address unintended social and cultural consequences. MICHELLE MARVIER Santa Clara University, Santa Clara, CA PETER KAREIVA Aquarium of the Pacific, Long Beach, CA

Oviposition response of the biocontrol agent Rhinocyllus conicus to resource distribution in its invasive host, Carduus nutans

Whether plant populations are limited by seed or microsite availability is a long-standing debate. However, since both can be important, increasing emphasis is placed on disentangling their relative importance and how they vary through space and time. Although uncommon, seed addition studies that include multiple levels of seed augmentation, and follow plants through to the adult stage, are critical to achieving this goal. Such data are also vital to understanding when biotic pressures, such as herbivory, influence plant abundance. In this study, we experimentally added seeds of a native thistle, Cirsium canescens, at four augmentation densities to plots at two long-term study sites and quantified densities of seedlings and reproductive adults over 9years. Recruitment to both seedling and adult stages was strongly seed-limited at both sites; however, the relative strength of seed limitation decreased with plant age. Fitting alternative recruitment functions to our data indicated that post-dispersal mortality factors were important as well. Strong density-dependent mortality limited recruitment at one site, while density-independent limitation predominated at the other. Overall, our experimental seed addition demonstrates that the environment at these sites remains suitable for C. canescens survival to reproduction and that seed availability limits adult densities. The results thus provide support for the hypothesis that seed losses due to the invasive weevil, Rhinocyllus conicus, rather than shifting microsite conditions, are driving C. canescens population declines. Shifts in the importance of density-dependent recruitment limitation between sites highlights that alternate strategies may be necessary to recover plant populations at different locations.

The species belonging to the weevil genus Rhinocyllus are studied. A new species, Rhinocyllus alpinus sp. nov. living on the inflorescences of Cirsium alsophilum (Pollini) Soldano (Asteraceae), is described from the central Italian Alps. A new subgenus, Rhinolarinus subgen. nov., is created for this new species. The following new synonymy is proposed: Curculio inquilinus Gyllenhal, 1827 syn. nov. of Rhinocyllus conicus (Froelich, 1792). Lectotypes for Curculio inquilinus Gyllenhal, 1827, Rhinocyllus olivieri Gyllenhal, 1835 and Rhinocyllus oblongus Capiomont, 1873 are designated. Due to lack of type specimens and information subsequently to its description, Rhinocyllus turkestanicus Desbrochers des Loges, 1900 is considered as species incertae sedis. Keys to the subgenera and to the species of Rhinocyllus s. str. are provided and taxonomical differences are illustrated.Key words: Rhinocyllus, Rhinolarinus, new subgenus, new species, Cirsium, Italy

Cirsium ownbeyi is a habitat-specific, endemic, polycarpic thistle in northwest Colorado, northeast Utah, and southwest Wyoming. In 1998, seven C. ownbeyi populations, which ranged from 4 to >30,000 plants, were known from Wyoming. The population genetics of C. ownbeyi and the threat posed by an exotic flower head–feeding weevil, Rhinocyllus conicus, in Wyoming are unknown. Between 2014 and 2016, we visited Wyoming C. ownbeyi populations to determine (1) changes in population sizes since 1998, (2) extent of R. conicus use, and (3) amount and distribution of genetic variation within and between populations. We quantified densities of plants within populations by life stage, population spatial extents, and, for 10 plants per population, proportion of flower heads with R. conicus ovipositions. Data at 6 simple sequence repeat loci were also collected. Three C. ownbeyi populations were <10% of their 1998 estimated size, 3 populations were unchanged, and one population was substantially larger than in 1998. We found Rhinocyllus conicus oviposition in all Wyoming C. ownbeyi populations, and we interpret increasing use by this weevil over our monitoring period as indicating recent colonization. Low FST and FIS values suggest that levels of C. ownbeyi inbreeding were low and that there was considerable gene flow among populations. Genetic variation increased with popu lation size, although a small C. ownbeyi population was the most divergent. We conclude that C. ownbeyi was less abundant in Wyoming in 2015–2016 than was estimated in 1998. Causes of changes in population sizes are unknown and likely vary among populations. The positive relationship between population size and genetic diversity notwithstanding, protecting small populations can preserve unique local gene pools in this rare species.El Cirsium ownbeyi es un cardo policárpico endémico, específico del hábitat del noroeste de Colorado, del noreste de Utah y del sudoeste de Wyoming. En 1998, se conocían en Wyoming siete poblaciones de C. ownbeyi, que oscilaban entre 4 y >30,000 plantas. La genética de la población de C. ownbeyi y la amenaza que representa el gorgojo de flores exóticas (Rhinocyllus conicus) en Wyoming, son desconocidas. Entre los años 2014 y 2016 visitamos las poblaciones de C. ownbeyi de Wyoming para determinar (1) los cambios en el tamaño poblacional desde 1998, (2) la frecuencia de uso de R. conicus, (3) la cantidad y la distribución de la variación genética dentro y entre poblaciones. Cuantificamos las densidades poblacionales de las plantas según su estadio de vida, su extensión espacial y la proporción de inflorescencias con ovoposiciones de R. conicus, en diez plantas por población. También, se colectaron datos de secuencias repetidas simples de seis loci. Tres poblaciones de C. ownbeyi fueron <10% menores respecto al tamaño estimado en 1998, tres poblaciones no presentaron cambios y una población fue sustancialmente mayor a la de 1998. Encontramos ovoposición de Rhinocyllus conicus en todas las poblaciones de C. ownbeyi de Wyoming, por lo que suponemos un mayor uso durante nuestro período de monitoreo, sugiriendo colonización reciente. Valores bajos de FST y FIS sugieren que los niveles de endogamia fueron reducidos y que hubo un flujo genético considerable entre las poblaciones. La variación genética aumentó con el tamaño de la población, aunque una pequeña población de C. ownbeyi fue la más divergente. Concluimos que el C. ownbeyi fue menos abundante en Wyoming en los años 2015–2016 que lo que se estimó en el año 1998. Las causas de los cambios en el tamaño poblacional son desconocidas y es probable que varíen entre las poblaciones. A pesar de la relación positiva entre el tamaño de la población y la diversidad genética, la protección de poblaciones pequeñas puede ayudar a preservar la reserva genética local de estas especies exóticas.

Abstract The weevil Rhinocyllus conicus Froelich was introduced in Georgia, USA as a biological control agent of musk thistle, Carduus nutans L. (Asteraceae) in 1991. Musk thistle populations and R. conicus infestations were monitored over 10 yr at four sites in central Georgia. Musk thistle populations declined substantially as R. conicus populations increased at all sites, but thistle infestations remained problematic at some sites. A laboratory cage study found that R. conicus would lay eggs on flower buds of yellow thistle, Cirsium horridulum Michaux, bull thistle Cirsium vulgare (Savi) Tenore, and blessed milkthistle, Silybum marianum (L.) Gaertner in no-choice and choice tests with and without Ca. nutans. Carduus nutans was preferred over the other thistle species, but the other thistle species were acceptable for oviposition by R. conicus. One or more mixed field infestations of musk thistle and yellow, bull, or blessed milkthistles were sampled for R. conicus infestation. Rhinocyllus conicus was n...

1. How organisms locate their hosts is of fundamental importance in a variety of basic and applied ecological fields, including population dynamics, invasive species management and biological control. However, tracking movement of small organisms, such as insects, poses significant logistical challenges.2. Mass‐release and individual–mark–recapture techniques were combined in an individually mark–mass release–resight (IMMRR) approach to track the movement of over 2000 adult insects in an economically important plant–herbivore system. Despite its widespread use for the biological control of the invasive thistle Carduus nutans, the host‐finding behaviour of the thistle head weevil Rhinocyllus conicus has not previously been studied. Insects were released at different distances from a mosaic of artificially created host patches with different areas and number of plants to assess the ecological determinants of patch finding.3. The study was able to characterize the within‐season dispersal abilities and between‐patch movement patterns of R. conicus. Weevils found host plant patches over 900 m away. Large patches, with tall plants, situated close to the nearest release point had the highest first R. conicus resights. Patch area and plant density had no effect on the number of weevils resighted per plant; however, R. conicus individuals were more likely to disperse out of small patches and into large patches.4. By understanding how R. conicus locates host patches of C. nutans, management activities for the control of this invasive thistle can be better informed. A deeper mechanistic understanding of host location will also improve prediction of coupled plant–herbivore spatial dynamics in general.

We examined the presence of the exotic weevil Rhinocyllus conicus Fröelich on native thistles at high elevations in the Rocky Mountains of Colorado. We tested whether the distribution of R. conicus was related to elevation by performing 2 separate studies. First, transects along an elevation gradient were sampled in Rocky Mountain National Park and presence/absence of R. conicus was scored to infer relationships between environmental variables and weevil occurrence. Second, an experiment was conducted on Niwot Ridge to determine whether weevils were able to complete their reproductive cycle and overwinter at an elevation where they do not currently exist. Results of a logistic regression indicated that R. conicus presence was negatively correlated with elevation. In addition, weevils successfully reproduced at tree line, but climatic limitations suppressed their ongoing presence at this site. The ability of the weevils to utilize native thistles at tree line was unknown prior to this experiment, and our results suggest that within the context of climate warming, the range of R. conicus could expand to include more native alpine thistles in North America.

A systematic review focused by plant on non-target impacts from agents deliberately introduced for the biological control of weeds found significant non-target impacts to be rare. The magnitude of direct impact of 43 biocontrol agents on 140 non-target plants was retrospectively categorized using a risk management framework for ecological impacts of invasive species (minimal, minor, moderate, major, massive). The vast majority of agents introduced for classical biological control of weeds (>99% of 512 agents released) have had no known significant adverse effects on non-target plants thus far; major effects suppressing non-target plant populations could be expected to be detectable. Most direct non-target impacts on plants (91.6%) were categorized as minimal or minor in magnitude with no known adverse long-term impact on non-target plant populations, but a few cacti and thistles are affected at moderate (n = 3), major (n = 7) to massive (n = 1) scale. The largest direct impacts are from two agents (Cactoblastis cactorum on native cacti and Rhinocyllus conicus on native thistles), but these introductions would not be permitted today as more balanced attitudes exist to plant biodiversity, driven by both society and the scientific community. Our analysis shows (as far as is known), weed biological control agents have a biosafety track record of >99% of cases avoiding significant non-target impacts on plant populations. Some impacts could have been overlooked, but this seems unlikely to change the basic distribution of very limited adverse effects. Fewer non-target impacts can be expected in future because of improved science and incorporation of wider values. Failure to use biological control represents a significant opportunity cost from the certainty of ongoing adverse impacts from invasive weeds. It is recommended that a simple five-step scale be used to better communicate the risk of consequences from both action (classical biological control) and no action (ongoing impacts from invasive weeds).

Establishment, dispersal, and prevalence of Rhinocyllus conicus (Coleoptera: Curculionidae), a biological control agent of thistles, Carduus species (Asteraceae), in Argentina, with experimental information on its damage

Consumer communities are being re‐arranged through unprecedented rates of human‐mediated invasions and extinctions. Such changes in consumer diversity potentially alter community function and impact on resource populations. Although insect herbivore invasions are increasingly common, the influence of such species additions on native resident herbivore guilds, along with their individual and combined effects on native plant resources, are rarely investigated. Here, we used site‐to‐site and plant‐to‐plant variation in herbivore composition to examine how the addition of an invasive exotic weevil, Rhinocyllus conicus , combines with a guild of native floral herbivores (tephritid flies, pyralid moths) to influence two key components of herbivore community function – aggregate herbivore densities and cumulative levels of seed destruction – on a native thistle, Cirsium canescens . Invasion of a site by R. conicus more than doubled aggregate herbivore density, resulting in increased levels of seed destruction and a halving of seed production by the native thistle. Further, herbivore function was significantly higher on individual plants attacked by R. conicus , compared to plants attacked only by native herbivores. Insect densities and levels of seed destruction on plants attacked by multiple herbivore taxa never exceeded those observed for plants attacked by R. conicus alone, suggesting that increases in herbivore community function with invasion resulted from the inclusion of a functionally dominant insect rather than any complementarity effects. Some evidence for interference between insects emerged, with a trend towards reduced moth and weevil densities in two and three taxon mixtures compared with plants attacked by each taxon alone. However, density compensation was limited so that, overall, the addition of a novel herbivore to the floral guild was associated with a significant increase in herbivore community function and impact on seed production. The results suggest that invasion of a functionally dominant herbivore into an unsaturated recipient community can augment function within a resource guild.

Mechanisms underlying invasive species impacts remain incompletely understood. We tested the hypothesis that priority resource access by an invasive biocontrol weevil, Rhinocyllus conicus, intensifies and alters the outcome of competition with native floral herbivores over flower head resources of the non-target, native host plant Cirsium canescens, specifically with the predominant, synchronous tephritid fly Paracantha culta. Four main results emerged. First, we documented strong, asymmetric competition, with R. conicus out-competing P. culta. Second, weevil priority access to floral resources accelerated competitive suppression of P. culta. Evidence for competitive suppression with increased weevil priority included decreases in both the numbers and the total biomass of native flies, plus decreases in individual P. culta fly mass and, so, potential fitness. Third, we found evidence for three concurrent mechanisms underlying the competitive suppression of P. culta by R. conicus. Prior use of a flower head by R. conicus interfered with P. culta pre-oviposition behavior. Once oviposition occurred, the weevil also reduced fly post-oviposition performance. Preemptive resource exploitation occurred, shown by the significant effect of flower head size on the total number of insects developing and in the magnitude of R. conicus effects on P. culta. Interference also occurred, shown by a spatial shift of surviving P. culta individuals away from the preferred receptacle resources as R. conicus priority increased. Finally, fourth, using an individual-based model (IBM), we found that the competitive interactions documented have the potential for imposing demographic consequences, causing a reduction in P. culta population sizes. Thus, priority resource access by an invasive insect increased competitive impact on the predominant native insect in the invaded floral guild. This study also provides the first experimental evidence for non-target effects of a weed biological control agent on an associated native insect herbivore.

Classical weed biocontrol has long been criticised, worldwide, for a lack of post-release long-term monitoring. This paper reports on a rare opportunity to follow up, three agents and 35 years later, a biocontrol programme against nodding thistle (Carduus nutans) in New Zealand. In New Zealand, the establishment of all three agents has been reported previously, but impacts have only been assessed for the receptacle weevil (Rhinocyllus conicus). This paper reports on the abundance of the nodding thistle crown weevil (Trichosirocalus horridus), and on the abundance, phenology and impact of the seed predators, R. conicus and Urophora solstitialis, in Canterbury, based on field surveys and, for the seed predators, also from a cultivated population. Trichosirocalus horridus was abundant at all field sites and on rosettes of all sizes. Seed destruction per larva by R. conicus (34 seeds per larva) was similar to previous estimates, but U. solstitialis achieved rates (21 seeds per larva) three times higher than estimates elsewhere. Population-level seed predation by R. conicus (14 and 4% in the cultivated and field populations, respectively), by U. solstitialis (9 and 7%) and cumulatively by both (23 and 11%) is insufficient to cause nodding thistle populations in New Zealand to decline. The results show that the introduction of multiple biocontrol agents does not guarantee increased pressure on a target weed host population.