Cyphocleonus Achates Research Articles

A 20-year evaluation of successes with biological control of spotted knapweed (Centaurea stoebe) in Colorado

Spotted knapweed (Centaurea stoebe) is an invasive plant introduced into North America from Europe that has colonized diverse plant communities across the temperate zone of North America. The impacts of classical biological control insects on this plant have been identified as harmful (benefiting the plant), inconsequential, or exhibiting effective controls. Here we present results that measured C. stoebe densities in Colorado subjected to small numbers of insect releases in 2001 and monitored for density effects on the plant from 2007 to 2020. Seed head weevils (Larinus spp), gall flies (Urophora spp) and root weevils (Cyphocleonus achates) were also measured. In one monitoring transect with ample native perennial and introduced grasses, knapweed stem densities declined over a 14-year interval from about 50 stems/m2 to 0/m2. At a second site that was disturbed by flood scour midway through the study, stems declined over a 13 year interval from about 40 stems/m2 to under 20 stems/m2. At a site lacking C. achates, stem densities were measured at 40 stems per m2 in 2020, the same densities reported at other sites in 2008. No statistically detectable directional shifts in growing season precipitation or temperatures have occurred across the 2001–2020 interval, and the strongest relationships among variables measured here were directional declines in stem and rosette densities of the knapweed. Seeds/seed head showed positive relationships with precipitation but negative relationships with seed head Larinus spp densities, which in turn showed no directional trends in abundance. The outcome of biological control releases at these Colorado sites for C. stoebe ranged from excellent to negligible, and our work supports the context specific model of biological control, i.e., biological control in conjunction with plant competition are the essential components for reduction of C. stoebe densities.

Cleonis pigra (Scopoli, 1763) (Coleoptera: Curculionidae: Lixinae): Morphological Re-Description of the Immature Stages, Keys, Tribal Comparisons and Biology.

Mature larvae and pupae of Cleonis pigra (Scopoli, 1763) (Curculionidae: Lixinae: Cleonini) are morphologically described in detail for the first time and compared with known larvae and pupae of other Cleonini species. The results of measurements and characteristics most typical for larvae and pupae of Cleonini are newly extracted and critically discussed, along with some records given previously. Keys for the determination of selected Cleonini species based on their larval and pupal characteristics are attached. Dyar’s law was used for the estimation of a number of larval instars of C. pigra. Descriptions of habitats, adult behavior, host plants, life cycle, and biotic interactions are reported here. Adults and larvae feed on plants from the Asteraceae family only (genera Carduus, Cirsium, Centaurea, and Onopordum). Oviposition occurs on the base of the plant stem or the root neck. In the process of larval development, a fusiform gall forms. C. pigra and Cyphocleonus achates can coexist in the same locality. In open habitats, the weevils become the prey of carnivorous animals.

Impact of Cyphocleonus achates on diffuse knapweed and its interaction with Larinus minutus

The root weevil Cyphocleonus achates (Fahr.), and the seed-head weevil, Larinus minutus Gyll. were introduced into North America as part of a classical biological control program against spotted (Centaurea stoebe L. ssp. micranthos (Gugler) Hayek) and diffuse knapweed (Centaurea diffusa Lamarck). The impact of C. achates alone, or in combination with L. minutus, has not been assessed on diffuse knapweed. In a field study, densities of C. achates were initially manipulated to measure its impact on the density, size, shape, and reproductive potential of diffuse knapweed. Subsequently L. minutus was released at a constant rate across all experimental treatments to determine how attack by this species was affected by previous impacts of C. achates. C. achates reduced plant density by 58%, height by 29%, seed-head diameter by 4.6% as well as the number of seed heads per plant through its effect on plant height. Rosettes attacked by C. achates also produced more flowering stems than unattacked plants and these plants produced fewer seed heads than unattacked plants after accounting for plant size. The reductions in seed-head size had a small negative effect on attack rates by L. minutus but this was generally balanced by a positive effect of reduced seed-head numbers on L. minutus attack rates. After accounting for direct impacts of root herbivory, attack rates by L. minutus were still significantly lower in C. achates plots. We conclude that the negative effects of C. achates on diffuse knapweed outweigh the potential negative interactions with L. minutus and that the two species will likely have additive effects when released in combination at diffuse knapweed sites.

Population‐level compensation impedes biological control of an invasive forb and indirect release of a native grass

The intentional introduction of specialist insect herbivores for biological control of exotic weeds provides ideal but understudied systems for evaluating important ecological concepts related to top-down control, plant compensatory responses, indirect effects, and the influence of environmental context on these processes. Centaurea stoebe (spotted knapweed) is a notorious rangeland weed that exhibited regional declines in the early 2000s, attributed to drought by some and to successful biocontrol by others. We initiated an experiment to quantify the effects of the biocontrol agent, Cyphocleonus achates, on Ce. stoebe and its interaction with a dominant native grass competitor, Pseudoroegneria spicata, under contrasting precipitation conditions. Plots containing monocultures of each plant species or equal mixtures of the two received factorial combinations of Cy. achates herbivory (exclusion or addition) and precipitation (May-June drought or "normal," defined by the 50-year average) for three years. Cy. achates herbivory reduced survival of adult Ce. stoebe plants by 9% overall, but this effect was stronger under normal precipitation compared to drought conditions, and stronger in mixed-species plots compared to monocultures. Herbivory had no effect on Ce. stoebe per capita seed production or on recruitment of seedlings or juveniles. In normal-precipitation plots of mixed composition, greater adult mortality due to Cy. achates herbivory resulted in increased recruitment of new adult Ce. stoebe. Due to this compensatory response to adult mortality, final Ce. stoebe densities did not differ between herbivory treatments regardless of context. Experimental drought reduced adult Ce. stoebe survival in mixed-species plots but did not impede recruitment of new adults or reduce final Ce. stoebe densities, perhaps due to the limited duration of the treatment. Ce. stoebe strongly depressed P. spicata reproduction and recruitment, but these impacts were not substantively alleviated by herbivory on Ce. stoebe. Population-level compensation by dominant plants may be an important factor inhibiting top-down effects in herbivore-driven and predator-driven cascades.

Influence of plant phenostage and ploidy level on oviposition and feeding of two specialist herbivores of spotted knapweed, Centaurea stoebe

A caged field experiment was used to determine how Centaurea stoebe L. phenostage (rosette, single-stem, multiple-stem) and ploidy level (diploid=2× and tetraploid=4×) influence oviposition and feeding of two biological control agents, Agapeta zoegana (Lep.: Cochylidae) and Cyphocleonus achates (Col.: Curculionidae). Ploidy level did not influence oviposition patterns of A. zoegana but rosette and one-stem plants had significantly more eggs than multiple-stem (4×) plants. Differences in oviposition levels did not translate into differences in larval densities, but 2× plants (particularly large one-stem plants) had significantly more larvae than 4× plants. There was a significant positive correlation between numbers of larvae and root diameter. Ploidy level and phenostage both had a significant effect on C. achates feeding damage, with adults feeding more frequently on multiple-stem plants. No C. achates larvae were observed when the roots were dissected. Furthermore, the generalist herbivore Arion lusitanicus, naturally present in the garden plots, was predominantly associated with young rosette plants, a stage at which survival rate is acknowledged to be the most important determinant of knapweed density. These results indicate that the combined damage caused by A. zoegana and C. achates, superimposed on damage caused by generalist herbivores in the local community, could provide effective control for C. stoebe.

The lesser of two weevils: physiological responses of spotted knapweed (Centaurea stoebe) to above- and belowground herbivory by Larinus minutus and Cyphocleonus achates

The physiological responses of plants to variable levels of root and shoot herbivory in the field may yield valuable insights regarding potential compensation or tolerance responses for herbivory. In an infestation of Centaurea stoebe (spotted knapweed) located in the Colorado foothills, we measured physiology, biomass, and flower production of individual plants undergoing a natural range of herbivory by the above- and belowground biological control insects, Larinus minutus and Cyphocleonus achates. Over the growing season, net carbon assimilation rate, transpiration, stomatal conductance, and intercellular leaf [CO2] (C i) all decreased, while water use efficiency increased. The decrease in these physiological traits was due to an increase in the intensity of L. minutus damage over time; effects of C. achates root damage to plant physiology, including transpiration were only marginally significant. The effects of these two species on plant physiology were not interactive, and Larinus minutus was found to exert larger negative effects on this invasive plant in terms of plant physiology and potential reproductive output than C. achates. While previous studies have shown C. achates to have significant negative effects on population densities of spotted knapweed, the addition of Larinus minutus to the suite of insects used in biological control of spotted knapweed should facilitate continued or enhanced reduction in densities of this noxious weed.

Reconciling contradictory findings of herbivore impacts on spotted knapweed (Centaurea stoebe) growth and reproduction

Substantial controversy surrounds the efficacy of biological control insects to reduce densities of Centaurea stoebe, a widespread, aggressive invasive plant in North America. We developed a graphical model to conceptualize the conditions required to explain the current contradictory findings, and then employed a series of manipulations to evaluate C. stoebe responses to herbivores. We manipulated soil nitrogen and competition in a field population and measured attack rates of a foliage and seed feeder (Larinus minutus), two gall flies (Urophora spp.), and a root feeder (Cyphocleonus achates), as well as their effects on the growth and reproduction of C. stoebe. Nitrogen limitation and competing vegetation greatly reduced C. stoebe growth. L. minutus most intensively reduced seed production in low-nitrogen soils, and removal of neighboring vegetation increased Larinus numbers per flower head and the percentage of flowers attacked by 15% and 11%, respectively. Cyphocleonus reduced flower production and aboveground biomass over two years, regardless of resources or competition. Our results, in conjunction with other published studies, demonstrate that positive, neutral, and negative plant growth responses to herbivory can be generated. However, under realistic field conditions and in the presence of multiple herbivores, our work repudiates earlier studies that indicate insect herbivores increase C. stoebe dominance.

Additive effects of aboveground and belowground herbivores on the dominance of spotted knapweed (Centaurea stoebe)

Spotted knapweed (Centaurea stoebe) is found in over 3 million ha of rangeland and forests across North America, and evidence supporting the use of biological control as a regional method to reduce infestations and their associated impacts remains inconclusive. Several species of insects have been reported to reduce plant densities in some areas; however, rigorous studies that test combinations of these species and the influence of resource availability are lacking. We examined the singular and combined effects of herbivory by a root weevil (Cyphocleonus achates) and a flower head weevil (Larinus minutus) on the growth and flower production of C. stoebe. We also manipulated soil resource fertility as an additional factor that could explain the outcomes of contradictory biological control herbivore effects on C. stoebe. In a greenhouse study, herbivory by C. achates decreased flower production for plants across all resource environments. In a caged common garden study, the negative effects of herbivory also did not interact with soil nutrient status. However, the presence of plant competition further decreased knapweed growth, and the negative effects of concurrent herbivory by C. achates and L. minutus on plant biomass and flower production were additive. Derived within the context of variable levels of soil nutrient availability and competing vegetation, these results support the cumulative stress hypothesis and the contention that combined above- and belowground herbivory can reduce spotted knapweed densities and reduce the ecological and economic impacts of this species in rangelands of western North America.

Reconciling contradictory findings of herbivore impacts on the growth and reproduction of spotted knapweed (Centaurea stoebe)

Substantial controversy surrounds the efficacy of biological control insects to reduce densities of Centaurea stoebe, a widespread, aggressive invasive plant in North America. We developed a graphical model to conceptualize the conditions required to explain the current contradictory findings, and then employed a series of manipulations to evaluate C. stoebe responses to herbivores. We manipulated soil nitrogen and competition in a field population and measured attack rates of a foliage and seed feeder (Larinus minutus), two gall flies (Urophora spp.), and a root feeder (Cyphocleonus achates), as well as their effects on the growth and reproduction of C. stoebe. Nitrogen limitation and competing vegetation greatly reduced C. stoebe growth. L. minutus most intensively reduced seed production in low-nitrogen soils, and removal of neighboring vegetation increased Larinus numbers per flower head and the percentage of flowers attacked by 15% and 11%, respectively. Cyphocleonus reduced flower production and aboveground biomass over two years, regardless of resources or competition. Our results, in conjunction with other published studies, demonstrate that positive, neutral, and negative plant growth responses to herbivory can be generated. However, under realistic field conditions and in the presence of multiple herbivores, our work repudiates earlier studies that indicate insect herbivores increase C. stoebe dominance.

Revised methods for the mass-rearing of the spotted knapweed biological control agent, Cyphocleonus achates (Coleoptera: Curculionidae), in field corrals

Cyphocleonus achates (Fahraeus), a root-feeding weevil introduced from Eurasia, is an effective biological control agent against spotted knapweed, Centaurea stoebe L. ssp. micranthos. Because C. achates is univoltine and does not fly, distribution of the weevil has been slow. To hasten the weevil's distribution, a rearing effort using field corrals was initiated at a facility in Corvallis, Montana. Procedures for mass-rearing the weevil in field corrals are described, with an emphasis on improvements over earlier methods. The described field-corral approach is effective and appropriate for producing C. achates for distribution in the western United States.

Comparison of Larval Development and Overwintering Stages of the Spotted Knapweed Biological Control AgentsAgapeta zoegana(Lepidoptera: Tortricidae) andCyphocleonus achates(Coleoptera: Curculionidae) in Montana Versus Eastern Europe

Larval development of insects introduced for biological control of invasive weeds may be constrained if the new climate is more extreme than in their native range. We surveyed larval development in Agapeta zoegana L. and Cyphocleonus achates (Fahraeus), two species of biological control insects introduced from eastern Europe against spotted knapweed in western North America. We dissected spotted knapweed roots collected from five sites in western Montana over 6 yr either in late fall or early spring and measured larval head capsule size to determine the overwintering instar stage. Development of A. zoegana was estimated equally well with late fall or early spring root collections, but C. achates rate of development may be underestimated using fall samples. The larvae of neither species entered diapause in as advanced an instar in western Montana as reported for their native range. Most A. zoegana larvae reached the third (26%) or fourth (20%) instar at diapause, with only 15% reaching sixth instars, as they typically do in their native Eurasia. Almost all (94%) C. achates overwintered as first instars, with most of the remaining (4%) being eggs, some of which were viable in the spring. Only a small number (2%) of C. achates larvae overwintered as second instars, the common overwintering stage in their native range. Slower development may explain, in part, why A. zoegana only has one generation per year in Montana compared with two to three generations per year in Europe.

Effect of Summer Drought Relief on the Impact of the Root Weevil <I>Cyphocleonus achates</I> on Spotted Knapweed

A recent decline in spotted knapweed, Centaurea stoebe L. subsp. micranthos (Asteraceae), has been observed in parts of western Montana. The release of the biological control agent Cyphocleonus achates (Fahraeus) is thought to contribute to the decline, but persistent drought since at least 1999 may be an additional factor. We conducted outdoor plot experiments to test the relative impacts of C. achates weevils and summer drought relief on spotted knapweed survival and growth. Groups of spotted knapweed transplants were assigned to one of four weekly water addition treatments (no added water, and 0.25, 0.5 or full recovery of plant water deficit, where "deficit" refers to potential evapotranspiration minus rainfall) in May to August 2004 and June to August 2005 and to either exposure to or protection from C. achates. In June of each subsequent year (2005 and 2006), plants were harvested and growth attributes that reflect plant vigor were measured. Drought indices showed that throughout the time of the study until January 2006, western Montana was in drought alert or severe drought. Summer drought relief had no effect on aboveground biomass and plant height of knapweed plants in subsequent years, but feeding by C. achates larvae reduced these two measures of plant vigor. Knapweed plants resuming growth after the drought ended in spring 2006 were significantly larger than those resuming growth under drought conditions in spring 2005. Spring drought may reduce knapweed growth, but C. achates reduced knapweed growth regardless of drought conditions.

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of <I>Cyphocleonus achates</I> (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

The root-feeding weevil Cyphocleonus achates (Fahraeus) is a promising biological control agent for managing the exotic, invasive weed spotted knapweed. The objective of this study was to compare the relative and potentially interactive effects of competition and specialized herbivory on spotted knapweed fitness. Competition was assessed through three grass seeding treatments: a nonseeded control and seeding to long-lived wheatgrasses originating either from North America, Pseudoroegneria spicata (Pursh) A. Love ssp. spicata, or Europe, Thinopyrum intermedium (Host) Barkworth and D.R. Dewey. Comparisons were made of above-ground biomass, stem length, number of flowers, and life stage densities for spotted knapweed plants either infested or free of C. achates and growing under one of the grass seeding treatments. Infested adult spotted knapweed plants had lower shoot weight, stem length, and flowers/plant. The density of adult spotted knapweed plants was lower in grass-seeded plots than in nonseeded control plots. Results indicate C. achates herbivory is correlated with a reduction in measures of fitness and reproductive potential in field populations of spotted knapweed in North America. Furthermore, these results suggest that this agent might play a significant role in reducing spotted knapweed populations when competitive grasses are also present.

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Influence of Herbivory and Competition on Invasive Weed Fitness: Observed Effects of Cyphocleonus achates (Coleoptera: Curculionidae) and Grass-Seeding Treatments on Spotted Knapweed Performance

Compatibility of Two Herbicides withCyphocleonus achates(Coleoptera: Curculionidae) andAgapeta zoegana(Lepidoptera: Tortricidae), Two Root Insects Introduced for Biological Control of Spotted Knapweed

Field studies were conducted in 2001 through 2004 to assess the compatibility of two herbicides, 2,4-D (2,4-dichlorophenoxyacetic acid) and clopyralid (3,6-dichloropicolinic acid), with two root insects, Cyphocleonus achates (Fahraeus), and Agapeta zoegana L., introduced for biological control of spotted knapweed, Centaurea stoebe Lamarck subsp. micranthos (formerly C. maculosa) in Montana. Both herbicides were applied at the fall and spring rosette stage. In 2003, both herbicides reduced knapweed canopy cover by ≈98% compared with ≈85% in 2004. The number of live larvae of both insect species was significantly lower in treated plots than in controls in 2003. In 2004, the number of live C. achates larvae was significantly lower in treated plots than in controls at the fall application, but larval numbers were not different at the spring application. Larval numbers of C. achates were not different between application times in 2003 but were significantly lower in fall-treated plots than in spring-treated plots in 2004. Numbers of A. zoegana larvae were not different between treated plots and controls in 2004. Larval numbers of A. zoegana were significantly lower in fall-treated plots than in spring-treated plots in 2003, but there was no difference between application times in 2004. Larval numbers of each insect species were similar between herbicides in both years. We conclude that fall applications of both herbicides are not compatible with the two insects. Spring applications of the two herbicides may be compatible with both insect species if delayed until late spring.

Decline of spotted knapweed density at two sites in western Montana with large populations of the introduced root weevil, Cyphocleonus achates (Fahraeus)

Abstract Spotted knapweed is an important weed of rangeland in the northwestern United States and western Canada, and has been the focus of considerable biological control efforts. Cyphocleonus achates, a Eurasian root weevil, has been released as a biocontrol agent against the weed in many areas of Montana and the Pacific Northwest. Spotted knapweed plant density was monitored over a 11-year period (1993–2004) at two sites in western Montana where C. achates was released. Spotted knapweed density declined significantly over time at both sites (99 and 77%, respectively), after C. achates numbers increased dramatically at both sites. The average annual population growth rate of C. achates during 1993–1995 was 14% at the two sites, compared to 10% during 1995–1998. The weevil population radially expanded a distance of 73 m per year at one study site compared to 99 m per year at the other site. The estimated C. achates population increase at the two sites was similar: in 1998, the C. achates population estimate for one site was 90,776 in a 23 ha area of occupation, compared to 97,173 in a 29 ha area at the other site. Evidence suggests that C. achates played a major role in the spotted knapweed population decline. Following the decline of spotted knapweed, two annual exotic weeds, Bromus tectorum and Descurainia sophia, became major components of the new plant community, with B. tectorum becoming the dominant plant. By 2004, B. tectorum comprised 89 and 50% of the replacement vegetation at the two sites.

Growth and development of the knapweed root weevil, Cyphocleonus achates, on a meridic larval diet

Abstract Cyphocleonus achates , the knapweed weevil, is an effective biological control agent of the invasive weed, Centaurea maculosa Lam. A meridic diet was developed and tested for the rearing of the larval stage of this insect. Using this diet, C. achates was reared for over three generations, with the adults being offered knapweed plants for feeding and oviposition in greenhouse conditions. Slight or no differences were seen between insects reared on a standard meridic diet formulation and one containing knapweed tissues. The following life history parameters were monitored over the three generations: percent egg hatch (ranging from 42.9 to 59.1%), time to egg hatch (20.0–23.2 days), time to adult emergence (52.0–54.1 days), adult weights 3 days post-eclosion (101.9–117.0 mg), percent adult emergence (48.3–58.6%), and percent mortality/deformity in the different stages (with mortality occurring primarily in the early larval stages). Additionally, a study involving low temperature and short day conditions suggested that C. achates could be maintained for longer periods of time in larval diet cells when placed in growth-retarding conditions, although percent adult emergence was lower. External morphology was also studied in order to distinguish between the sexes to ensure that each adult cage had a similar ratio of females to males. Abdominal features were found to be the most dependable characteristics for use when determining the sex of adult C. achates .

Biological Control Insect Use of Fertilized and Unfertilized Diffuse Knapweed in a Colorado Grassland

The responses and impacts of five insect species that feed on Centaurea diffusa Lamarck, diffuse knapweed, to soil nitrogen and phosphorus additions were studied in grasslands east of the Colorado Front Range. We predicted that fertilization was unlikely to have a direct effect on herbivory but that if the insects preferentially select vigorous plants, increased tissue nitrogen, plant density, or plant mass resulting from fertilization would increase plant susceptibility to biological control insects. Fertilization caused a modest increase in the biomass of C. diffusa adults and the nutritive content of aboveground plant biomass. Larinus minutus Gyllenhal, a seed head weevil, was less abundant and exhibited reduced impacts on fertilized C. diffusa plants. Abundance of gall fly larvae, Urophora spp., was negatively influenced by the presence of L. minutus and had no detectable effect on seed abundance. No positive or negative impacts of root feeders on knapweed were observed, but the root-feeding Cyphocleonus achates (Fahraeus) selected plants in fertilized plots, whereas the root phytophage Sphenoptera jugoslavica Obenberger was more common on knapweed in unfertilized plots. Our results indicate that these specialist biological control insects exhibit little preference based on plant biomass, density, or nutritive quality. However, variables that affect the timing of knapweed seed production, such as fertilization, mowing, and grazing, reduce insect impacts and may explain some of the regional variation in effectiveness of these insects as biological controls on knapweed.

Comparative Efficacy of Adult and Larval Sampling for Detection of the Root-Boring InsectsAgapeta zoegana(Lepidoptera: Cochylidae) andCyphocleonus achates(Coleoptera: Curculionidae) Released for Biological Control of Spotted Knapweed

In biological control projects, establishment of released natural enemies is a key step and must be efficiently detected. We studied the relative efficacy of larval versus adult sampling to detect establishment on spotted knapweed, Centaurea maculosa Lamarck, of the two root feeding insects Agapeta zoegana L. and Cyphocleonus achates (Fahraeus). Larval sampling was based on excavation and dissection of plant roots. Adult sampling consisted of either sighting adults along transects at release sites or collection of adults by sweep netting. Recovery rates for A. zoegana were higher through adult visual sampling (54.8%) than through larval sampling via root dissection (43.0%). Adult visual sampling required less time (30 min/site) than did root dissection (130 min/site). Sweep net sampling, although having the lowest detection rate (38.1%), required even less time (10 min/site) and was the most effective method per unit time. In contrast, for C. achates, larval sampling was the most effective method, with a recovery rate of 35.6%, compared with 8.9% for adult visual sampling. Sweep netting was more effective than visual sampling, with a detection rate of 18.1%. Adult visual sampling required less time (44 min/site) than did root dissection (130 min/site). Sweep net sampling, although having the lowest detection rate (18.1%), required even less time (10 min/site) and was the most effective method per unit time. However, other factors such as weather, travel time, and training levels needed for sampling make root sampling a more effective method, in a larger sense, for both of these insects.

Use of Picloram to Enhance Establishment of <I>Cyphocleonus achates</I> (Coleoptera: Curculionidae)

Abstract Herbicidal control of spotted knapweed, Centaurea maculosa Lamarck, is rarely cost-effective, and sustainable control may require an integrated approach. Cyphocleonus achates (Fahraeus) is a flightless root-feeding weevil of Eurasian origin that has been introduced into North America for biological control of spotted knapweed. We hypothesized that reducing the density of spotted knapweed using reduced rates of picloram would improve the establishment of C. achates. At two sites in western Montana, three adult weevil densities (none, three, and six weevils m−2) were released in the fall (1995 at site 1 and 1996 at site 2) in 2-m2 plots encircled with enclosures to prevent weevil escape. Six picloram rates (0, 0.03, 0.06, 0.09, 0.12 or 0.15 kg ha−1) were applied the following spring before weevil emergence in a randomized complete-block design with four replications (18 treatments per replication). Spotted knapweed density and spotted knapweed and grass cover were sampled in July each year followin...