Post-dispersal factors influence recruitment patterns but do not override the importance of seed limitation in populations of a native thistle.

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 9 years. 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.

Non-random food-web assembly at habitat edges increases connectivity and functional redundancy.

Habitat fragmentation dramatically alters the spatial configuration of landscapes, with the creation of artificial edges affecting community structure and dynamics. Despite this, it is not known how the different food webs in adjacent habitats assemble at their boundaries. Here we demonstrate that the composition and structure of herbivore-parasitoid food webs across edges between native and plantation forests are not randomly assembled from those of the adjacent communities. Rather, elevated proportions of abundant, interaction-generalist parasitoid species at habitat edges allowed considerable interaction rewiring, which led to higher linkage density and less modular networks, with higher parasitoid functional redundancy. This was despite high overlap in host composition between edges and interiors. We also provide testable hypotheses for how food webs may assemble between habitats with lower species overlap. In an increasingly fragmented world, non-random assembly of food webs at edges may increasingly affect community dynamics at the landscape level.

Community-level net spillover of natural enemies from managed to natural forest.

Edge effects in fragmented natural habitats may De exaceroateci by intensive land use in the surrounding landscape. Given that most managed systems have higher primary productivity than adjacent natural systems, theory suggests that bottom-up subsidized consumers are likely to spill over from managed to natural habitats. Furthermore, the magnitude of spillover is likely to differ between generalist and specialist consumers, because of differences in their ability to use the full spectrum of resources. However, it is unknown whether there is indeed asymmetrical spillover of consumers between managed and natural habitats, and whether this is related to resource abundance or the trophic specialization of the consumer. We used flight intercept traps to measure spillover of generalist predators (Vespula wasps, Vespidae) and more specialist predators (106 species of parasitoids, Ichneumonidae and Braconidae) across habitat edges between native New Zealand forest and exotic plantation forest over a summer season. We found net spillover of both generalist and specialist predators from plantation to native forest, and that this was greater for generalists. To test whether natural enemy spillover from managed habitats was related to prey (caterpillar) abundance (i.e., whether it was bottom-up productivity driven, due to increased primary productivity), we conducted a large-scale herbivore reduction experiment at half of our plantation sites, by helicopter spraying caterpillar-specific insecticide over 2.5 ha per site. We monitored bidirectional natural enemy spillover and found that herbivore reduction reduced generalist but not specialist predator spillover. Trophic generalists may benefit disproportionately from high resource productivity in a habitat, and their cross-habitat spillover effects on natural food webs may be an important source of consumer pressure in mosaic landscapes.

Complementarity and redundancy of interactions enhance attack rates and spatial stability in host-parasitoid food webs.

Complementary resource use and redundancy of species that fulfill the same ecological role are two mechanisms that can respectively increase and stabilize process rates in ecosystems. For example, predator complementarity and redundancy can determine prey consumption rates and their stability, yet few studies take into account the multiple predator species attacking multiple prey at different rates in natural communities. Thus, it remains unclear whether these biodiversity mechanisms are important determinants of consumption in entire predator-prey assemblages, such that food-web interaction structure determines community-wide consumption and stability. Here, we use empirical quantitative food webs to study the community-wide effects of functional complementarity and redundancy of consumers (parasitoids) on herbivore control in temperate forests. We find that complementarity in host resource use by parasitoids was a strong predictor of absolute parasitism rates at the community level and that redundancy in host-use patterns stabilized community-wide parasitism rates in space, but not through time. These effects can potentially explain previous contradictory results from predator diversity research. Phylogenetic diversity (measured using taxonomic distance) did not explain functional complementarity or parasitism rates, so could not serve as a surrogate measure for functional complementarity. Our study shows that known mechanisms underpinning predator diversity effects on both functioning and stability can easily be extended to link food webs to ecosystem functioning.

Evaluating the use of common grasses by the wheat stem sawfly (Hymenoptera: Cephidae) and its native parasitoids in rangeland and Conservation Reserve Program grasslands.

The wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae), is a major pest of wheat (Triticum aestivum L., Poales: Poaceae) across the northern Great Plains of North America. Cephus cinctus has a wide host range, attacking numerous wild grasses and cultivated cereals in crop and grassland habitats, where it is, in turn, attacked by 2 native braconid parasitoids. Quantitative assessments of C. cinctus infestation and parasitism levels in different grass species across the full spectrum of available hosts are important in assessing the extent to which grasslands, or specific constituent grass species, may be reservoirs of pests or parasitoids moving into wheat. We quantified infestation and parasitism levels in over 25,000 stems collected from 17 grass species and wheat spanning 35 sites in central Montana, United States, over 2 yr. Infestation levels in 5 grass species, primarily wheatgrasses, were high (38%-65%) and similar to the levels observed in wheat (55%). In contrast, the majority of grass species (12 of 17) had significantly lower levels of infestation (<10%), suggesting that most grasses are not important reservoirs of C. cinctus. Parasitism levels in highly infested wheatgrasses were, on average, 3 times higher than those in cultivated wheat, suggesting that these grasses could provide important conservation habitat for parasitoids. Future work examining the relative performance of pests and parasitoids in these grasses will be important in gauging their relative value as plant materials to bolster parasitoid conservation in reseeded grassland habitats.

On-farm harvest timing effects on alfalfa weevil across the Intermountain West region of the United States.

Alfalfa (Medicago sativa L.) is an economically important commodity in the Intermountain Western United States. A major concern for alfalfa producers in this region is the alfalfa weevil (Hypera postica Gyllenhal). Insecticide resistance development coupled with regulatory changes in pesticide use has resulted in renewed interest by producers in non-chemical control methods such as cultural control. One such cultural control method is early harvest, which consists of producers timing their harvests early in the season to decrease alfalfa weevil damage. This method is thought to be effective by exposing weevil larvae to adverse conditions before significant damage occurs. Still, early harvest can be difficult to employ because recommendations are often vague. To better understand how early harvest impacts both alfalfa weevils and their natural enemies and how producers are using this method across the Intermountain Western United States, we conducted a study in alfalfa production fields in Colorado, Montana, and Wyoming over three growing seasons. We determined that the timing of the initial alfalfa harvest spanned more than 1 month across fields, and alfalfa plant stage at harvest ranged from late vegetative to early bloom. Harvest was more impactful on reducing alfalfa weevil densities the earlier it was implemented. Removing windrows in a timely manner is likely useful to further decrease alfalfa weevil densities. Harvest timing was not associated with parasitism rates of alfalfa weevil, but higher parasitism rates were associated with lower post-harvest alfalfa weevil densities. This work has increased our understanding of early harvest in an on-farm setting and to improve recommendations for producers across the Intermountain Western United States.

Landscape Composition and Management History Affect Alfalfa Weevil but not its Parasitoid.

It is widely recognized that both local and landscape-scale factors can be important drivers of crop pests, natural enemies, and biocontrol services. However, recent syntheses have found that landscape effects are inconsistent across study systems, highlighting the need for system-specific research to guide management decisions. In particular, studies conducted in perennial crops and that examine landscape configuration, not just composition, are especially lacking. We studied the impact of local and landscape factors on alfalfa weevil Hypera postica and its parasitoid Bathyplectes curculionis. Although classical biological control efforts have largely suppressed H. postica in the eastern United States, it remains problematic in the western United States. We sampled 20 production alfalfa fields in southeastern Wyoming to estimate H. postica density, parasitism rates by B. curculionis, and vegetation at local scales. We used remotely sensed imagery to characterize both landscape composition and configuration surrounding each sampled field. We used a hypothesis-driven modeling approach to determine which model was most predictive of H. postica and parasitism rate by B. curculionis. Landscape composition was the best model to predict H. postica densities. Host density was the best predictor of parasitism rates by B. curculionis. Production fields that had received insecticide applications in the last 5 years had higher weevil densities than fields that had not received insecticide applications. Stand age was not associated with weevil density or parasitism rate. In conclusion, we found local, landscape, and management components to be important in this system.

Variation in reproductive mode across the latitudinal range of invasive Russian knapweed.

For invading species, reproduction is a critical determinant of population establishment as well as spread into new areas. When species have multiple modes of reproduction, the prevalence of different modes can influence management decisions. We used genetic markers to determine the prevalent method of recruitment for invasive Russian knapweed (Acroptilon repens). This species forms patches and can spread by both rhizomic growth and seed from outcrossing. We found no shared genotypes between 41 western North American populations, indicating at the macroscale, Russian knapweed is spreading via seed to distant locations. We also examined drivers of reproductive mode by comparing clonality with large-scale environmental factors across the invasion. We found a correlation between latitude and clonal versus seed reproduction, with clonality higher in northern latitude populations. This trend was associated most parsimoniously with decreasing maximum annual temperature and 30-year average of available growing degree days, and increasing soil organic carbon content. These results have management implications: if not properly temporally implemented, grazing or herbicide applications that create open spaces for recruitment may increase the likelihood of Russian knapweed patch persistence through seed, and recently released galling biological control agents in North America may be less effective in northern latitudes where Russian knapweed spread by seed is less prevalent.

Effects of Landscape Composition on Wheat Stem Sawfly (Hymenoptera: Cephidae) and Its Associated Braconid Parasitoids.

Several agroecological and integrated pest management strategies focus on landscape management to increase complexity and foster biodiversity. However, landscape complexity does not always enhance biological control and in some cases may lead to increased pest populations. We examined the prevalence of two Bracon parasitoids, Bracon cephi Gahan and Bracon lissogaster Muesebeck (Hymenoptera: Braconidae), and their host the wheat stem sawfly Cephus cinctus Norton, a major pest of wheat. We assessed the degree of noncrop and crop host plant use and responses to landscape composition. We found no instances of parasitism by either Bracon species in our three-year, statewide winter wheat survey but found small populations of Bracon in noncrop landscapes throughout eastern and western Colorado. We used model selection to examine how local (500 m scale) and landscape (5 km scale) cover of suitable noncrop and crop habitats potentially affects abundances of Bracon and wheat stem sawfly. Our best fit model for wheat stem sawfly suggests that a decrease in noncrop cover at the landscape scale leads to an increase in wheat stem sawfly infestation. Our best fit model for Bracon parasitism suggests an increase in wheat cover at the local level results in the greatest increase in the odds of parasitism by either species of Bracon. Herbaceous cover at local and landscape scales were also significant predictors of Bracon parasitism. The results of this study suggest that pest and natural enemies respond differently to landscape composition and these responses should be evaluated before management decisions are made.

Aphid Honeydew Enhances Parasitoid Longevity to the Same Extent as a High-Quality Floral Resource: Implications for Conservation Biological Control of the Wheat Stem Sawfly (Hymenoptera: Cephidae).

Providing sugar resources for parasitoids is an important component of habitat management approaches to bolster biological control. We screened three flowering cover crop species, and one aphid species, for their potential to increase the longevity of the parasitoid wasp, Bracon cephi (Gahan) (Hymenoptera: Braconidae), an important biological control agent of the wheat stem sawfly, Cephus cinctus Norton (Hymenoptera: Cephidae). We found that buckwheat and honeydew from the cereal aphid, Rhopalosiphum padi (Linnaeus) (Hemiptera: Aphididae), increased longevity of B. cephi females by over threefold, while longevity on sunflower and coriander was not significantly different from controls on wheat. The results suggest that incorporating buckwheat into cover crop mixes could enhance parasitoid performance. However, the finding that honeydew associated with a common aphid in wheat provides a suitable resource suggests that a better understanding of the varying quality, and spatial and temporal availability, of aphid honeydew will be a critical consideration in evaluating the potential benefits of managing floral resources for parasitoid conservation in this system.

Modeling the combined impacts of host plant resistance and biological control on the population dynamics of a major pest of wheat.

BACKGROUND: Single-tool approaches often fail to provide effective long-term suppression of pest populations, such that combining several tools into an integrated management strategy is critical. Yet studies that harness the power of population models to explore the relative efficacy of various management tools and their combinations remain rare. We constructed a Leslie matrix population model to evaluate the potential of crop resistance, acting alone or in combination with biological control, to reduce populations of the wheat stem sawfly, Cephus cinctus Norton, a major pest of wheat in North America. RESULTS: Our model projections indicated that crop resistance reduced, but did not stop, C. cinctus population growth, suggesting that implementing multiple management tools will be necessary for longer term control of this pest. The levels of parasitism needed to curtail population growth were much lower in model projections for resistant solid-stemmed compared with susceptible hollow-stemmed cultivars (22% versus 86%). Furthermore, even when accounting for the reduced levels of parasitism observed in resistant cultivars, projected population growth rates for C. cinctus were always lower in resistant compared with susceptible wheat cultivars. CONCLUSION: Despite some empirical evidence for antagonistic interactions between resistance and biological control, our models suggest that combining these two approaches will always reduce population growth rates to lower levels than implementing either strategy alone. More work focused on integrating biological control into crop resistance breeding programs, and determining how these approaches affect performance of limiting life stages, will be important to optimize sustainable approaches to integrated pest management in this system and more broadly. Published 2020. This article is a U.S. Government work and is in the public domain in the USA.

Quantifying Temporal Variation in the Benefits of Aphid Honeydew for Biological Control of Alfalfa Weevil (Coleoptera: Curculionidae).

Sugar feeding by biological control agents, such as parasitoid wasps, may enhance their ability to control crop pests, although its importance is likely to vary greatly through space and time. Here we quantified temporal variation in the potential importance of sugar resources associated with honeydew secreted by the pea aphid (Acyrthosiphon pisum (Harris) (Hemiptera: Aphididae)) in determining levels of parasitism of the alfalfa weevil (Hypera postica (Gyllenhal) (Coleoptera: Curculionidae)) by its dominant parasitoid, Bathyplectes curculionis (Thomson) (Hymenoptera: Ichneumonidae) across irrigated alfalfa fields in Montana, United States over 5 yr. A positive association between parasitism of H. postica and A. pisum densities at the across-site scale was observed in 2 of 5 yr, with parasitism increasing twofold to fourfold over gradients in A. pisum density. The relationship was strongest in the 2 yr of lowest parasitoid relative to host densities, when increases in per capita effects of individual parasitoids would be expected to be particularly important. Acyrthosiphon pisum densities were at their lowest in these same years, suggesting that they may generally be sufficiently abundant that parasitoids are not limited by sugars in most years. This conclusion is supported by results of anthrone tests which revealed a high level of sugar-fed parasitoids (>50%) in a year of high aphid abundance. More studies, such as this one, that explore the frequency with which increasing sugar resource availability actually enhances parasitism levels in the field will be critical to gauge the broader potential of sugar resource addition (e.g., through flowering strips, banker plants or sugar sprays) to bolster biological control.

Do Floral Resources Benefit the Herbivorous Sawfly, Cephus cinctus (Hymenoptera: Cephidae), a Major Pest of Wheat in North America?

Leveraging floral resources to promote biological control requires carefully screening prospective floral species for their suitability not just for natural enemies, but also for targeted pests. Here we examined the influence of access to various sugar resources on Cephus cinctus Norton (Hymenoptera: Cephidae), a major pest of wheat in the northern Great Plains of North America. We conducted greenhouse studies to examine the effect of access to a honey-sucrose solution, three flowering plant species, and aphid honeydew, on the longevity and potential fecundity of C. cinctus. Cephus cinctus longevity increased significantly (females living 1.18-1.25 times as long) in treatments with buckwheat (Fagopyrum esculentum Moench (Polygonales: Polygonaceae)) flowers compared with controls. In contrast, there was no significant effect of two additional mustard species (Capparales: Brassicaceae) tested, canola (Brassica napus L.) and white mustard (Sinapis alba L.), or aphid (Myzus persicae Sulzer (Hemiptera: Aphididae)) honeydew on C. cinctus longevity. Access to buckwheat did not significantly increase the cumulative number of eggs laid by C. cinctus, suggesting that sugar feeding on this plant is unlikely to increase pest impacts. To our knowledge, this is the first published experimental evidence that access to floral resources can increase the adult lifespan of C. cinctus. The results re-inforce previous observations that sugar feeding may be common in Cephus spp. and other Cephidae. Our results further suggest that screening prospective floral species being considered for conservation biological control programs against both C. cinctus and potential parasitoids will be an important precautionary measure in future work.

Decoupled recovery of ecological communities after reclamation.

Grassland restoration is largely focused on creating plant communities that match reference conditions. However, these communities reflect only a subset of the biodiversity of grassland systems. We conducted a multi-trophic study to assess ecosystem recovery following energy development for oil and gas extraction in northern US Great Plains rangelands. We compared soil factors, plant species composition and cover, and nematode trophic structuring between reclaimed oil and gas well sites ("reclaims") that comprise a chronosequence of two-33 years since reclamation and adjacent, undeveloped rangeland at distances of 50 m and 150 m from reclaim edges. Soils and plant communities in reclaims did not match those on undeveloped rangeland even after 33 years. Reclaimed soils had higher salt concentrations and pH than undeveloped soils. Reclaims had lower overall plant cover, a greater proportion of exotic and ruderal plant cover and lower native plant species richness than undeveloped rangeland. However, nematode communities appear to have recovered following reclamation. Although total and omni-carnivorous nematode abundances differed between reclaimed well sites and undeveloped rangeland, community composition and structure did not. These findings suggest that current reclamation practices recover the functional composition of nematode communities, but not soil conditions or plant communities. Our results show that plant communities have failed to recover through reclamation: high soil salinity may create a persistent impediment to native plant growth and ecosystem recovery.

Oilfield Reclamation Recovers Productivity but not Composition of Arthropod Herbivores and Predators.

Arthropods are key components of grassland ecosystems. Though arthropod communities are often strongly influenced by plant communities, plants and arthropods may respond differently to disturbance. Studying plant responses alone may, therefore, not fully capture altered ecosystem dynamics; thus multi-trophic approaches are critical to fully understand ecosystem responses to disturbance. Energy development is a large-scale driver of disturbance in northern Great Plains rangelands, and recovery of arthropod communities following reclamation is not well understood. We sampled Orthoptera and spiders in western North Dakota, United States, in 2016. Samples were collected from 14 reclaimed oil well sites ('reclaims') 2-33 yr since reclamation, and native prairie at two distances (50 and 150 m) from reclaim edges. Overall Orthopteran and spider abundances on reclaims and native prairie did not differ; however, Orthopteran community composition and species abundances were distinct on reclaims versus native prairie, including increased abundances of Melanoplus femurrubrum (De Geer) (Orthoptera: Acrididae) (a noted crop pest) on reclaims. In contrast, NMS analyses revealed no differences in spider community composition between reclaims and native prairie, although abundances of one group (Salticidae) strongly decreased on reclaims. We present one of the first studies to investigate impacts of energy development and reclamation on arthropod communities. While reclamation efforts successfully recovered abundances and biomass of arthropod herbivores and predators, Orthopteran (but not spider) community composition on reclaims has not recovered to match that of intact prairie even 30 yr after reclamation. These findings suggest that energy development may have long-term or potentially irreversible impacts to rangeland arthropod communities.

Variation in herbivore-mediated indirect effects of an invasive plant on a native plant.

Theory predicts that damage by a shared herbivore to a secondary host plant species may either be higher or lower in the vicinity of a preferred host plant species. To evaluate the importance of ecological factors, such as host plant proximity and density, in determining the direction and strength of such herbivore-mediated indirect effects, we quantified oviposition by the exotic weevil Rhinocyllus conicus on the native wavyleaf thistle Cirsium undulatum in midgrass prairie on loam soils in the upper Great Plains, USA. Over three years (2001-2003), the number of eggs laid by R. conicus on C. undulatum always decreased significantly with distance (0-220 m) from a musk thistle (Carduus nutans L.) patch. Neither the level of R. conicus oviposition on C. undulatum nor the strength of the distance effect was predicted by local musk thistle patch density or by local C. undulatum density (<5 m). The results suggest that high R. conicus egg loads on C. undulatum near musk thistle resulted from the native thistle's co-occurrence with the coevolved preferred exotic host plant and not from the weevil's response to local host plant density. Mean egg loads on C. undulatum also were greater at sites with higher R. conicus densities. We conclude that both preferred-plant proximity and shared herbivore density strongly affected the herbivore-mediated indirect interaction, suggesting that such interactions are important pathways by which invasive exotic weeds can indirectly impact native plants.

Facilitative and competitive interaction components among New England salt marsh plants.

Intra- and interspecific interactions can be broken down into facilitative and competitive components. The net interaction between two organisms is simply the sum of these counteracting elements. Disentangling the positive and negative components of species interactions is a critical step in advancing our understanding of how the interaction between organisms shift along physical and biotic gradients. We performed a manipulative field experiment to quantify the positive and negative components of the interactions between a perennial forb, Aster tenuifolius, and three dominant, matrix-forming grasses and rushes in a New England salt marsh. Specifically, we asked whether positive and negative interaction components: (1) are unique or redundant across three matrix-forming species (two grasses; Distichlis spicata and Spartina patens, and one rush; Juncus gerardi), and (2) change across Aster life stages (seedling, juvenile, and adult). For adult Aster the strength of the facilitative component of the matrix-forb interaction was stronger than the competitive component for two of the three matrix species, leading to net positive interactions. There was no statistically significant variation among matrix species in their net or component effects. We found little difference in the effects of J. gerardi on Aster at later life-history stages; interaction component strengths did not differ between juveniles and adults. However, mortality of seedlings in neighbor removal plots was 100%, indicating a particularly strong and critical facilitative effect of matrix species on this forb during the earliest life stages. Overall, our results indicate that matrix forming grasses and rushes have important, yet largely redundant, positive net effects on Aster performance across its life cycle. Studies that untangle various components of interactions and their contingencies are critical to both expanding our basic understanding of community organization, and predicting how natural communities and their component parts will respond to environmental change.

Host Plants of the Wheat Stem Sawfly (Hymenoptera: Cephidae).

Wheat stem sawfly (Cephus cinctus Norton) is a pest of economic importance across much of the wheat (Triticum aestivum L.)-growing areas of the western Great Plains of North America as well as an ecologically important insect owing to its wide range of grass hosts. Little research has been published regarding the noncultivated native and invasive grasses attacked by this insect. Knowledge of the complete host range of C. cinctus can inform future research about potential new sources of genetic resistance, improve understanding of the biology and spread of natural enemies, and better define this insect's role in grassland and agricultural systems. The aim of this review is to compile a checklist of reported host plants of C. cinctus and present data from an extensive survey of noncultivated hosts used by C. cinctus.

Spillover edge effects: the dispersal of agriculturally subsidized insect natural enemies into adjacent natural habitats.

The cross-edge spillover of subsidized predators from anthropogenic to natural habitats is an important process affecting wildlife, especially bird, populations in fragmented landscapes. However, the importance of the spillover of insect natural enemies from agricultural to natural habitats is unknown, despite the abundance of studies examining movement in the opposite direction. Here, we synthesize studies from various ecological sub-disciplines to suggest that spillover of agriculturally subsidized insect natural enemies may be an important process affecting prey populations in natural habitat fragments. This contention is based on (1) the ubiquity of agricultural-natural edges in human dominated landscapes; (2) the substantial literature illustrating that crop and natural habitats share important insect predators; and (3) the clear importance of the landscape matrix, specifically distance to ecological edges, in influencing predator impacts in agroecosystems. Further support emerges from theory on the importance of cross-boundary subsidies for within site consumer-resource dynamics. In particular, high productivity and temporally variable resource abundance in agricultural systems are predicted to result in strong spillover effects. More empirical work examining the prevalence and significance of such natural enemy spillover will be critical to a broader understanding of fragmentation impacts on insect predator-prey interactions.

Invasive insect abundance varies across the biogeographic distribution of a native host plant.

Studies of biogeographic variation in species abundances are fundamental to understanding and predicting the impacts of invasive exotic species. We quantified the abundance of the introduced and now invasive biocontrol weevil, Rhinocyllus conicus, on a newly adopted native host plant, Cirsium canescens (Platte thistle), across the plant's distributional range. We used regression and structural equation analyses to examine variation in weevil abundance at 92-108 sites over three years in relation to variation in abiotic and biotic parameters hypothesized to be important in insect or plant dynamics and distribution. We found that R. conicus now occurs throughout the majority of the range of C. canescens, even in the center of the native plant's distribution where its coevolved, targeted weed host (Carduus nutans, musk thistle) is absent. In fact, weevil densities were greater in the center of the native plant's distribution in the Sand Hills formation than in peripheral sand patches closer to areas where the targeted 'exotic thistle is common. None of the macroclimatic variables examined were consistent predictors of the large-scale variation in weevil abundance on C. canescens. In addition to biogeographic position, the only other consistent predictor of weevil densities across sites was the number of flower heads per C. canescens plant. These results exclude the "spillover" hypothesis to explain nontarget feeding on this newly adopted native host species. Instead, the results are consistent with the alternative hypothesis that exotic weevil abundance on C. canescens is related to the local availability of native floral resources. Because C. canescens densities have declined with increases in R. conicus at sites central in the plant's distribution, these results suggest that isolated, peripheral populations of C. canescens are likely to be critical for persistence of Platte thistle. More generally, this study suggests that the persistence of a native species that is impacted by an exotic natural enemy may require preservation of populations in habitats outside the optimal portion of the native species' distribution.