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Impacts of the introduced biocontrol agent, Rhinocyllus conicus (Coleoptera: Curculionidae), on the seed production and population dynamics of Cirsium ownbeyi (Asteraceae), a rare, native thistle
Biological Control 55 (2010) 79–84
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Impacts of the introduced biocontrol agent, Rhinocyllus conicus (Coleoptera: Curculionidae), on the seed production and population dynamics of Cirsium ownbeyi (Asteraceae), a rare, native thistle Michelle E. DePrenger-Levin a,*, Thomas A. Grant III b, Carol Dawson c a b c
Denver Botanic Gardens, Research and Conservation Department, USA Colorado State University, Graduate Degree Program in Ecology (GDPE) and the Department of Forestry, Rangeland and Watershed Science, USA US Bureau of Land Management (BLM) – Colorado State Office, USA
a r t i c l e
i n f o
Article history: Received 17 November 2009 Accepted 19 July 2010 Available online 23 July 2010 Keywords: Carduus nutans Cirsium ownbeyi Colorado Native plants Non-target effects Rhinocyllus conicus
a b s t r a c t The release of non-native insects to control noxious weeds is commonly used to combat invasions without disturbing the environment through chemical or mechanical methods. However, introduced biological control agents can have unintended effects. This study was initiated to evaluate potential non-target effects of the flowerhead weevil, Rhinocyllus conicus Frölich, on Cirsium ownbeyi S.L. Welsh, a rare, native and short-lived perennial thistle in northwestern Colorado, northeastern Utah, and southwestern Wyoming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has naturalized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate (k) from a count-based population viability analysis of the population was 1.03; however, large inter-year variation indicates this rare species is still vulnerable to local extirpation. The biocontrol weevil consistently damaged the developing seeds over the course of the study independent of changes in overall population size and variation in the number of flowering individuals. The target species, Carduus nutans L. (musk thistle) is generally absent near the study plots, which may limit the population levels of R. conicus that can be sustained in this area. Although R. conicus utilizes C. ownbeyi as a host plant, the late flowering period of this native thistle and the small size of the flower heads may limit the demographic impact of R. conicus on C. ownbeyi. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The Eurasian flowerhead weevil, Rhinocyllus conicus Frölich, was introduced in 1963 as a classical biological control agent against a suite of invasive thistles, especially Carduus spp. (Kok and Surles, 1975; Turner et al., 1987; Kok, 2001). Successful or substantial biological control of musk thistle (Carduus nutans L.) has been noted in much of the United States (Goeden and Ricker, 1977; Puttler et al., 1978; Batra, 1980; Littlefield, 1991; Lambdin and Grant, 1992; Boldt and Jackman, 1993; Buntin et al., 1993). Substantial non-target impacts to native North American thistles have also occurred (Pemberton, 2000). While R. conicus was known to feed on Carduus, Cirsium, Silybum, and Onopordum species in its native European range, it was not expected to feed significantly on the native North American thistles. Initial R. conicus releases on Carduus nutans showed populations of the weevil increased greatly in range but not in density (Hodgson and Rees, 1976; Reed et al., 2006). * Corresponding author. Address: 909 York Street, Denver, CO 80206. Fax: +1 720 865 3683. E-mail address: [email protected] (M.E. DePrenger-Levin). 1049-9644/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2010.07.004
Evidence of R. conicus on native Cirsium spp. was reported in growing numbers of states including southwestern Montana in 1974 (Rees, 1977) and California in 1982 (Turner et al., 1987; Louda et al., 1997). Biocontrol releases continued through the 1970s despite knowledge that the weevil’s host plants included native North American thistles (Rees, 1977; Harris, 1988; Louda et al., 1997; Pemberton, 2000 and references therein). As confirmation of R. conicus feeding on native thistles spread across North America, so did concerns regarding the possibility of some native species becoming extinct (Louda et al., 1997, 1998, 2005; Rose et al., 2005). Pemberton (2000) reports that 22 of 90 North American Cirsium spp. are known hosts of R. conicus including Cirsium canescens Nutt., C. centaureae (Rydb.) K. Schum., C. hillii (Canby) Fernald, C. pulchellum (Greene) Woot. & Standl., C. tweedyi (Rydb.) Petrak, C. undulatum (Nutt.) Spreng., and C. undulatum (Nutt.) Spreng. var. tracyi (Rydb.) S.L. Welsh (Louda et al., 1997; Louda and O’Brien, 2002; Rose et al., 2005; Russell and Louda, 2005; Sauer and Bradley, 2008). Many states have ceased further releases of the biocontrol agent after observing damage on nontarget native species (Sauer and Bradley, 2008). However, R. conicus populations continue to spread naturally and more information is
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needed on the quantitative impacts of R. concius on rare Cirsium spp. to understand the potential threats to native thistles. This study documents a population consisting of over twelve sub-populations of C. ownbeyi in northern Colorado in order to quantify basic demographic parameters and presence and magnitude of any effects of the biocontrol weevil, R. conicus, on achene production and viability and therefore to determine if management intervention is needed to conserve the population of plants or even the species as a whole. 2. Materials and methods 2.1. Natural history The weevil, Rhinocyllus conicus, damages thistles by feeding and pupating within the receptacles of the capitula (Shorthouse and Lalonde, 1984; Jongejans et al., 2006). In spring, the adult female weevil deposits eggs on the outside of developing inflorescences’ involucral bracts, covering them with a hard coating of masticated particles of the host plant material (Shorthouse and Lalonde, 1984). The oviposition period lasts approximately five weeks (Buntin et al., 1993; Roduner et al., 2005). When the eggs hatch, the larvae burrow through the bracts into the head of florets where they feed, develop, and remain during pupation. Dissection of capitula reveals that larval tunnels and chambers in the receptacle disrupt achene development by severing vascular bundles that would otherwise transport nutrients to the developing seeds (Shorthouse and Lalonde, 1984). Achenes will be referred to as seeds hereafter. Studies of the effects of R. conicus larvae on the seedheads of musk thistles have identified reductions in seed production as the number of R. conicus increased (Rees, 1991; Louda et al., 1997). The non-target host plant, Cirsium ownbeyi, is a polycarpic, short-lived perennial herb that is known from approximately 30 extant populations, containing an estimated 25,000 individuals in Moffat County, Colorado, the Uintah basin of northeastern Utah, and southwestern Wyoming (Spackman et al., 1997). It occupies an elevational range of 1600–1900 m and its rarity is not believed to have anthropogenic origins. Cirsium ownbeyi is found in juniper, sagebrush, and riparian communities usually consisting of gravelly alluvium, talus, sandstone or occasionally limestone (Spackman et al., 1997). It flowers in June to August (Keil, 2006) and can return to a non-flowering, vegetative state in subsequent years. Cirsium ownbeyi was designated as a sensitive species by the Bureau of Land Management (BLM, US Department of the Interior) in 1987 (US Fish and Wildlife Service, 1996). The study site was established in 1998 by Denver Botanic Gardens and the BLM and is in the vicinity of Cross Mountain Canyon, a BLM wilderness study area in Moffat County, Colorado. The study site was located in the eastern-most portion of Cross Mountain Canyon, a deep canyon formed by the Yampa River. The rocky slopes of the south side of Cross Mountain Canyon (north-facing) provide mesic microhabitats that can support moderate numbers of C. ownbeyi. 2.2. Field protocols Surveys in 1988 and 1998 located the Cross Mountain Canyon population of C. ownbeyi and subsequently verified it as a population of sufficient size to implement population monitoring and evaluation of insect floral herbivory. The steep and rugged terrain of the canyon makes surveying the area difficult, and acts as a natural barrier to anthropogenic disturbance from recreational users of the canyon. At Cross Mountain Canyon, plants are often found in erosional channels of steep hillsides and in areas of active surface rockslides.
In 1998, all plants found within an approximately 600 100 m section of the canyon were flagged and circular plots 5 m in diameter were demarcated in 12 areas with the largest concentration of plants. Four of the original plots could not be located in subsequent years. Two of these were ‘lost’ in 1999 (of which one was relocated in 2005) and one each in 2000 and 2005. Another two plots could not be located in 2003 but were relocated in 2004. At different times during the study, three plots needed to be retagged due to rockslides covering the original markers. An additional plot was added to the set in 1999 so that ten plots remained at the end of the study, of which six plots had consecutive monitoring data on C. ownbeyi density and demographics for all eight years of the study. The center of each plot was marked and the position of each of the surrounding plants was recorded by measuring its distance and compass bearing. Plants were considered the same individual if the location measurement for the preceding or subsequent year was within seven degrees and 10 cm. For each plant the life stage (seedling, vegetative or reproductive) was recorded. The capitula were then collected and stored separately by positions on the plant. Plots were visited once a year during the flowering period. Due to this, seedlings bearing cotyledons were missed during data collection. New individuals within a plot were recorded as seedlings, reflecting their status during the interval between annual samples. To quantify seed production and weevil occurrence, an entire flowering stalk, including all capitula, was collected each August from three randomly selected reproductive plants within each plot, except in 2002 and 2003 when no seeds were collected because a severe drought limited reproduction. Only three capitula could be collected in 2004. In 2004 and 2005, flowers were dissected on site so that the seeds could be scattered in the vicinity of the parent plants and thereby minimize experimental disruption of reproduction. Capitulum samples collected from 1999 to 2001 were dissected at Denver Botanic Gardens. Using previously published methods (Surles and Kok, 1978; Louda and Potvin, 1995; Louda, 1998) the following measurements were made for each capitulum: weight (g) (the processing of samples in the field precluded the weighing of capitula in 2004–2005), diameter (mm), egg casings, number of first instar weevil entry holes, developmental stage of seed (undeveloped, aborted, developed), presence of insect damage on individual seed, seed color (1–5 scale), seed length (mm), number of weevil pupal chambers per capitulum, number of adult weevils present per capitulum, and whether the capitulum had shed its seeds prior to collection. In 2005 only the number of first instar entry holes and the diameter of the capitula were recorded. Although the larvae of moths and flies as well as several aphids, chrysomelids, myrids, and grasshoppers may feed on Cirsium spp. (Rose et al., 2005), there was no evidence of damage other than that attributed to R. conicus in any of the samples. Each capitulum was sorted into the following categories: seeds dispersed, partially dispersed, or not dispersed. Most capitula were collected before seed dispersal. Viable seed per capitulum were counted as was seed damaged by the weevil. Undeveloped, aborted and insect damaged seed are consistently darker in color than healthy, viable mature seeds which are light brown or tan (Shorthouse and Lalonde, 1984). Color of the seeds was used in conjunction with physical damage to the seeds to determine the percent damaged by weevils. Both developed and undeveloped seeds were categorized as viable when they were neither physically deformed due to predation nor in the two darkest color categories (4 or 5). Maturation of C. ownbeyi seed continues into September, therefore, the undeveloped undamaged seeds collected in August were treated as potentially viable seeds in the analyses. Viable seeds collected before 2004 are stored at the National Center for Genetic Resources Preservation (US Department of Agriculture, Fort Collins
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Colorado) as part of the Center for Plant Conservation’s National Collection of Endangered Plants and are available to qualified programs for research or restoration projects. 2.3. Data analysis Repeated measures analysis of variance (RM ANOVA) was used to analyze change in population size over time. Correlations were used to examine the number of oviposited eggs, differences in the percent of reproductive and vegetative individuals and the percent of seed with physical damage. Unpaired t-tests assuming equal variance were used to compare the number of seed in capitula with eggs to those lacking eggs. Logistic regression was used to compare the number of capitula containing seed with physical damage to capitula without damaged seeds across the weight distribution of capitula. Linear regression was used to determine relationships between the percent of seed with physical damage and the number of eggs per capitulum. Analyses were conducted using SAS/JMP INÒ software (Littell et al., 1996). Growth rate (k) and extinction predictions were made following Morris and Doak (2003) using a count-based population viability analysis in R (R Development Core Team, 2008). To calculate the growth rate (k) the total count of individuals from each plot with eight years of consecutive data was summed. The log of each yearly transition was averaged and a 95% confidence interval on the estimate of k was calculated using a linear regression of the yearly log population growth rate on the amount of time elapsed with the intercept forced through zero (Morris and Doak, 2003). 3. Results 3.1. Population monitoring Plant numbers fluctuated in the plots over time but did not change significantly over the eight years of the study (F(1,4) = 5.90, p = 0.0721) (Fig. 1). The average population size dropped to a low of 9.9 plants per plot in 2002 and increased to a high of 17.4 plants per plot in 2005. Although new individuals, considered seedlings, were documented in all but two years of the study, recruitment of individuals was generally uncommon in the study plot. Vegetative individuals made up over 83% of the individuals observed over the course of the study, reproductive individuals composed 14% and seedlings composed 3% of the individuals (Fig. 1). Most (64%) of the individual plants, tracked by direction and distance from the center marker, lived for two years.
Fig. 1. Average Cirsium ownbeyi S.L. Welsh individuals per plot over the eight years of monitoring. Bars indicate +/ SE.
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The longest life span observed was for two individuals which had persisted for six years and which were both still alive when monitoring stopped. Based on data collected from individuals within the six plots with data for all eight years, the average population growth rate (k) was 1.03 with a 95% confidence interval ranging from k = 0.78 to 1.37. The average number of plants per plot fluctuated by as much as 51% in a single year interval amounting to a change of 7.6 plants per plot. The number of individual plants (171) remaining in 2005 and found in all plots with consecutive data were used to model the population’s viability (count-based) and probability of becoming functionally extirpated (quasi-extinction). If the population is considered functionally extirpated when it falls to two or fewer individuals, and the growth rate remains nearly flat, then the population has approximately a 30% chance of becoming extirpated in 50 years (Fig. 2). 3.2. Capitula, casings and seeds Measurements on capitula are summarized in Table 1. From 1999 through 2001, each reproductive plant produced a mean ± SE of 6 ± 0.99 viable seeds per plant compared to an overall average of 83 seeds produced per reproductive plant per year. Including all seeds (viable, damaged, and undeveloped or aborted) collected in 1999–2004, a mean of 15.4 ± 1.7 seeds were found in the primary terminal capitulum, 12.5 ± 2.1 in the secondary terminal capitulum, and 13.5 ± 2.8 and 15.9 ± 4.9 in the terminal capitula of the successive lateral branches. Overall, 1999 had the highest average number of seeds per capitulum (18.04 ± 2.3). There were significantly fewer seeds (7.3 ± 0.79 per capitulum) over the subsequent four years in which seeds were counted (F(3,25) = 3.391, p = 0.034, r2 = 0.262) (Table 1), with a mean of only 4.0 ± 0.78 viable seed per capitulum during that period. Capitula with no weevil damage produced significantly more seed (16.4 ± 1.4) than capitula with one or more larval entry holes (9.7 ± 1.6), (t = 3.2, p = 0.0017). Of the capitula analyzed in the laboratory, 43% had eggs (Table 1). Heavier capitula were significantly more likely to contain seed damaged by the weevil than lighter capitula (p < 0.001, r2 = 0.17). The number of weevil eggs per capitulum significantly decreased
Fig. 2. Probability of quasi-extinction of Cirsium ownbeyi S.L. Welsh based on the number of individual plants in a plot and a 50 year timeframe. Quasi-extinction is defined as fewer than two individuals.
10.18 (0.41) 49 0.13 (0.01) 51 2.88 (0.51) 51 0.43 (0.07) 51 8.83 (0.43) 37 0.10 (0.01) 47 1.60 (0.54) 47 0.43 (0.07) 47 9.86 (0.45) 43 0.15 (0.01) 43 1.98 (0.56) 43 0.44 (.08) 43 11.6 (1.33) 3 - 1.00 (2.12) 3 0.33 (.29) 3 38.4 39.8 26.9 14.3
(6.21) 23 30.53 (9.66) 15 47.33 (8.47) 9 18.04 (8.26) 13 49.00 (18.70) 4 35.78 (8.47) 9 14.00 (7.02) 18 – – 26.89 (5.99) 18 11.26 (17.2) 3 – – 14.33 (14.67) 3 14.33
(2.30) 49 20.70 (3.37) 27 14.77 (2.74) 22 6.37 (1.31) (2.65) 37 22.00 (4.13) 18 6.42 (2.94) 19 2.13 (1.36) (2.46) 43 13.88 (3.58) 24 7.95 (2.94) 19 3.51 (1.42) (9.30) 3 21.00 (12.38) 2 1 (12.84) 1 0.33 (5.38)
51 47 43 3
4. Discussion
1999 2000 2001 2004
Mean (SE) N Mean (SE) N Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
Average capitula damaged by weevil Average eggs/ capitulum Average capitulum weight (g) Average Percent of Average # of Average # of seeds/capitulum seeds/capitulum seeds damaged capitulum diameter (cm) by weevil damaged weevil present by weevil Average # of seeds/capitulum no weevil Average # of seeds per capitulum Average # of seeds/plant weevil present Average # of seeds/plant no weevil Average seeds per plant Year
Table 1 Summary seed data per Cirsium ownbeyi plants and per capitula collected in August of 1999–2001 and 2004. All seed, viable and damaged, are included.
as the density of plants found in the plots increased (F(1250) = 6.12, p = 0.014, r2 = 0.024) while the percent of seeds with visible damage was not significantly correlated with plant density. A linear regression of the sum of all seeds damaged by the weevil on the number of eggs per capitulum showed a significant positive relationship (F(1142) = 83.24, p < 0.0001, r2 = 0.37). A repeated measures analysis of variance determined that the number of larval entry holes per capitulum did not vary significantly between years (F(1140) = 1.16, p = 0.3261) (Fig. 3). Overall, 35.5% of potentially viable seed were either physically deformed or a darker color (4 or 5) because of weevil larvae, and a 95% reduction in viable seed was documented in capitula affected by the weevil compared to the potential viable seed produced in capitula that were weevil-free. It has been shown that on Carduus nutans, the number of egg casings tended to decrease on successively lower lateral capitula (Puttler et al., 1978; Denslow and D’Antonio, 2005). However, on C. ownbeyi there was no significant linear relationship between the positions of the capitula and the amount of insect damage (F(6250) = 0.8035, p = 0.568). However, a general pattern was observed of decreasing number of first instar entry holes and lower likelihood to have first instar entry holes on successively lower lateral and later developing capitula.
44.11% 26.89% 31.03% 3.30%
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N
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Despite the limited number of reproductive plants, population fluctuations, low levels of recruitment and moderate to high levels of insect damage to seed (35.5%) and capitula (43%), the Cross Mountain Canyon population of C. ownbeyi appears to be stable in the short-term. The goal of this study was to determine if the introduced biocontrol agent, R. conicus, is a threat to the survival of C. ownbeyi. Collection of plant demographic and insect herbivory data is essential to the decision making processes concerning management of this plant species and management of the biocontrol’s non-target effects. The discovery of R. conicus in the capitula of C. ownbeyi represents a potential threat to the seed production and long-term viability of this population. Since R. conicus has been released for biological control use, it has been documented to migrate as much as 3.2 km from its origin of release in one year and as much as 32 km in six years (Kok and Surles, 1975; Puttler et al., 1978). Movement of R. conicus to nontarget species has been reported as a ‘‘spillover effect” with the largest weevil egg densities on capitula closest to C. nutans populations (Rand and Louda, 2004). Despite the lack of the target species in and around the study plots, R. conicus was able to migrate to these populations of C. ownbeyi. The population size of R. conicus appears to be stable on C. ownbeyi. The weevil had first been observed in Cross Mountain Canyon at least ten years before the start of this study. While R. conicus populations have been shown to exhibit exponential growth rates in the first years of invasion (Louda, 1998), the population at this site has likely reached its carrying capacity. Movement to non-target Cirsium species in low densities has been observed (Laing and Heels, 1978; Louda, 1998). While the presence of the biocontrol agent in other populations of C. ownbeyi was not addressed by this study, unpublished data documented R. conicus in only one of 22 studied C. ownbeyi populations in adjacent Dinosaur National Monument (Gary J. Dodge, 2005). The population of C. ownbeyi at Cross Mountain Canyon fluctuated within the study plots over the course of the study, yet overall experienced a relatively flat growth rate (k = 1.03). Natural surface disturbances (rockslides) continually modify the habitat and often cover known occurrences of C. ownbeyi with talus and scree which tends to destroy existing populations while potentially creating habitat for the establishment of new ones. Large yearly fluctua-
M.E. DePrenger-Levin et al. / Biological Control 55 (2010) 79–84
Fig. 3. Number of first instar boring holes per capitulum per year of inflorescence collection. Whiskers represent the upper fence and points are potential outliers.
tions in population size contribute to the wide confidence interval of k (0.78 to 1.37). This could indicate an episodic or cyclical pattern of seedling recruitment based upon environmental factors not analyzed in this study. These factors indicate that despite the flat growth rate, this population is at great risk of extirpation should multiple years of decline occur. A count-based population viability analysis indicated that the population in 2005 had approximately a 30% chance of extirpation in 50 years. However, if the population size drops as it did by 40% in one year, the threat of local extirpation increases. There is enough variability seen in yearly population size to warrant some concern that this species will become locally extirpated. This count-based viability analysis did not take into account seeds in the soil seed bank which would buffer the threat of local extirpation. A soil seed bank likely exists given the documented increases in population size. The flowerhead weevil, R. conicus, has had a negative effect on C. ownbeyi’s seed development although not on the population size at Cross Mountain Canyon. Rhinocyllus conicus reduces seed set in the capitula it utilizes by 95%; however, it utilizes less than half of the capitula available leaving an average of 83 seed produced per reproductive plant per year. Of these seeds, only an average of six seed per reproductive plant was observed to be potentially viable. The 2005 study by Gary J. Dodge (unpublished data) found no mature seeds in C. ownbeyi capitula infested with the biocontrol agent compared to a mean of 14.98 ± 1.07 seeds per uninfested capitula. Louda (2000) documents that R. conicus has caused an 80% reduction of viable seeds in the common Platte thistle (Cirsium canescens Nutt.) after only four years of exposure to the weevils. Rhinocyllus conicus may not cause a decline in population size but may dampen population increases through reduced reproductive output which could cause local extirpation due to the small population size and environmental stochasticity. The phenology of C. ownbeyi and size of its capitula may restrain the negative effect of R. conicus. Cirsium ownbeyi flowers later in the season than the target host resulting in little overlap between flowering and ovipositing by R. conicus and does not co-occur with other potential host thistles at the Cross Mountain Canyon site that could support a population of R. conicus. Completion of ovipositing has been recorded at different times in different climates (i.e., late June in Nebraska and early August in Utah) (Smith and Kok, 1987;
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Youssef and Evans, 1994; Roduner et al., 2005; Russell and Louda, 2005). In addition, capitula of Carduus ssp. which develop late in the flowering season experience diminished levels of weevil feeding damage (Hodgson and Rees, 1976; Boldt and Jackman, 1993). Later flowering capitula of C. ownbeyi were never observed to contain egg casings. Additional secondary (non-apical) inflorescences may develop after the weevil has finished its ovipositing period thus limiting the time period for phenological synchrony between the plant and weevil (Surles and Kok, 1977; Feldman, 1997; Russell and Louda, 2005). The weak negative correlation of first instar entry holes and the number of developed seeds could be due the small size of C. ownbeyi capitula compared to the target species. Rhinocyllus conicus appears to be limited in the amount of larvae that can survive in one capitulum (Hodgson and Rees, 1976; Goeden and Ricker, 1985). Carduus nutans, however, produces capitula that are nearly 4 times as large as C. ownbeyi (Kok et al., 1986). No more than two larvae were found in one C. ownbeyi capitulum which may afford it the advantage of being a smaller and later flowering species thus limiting the size of the R. conicus population that can be supported by the presence of only one thistle species. The consistent level of impact by R. conicus despite increases in the density of C. ownbeyi could be due to the general absence of the target species, Carduus nutans. When the weevil first established at the Cross Mountain site it is possible there was a decline in the C. ownbeyi population similar to that seen in other Cirsium spp. (Louda, 1998). However, the stable weevil population does not appear to reduce the population size of C. ownbeyi in the short-term but may limit any population growth. Climate change could affect the phenological synchrony of these species and dramatically increase the damage R. conicus exerts on C. ownbeyi. The combination of population size fluctuations and a steady amount of damage by the biocontol puts the long-term viability of C. ownbeyi at risk. If evidence of population extirpation becomes apparent due to the effects of R. conicus, the BLM can take additional action with these data in mind to conserve this species by limiting the spread of the biocontrol agent.
5. Role of the funding source Denver Botanic Gardens provided support for this project along with funding through an Assistance Agreement from the Colorado state office of the Bureau of Land Management (BLM) from 1998– 2005. The BLM conducted surveys to identify the population of Cirsium ownbeyi at Cross Mountain Canyon and developed the study design in 1998. BLM staff collected data with Denver Botanic Gardens in 2000 and were involved in early data analysis. BLM staff also contacted Dr. Svata Louda in April 1999 to use her protocol for seed head collection and analysis. The BLM Colorado State Botanist was consulted on study design and was given annual reports. The BLM did not influence the decision to submit the paper for publication but was given the paper for review before submission.
Acknowledgments We acknowledge Carol Spurrier, a BLM botanist, who provided funding and helped set up the experimental design. We thank Dr. Svata Louda for providing her protocol for seed head analysis and Dr. Jennifer Neale for assistance editing this manuscript. We would also like to acknowledge the many interns and Denver Botanic Gardens’ staff who assisted with summer field work: Bethany Demarco, Dr. Peter Gordon, Chris Story, Nathan Keller, Sierra Smith, Kristin Schou, Jonathan Almond, Cathy Steward, Chris Malone, Aaron Shiels, Michael Denslow, Shannon D. Fehlberg, and R. Powers.
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Louda, S.M., Rand, T.A., Arnett, A.E., McClay, A.S., Shea, K., McEachern, A.K., 2005. Evaluation of ecological risk to populations of a threatened plant from an invasive biocontrol insect. Ecological Applications 15, 234–249. Louda, S.M., Simberloff, D., Boettner, G., Connor, J., Kendall, D., 1998. Insights from data on the nontarget effects of the flowerhead weevil. Biocontrol News and Information 19, 70–71. Morris, W.F., Doak, D.F., 2003. Quantitative Conservation Biology: Theory and Practice of Population Viability Analysis. Sinauer Associates, Sunderland, Masschusetts, USA. Pemberton, R.W., 2000. Predictable risk to native plants in weed biological control. Oecologia 125, 489–494. Puttler, B., Long, S.H., Peters, E.J., 1978. Establishment in Missouri of Rhinocyllus conicus for the biological control of musk thistle (Carduus nutans). Weed Science 26, 188–190. R Development Core Team, 2008, R: A language and environment for statistical computing, R Foundation for Statistical Computing, Vienna, AustriaURL: http:// www.R-project.org (ISBN 3-900051-00-3). Rand, T.A., Louda, S.M., 2004. Exotic weed invasion increases the susceptibility of native plants attack by a biocontrol herbivore. Ecology 85, 1548–1554. Reed, C.C., Larson, D.L., Larson, J.L., 2006. Canada thistle biological control agents on two South Dakota wildlife refuges. Natural Areas Journal 26, 47–52. Rees, N.E., 1977. Impact of Rhinocyllus conicus (Coleoptera, Curculionidae) on thistles in Southwestern Montana. Environmental Entomology 6, 839–842. Rees, N.E., 1991. Biological Control of Thistles. In: James, L.F. et al. (Eds.), Noxious Range Weeds. Westview Press, Boulder, pp. 264–273. Roduner, M.A., Mulder, P.G., Cuperus, G.W., Stritzke, J.F., Payton, M.E., 2005. Plant growth parameters of musk thistle Carduus nutans and egg distribution patterns of Rhinocyllus conicus on their blooms. Southwestern Entomologist 30, 93–103. Rose, K.E., Louda, S.M., Rees, M., 2005. Demographic and evolutionary impacts of native and invasive insect herbivores on Cirsium canescens. Ecology 86, 453– 465. Russell, F.L., Louda, S.M., 2005. Indirect interaction between two native thistles mediated by an invasive exotic floral herbivore. Oecologia 146, 373–384. Sauer, S.A., Bradley, K.L., 2008. First record for the biological control agent Rhinocyllus conicus (Coleoptera: Curculionidae) in a threatened native thistle, Cirsium hillii (Asteraceae), in Wisconsin, USA. Entomological News 119, 90–95. Shorthouse, J.D., Lalonde, R.G., 1984. Structural damage by Rhinocyllus conicus (Coleoptera, Curculionidae) within the flowerheads of nodding thistle. Canadian Entomologist 116, 1335–1343. Smith, L.M., Kok, L.T., 1987. Influence of temperature on oviposition, quiescence, and mortality of Rhinocyllus conicus (Coleoptera: Curculionidae). Environmental Entomology 16, 971–974. Spackman, S., Jennings, B., Coles, C., Dawson, D., Minton, M., Kratz, A., Spurrier, C. 1997. Colorado Rare Plant Field Guide. Prepared for the Bureau of Land Management, The US Forest Service, and the US Fish and Wildlife Service by the Colorado Natural Heritage Program. Surles, W.W., Kok, L.T., 1977. Ovipositional preference and synchronization of Rhinocyllus conicus (Coleoptera, Curculionidae) with Carduus compositae nutans and C. acanthoides. Environmental Entomology 6, 222–224. Surles, W.W., Kok, L.T., 1978. Carduus thistle seed destruction by Rhinocyllus conicus. Weed Science 26, 264–269. Turner, C.E., Pemberton, R.W., Rosenthal, S.S., 1987. Host utilization of native Cirsium thistles (Asteraceae) by the introduced weevil Rhinocyllus conicus (Coleoptera: Curculionidae) in California. Environmental Entomology 16, 111– 115. US Fish and Wildlife Service, 1996. Endangered and threatened wildlife and plants; notice of final decision on identification of candidates for listing as endangered or threatened. Federal Register 61, 64481–64485. Youssef, N.N., Evans, E.W., 1994. Exploitation of Canada thistle by the weevil Rhinocyllus conicus (Coleoptera: Curculionidae) in northern Utah. Environmental Entomology 23, 1013–1019.
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Biological Control journal homepage: www.elsevier.com/locate/ybcon
Impacts of the introduced biocontrol agent, Rhinocyllus conicus (Coleoptera: Curculionidae), on the seed production and population dynamics of Cirsium ownbeyi (Asteraceae), a rare, native thistle Michelle E. DePrenger-Levin a,*, Thomas A. Grant III b, Carol Dawson c a b c
Denver Botanic Gardens, Research and Conservation Department, USA Colorado State University, Graduate Degree Program in Ecology (GDPE) and the Department of Forestry, Rangeland and Watershed Science, USA US Bureau of Land Management (BLM) – Colorado State Office, USA
a r t i c l e
i n f o
Article history: Received 17 November 2009 Accepted 19 July 2010 Available online 23 July 2010 Keywords: Carduus nutans Cirsium ownbeyi Colorado Native plants Non-target effects Rhinocyllus conicus
a b s t r a c t The release of non-native insects to control noxious weeds is commonly used to combat invasions without disturbing the environment through chemical or mechanical methods. However, introduced biological control agents can have unintended effects. This study was initiated to evaluate potential non-target effects of the flowerhead weevil, Rhinocyllus conicus Frölich, on Cirsium ownbeyi S.L. Welsh, a rare, native and short-lived perennial thistle in northwestern Colorado, northeastern Utah, and southwestern Wyoming. C. ownbeyi represents one of 22 known native hosts on which this introduced weevil has naturalized. The study population remained stable over the eight years of the study despite floral damage by the biocontrol beetle. The growth rate (k) from a count-based population viability analysis of the population was 1.03; however, large inter-year variation indicates this rare species is still vulnerable to local extirpation. The biocontrol weevil consistently damaged the developing seeds over the course of the study independent of changes in overall population size and variation in the number of flowering individuals. The target species, Carduus nutans L. (musk thistle) is generally absent near the study plots, which may limit the population levels of R. conicus that can be sustained in this area. Although R. conicus utilizes C. ownbeyi as a host plant, the late flowering period of this native thistle and the small size of the flower heads may limit the demographic impact of R. conicus on C. ownbeyi. Ó 2010 Elsevier Inc. All rights reserved.
1. Introduction The Eurasian flowerhead weevil, Rhinocyllus conicus Frölich, was introduced in 1963 as a classical biological control agent against a suite of invasive thistles, especially Carduus spp. (Kok and Surles, 1975; Turner et al., 1987; Kok, 2001). Successful or substantial biological control of musk thistle (Carduus nutans L.) has been noted in much of the United States (Goeden and Ricker, 1977; Puttler et al., 1978; Batra, 1980; Littlefield, 1991; Lambdin and Grant, 1992; Boldt and Jackman, 1993; Buntin et al., 1993). Substantial non-target impacts to native North American thistles have also occurred (Pemberton, 2000). While R. conicus was known to feed on Carduus, Cirsium, Silybum, and Onopordum species in its native European range, it was not expected to feed significantly on the native North American thistles. Initial R. conicus releases on Carduus nutans showed populations of the weevil increased greatly in range but not in density (Hodgson and Rees, 1976; Reed et al., 2006). * Corresponding author. Address: 909 York Street, Denver, CO 80206. Fax: +1 720 865 3683. E-mail address: [email protected] (M.E. DePrenger-Levin). 1049-9644/$ - see front matter Ó 2010 Elsevier Inc. All rights reserved. doi:10.1016/j.biocontrol.2010.07.004
Evidence of R. conicus on native Cirsium spp. was reported in growing numbers of states including southwestern Montana in 1974 (Rees, 1977) and California in 1982 (Turner et al., 1987; Louda et al., 1997). Biocontrol releases continued through the 1970s despite knowledge that the weevil’s host plants included native North American thistles (Rees, 1977; Harris, 1988; Louda et al., 1997; Pemberton, 2000 and references therein). As confirmation of R. conicus feeding on native thistles spread across North America, so did concerns regarding the possibility of some native species becoming extinct (Louda et al., 1997, 1998, 2005; Rose et al., 2005). Pemberton (2000) reports that 22 of 90 North American Cirsium spp. are known hosts of R. conicus including Cirsium canescens Nutt., C. centaureae (Rydb.) K. Schum., C. hillii (Canby) Fernald, C. pulchellum (Greene) Woot. & Standl., C. tweedyi (Rydb.) Petrak, C. undulatum (Nutt.) Spreng., and C. undulatum (Nutt.) Spreng. var. tracyi (Rydb.) S.L. Welsh (Louda et al., 1997; Louda and O’Brien, 2002; Rose et al., 2005; Russell and Louda, 2005; Sauer and Bradley, 2008). Many states have ceased further releases of the biocontrol agent after observing damage on nontarget native species (Sauer and Bradley, 2008). However, R. conicus populations continue to spread naturally and more information is
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needed on the quantitative impacts of R. concius on rare Cirsium spp. to understand the potential threats to native thistles. This study documents a population consisting of over twelve sub-populations of C. ownbeyi in northern Colorado in order to quantify basic demographic parameters and presence and magnitude of any effects of the biocontrol weevil, R. conicus, on achene production and viability and therefore to determine if management intervention is needed to conserve the population of plants or even the species as a whole. 2. Materials and methods 2.1. Natural history The weevil, Rhinocyllus conicus, damages thistles by feeding and pupating within the receptacles of the capitula (Shorthouse and Lalonde, 1984; Jongejans et al., 2006). In spring, the adult female weevil deposits eggs on the outside of developing inflorescences’ involucral bracts, covering them with a hard coating of masticated particles of the host plant material (Shorthouse and Lalonde, 1984). The oviposition period lasts approximately five weeks (Buntin et al., 1993; Roduner et al., 2005). When the eggs hatch, the larvae burrow through the bracts into the head of florets where they feed, develop, and remain during pupation. Dissection of capitula reveals that larval tunnels and chambers in the receptacle disrupt achene development by severing vascular bundles that would otherwise transport nutrients to the developing seeds (Shorthouse and Lalonde, 1984). Achenes will be referred to as seeds hereafter. Studies of the effects of R. conicus larvae on the seedheads of musk thistles have identified reductions in seed production as the number of R. conicus increased (Rees, 1991; Louda et al., 1997). The non-target host plant, Cirsium ownbeyi, is a polycarpic, short-lived perennial herb that is known from approximately 30 extant populations, containing an estimated 25,000 individuals in Moffat County, Colorado, the Uintah basin of northeastern Utah, and southwestern Wyoming (Spackman et al., 1997). It occupies an elevational range of 1600–1900 m and its rarity is not believed to have anthropogenic origins. Cirsium ownbeyi is found in juniper, sagebrush, and riparian communities usually consisting of gravelly alluvium, talus, sandstone or occasionally limestone (Spackman et al., 1997). It flowers in June to August (Keil, 2006) and can return to a non-flowering, vegetative state in subsequent years. Cirsium ownbeyi was designated as a sensitive species by the Bureau of Land Management (BLM, US Department of the Interior) in 1987 (US Fish and Wildlife Service, 1996). The study site was established in 1998 by Denver Botanic Gardens and the BLM and is in the vicinity of Cross Mountain Canyon, a BLM wilderness study area in Moffat County, Colorado. The study site was located in the eastern-most portion of Cross Mountain Canyon, a deep canyon formed by the Yampa River. The rocky slopes of the south side of Cross Mountain Canyon (north-facing) provide mesic microhabitats that can support moderate numbers of C. ownbeyi. 2.2. Field protocols Surveys in 1988 and 1998 located the Cross Mountain Canyon population of C. ownbeyi and subsequently verified it as a population of sufficient size to implement population monitoring and evaluation of insect floral herbivory. The steep and rugged terrain of the canyon makes surveying the area difficult, and acts as a natural barrier to anthropogenic disturbance from recreational users of the canyon. At Cross Mountain Canyon, plants are often found in erosional channels of steep hillsides and in areas of active surface rockslides.
In 1998, all plants found within an approximately 600 100 m section of the canyon were flagged and circular plots 5 m in diameter were demarcated in 12 areas with the largest concentration of plants. Four of the original plots could not be located in subsequent years. Two of these were ‘lost’ in 1999 (of which one was relocated in 2005) and one each in 2000 and 2005. Another two plots could not be located in 2003 but were relocated in 2004. At different times during the study, three plots needed to be retagged due to rockslides covering the original markers. An additional plot was added to the set in 1999 so that ten plots remained at the end of the study, of which six plots had consecutive monitoring data on C. ownbeyi density and demographics for all eight years of the study. The center of each plot was marked and the position of each of the surrounding plants was recorded by measuring its distance and compass bearing. Plants were considered the same individual if the location measurement for the preceding or subsequent year was within seven degrees and 10 cm. For each plant the life stage (seedling, vegetative or reproductive) was recorded. The capitula were then collected and stored separately by positions on the plant. Plots were visited once a year during the flowering period. Due to this, seedlings bearing cotyledons were missed during data collection. New individuals within a plot were recorded as seedlings, reflecting their status during the interval between annual samples. To quantify seed production and weevil occurrence, an entire flowering stalk, including all capitula, was collected each August from three randomly selected reproductive plants within each plot, except in 2002 and 2003 when no seeds were collected because a severe drought limited reproduction. Only three capitula could be collected in 2004. In 2004 and 2005, flowers were dissected on site so that the seeds could be scattered in the vicinity of the parent plants and thereby minimize experimental disruption of reproduction. Capitulum samples collected from 1999 to 2001 were dissected at Denver Botanic Gardens. Using previously published methods (Surles and Kok, 1978; Louda and Potvin, 1995; Louda, 1998) the following measurements were made for each capitulum: weight (g) (the processing of samples in the field precluded the weighing of capitula in 2004–2005), diameter (mm), egg casings, number of first instar weevil entry holes, developmental stage of seed (undeveloped, aborted, developed), presence of insect damage on individual seed, seed color (1–5 scale), seed length (mm), number of weevil pupal chambers per capitulum, number of adult weevils present per capitulum, and whether the capitulum had shed its seeds prior to collection. In 2005 only the number of first instar entry holes and the diameter of the capitula were recorded. Although the larvae of moths and flies as well as several aphids, chrysomelids, myrids, and grasshoppers may feed on Cirsium spp. (Rose et al., 2005), there was no evidence of damage other than that attributed to R. conicus in any of the samples. Each capitulum was sorted into the following categories: seeds dispersed, partially dispersed, or not dispersed. Most capitula were collected before seed dispersal. Viable seed per capitulum were counted as was seed damaged by the weevil. Undeveloped, aborted and insect damaged seed are consistently darker in color than healthy, viable mature seeds which are light brown or tan (Shorthouse and Lalonde, 1984). Color of the seeds was used in conjunction with physical damage to the seeds to determine the percent damaged by weevils. Both developed and undeveloped seeds were categorized as viable when they were neither physically deformed due to predation nor in the two darkest color categories (4 or 5). Maturation of C. ownbeyi seed continues into September, therefore, the undeveloped undamaged seeds collected in August were treated as potentially viable seeds in the analyses. Viable seeds collected before 2004 are stored at the National Center for Genetic Resources Preservation (US Department of Agriculture, Fort Collins
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Colorado) as part of the Center for Plant Conservation’s National Collection of Endangered Plants and are available to qualified programs for research or restoration projects. 2.3. Data analysis Repeated measures analysis of variance (RM ANOVA) was used to analyze change in population size over time. Correlations were used to examine the number of oviposited eggs, differences in the percent of reproductive and vegetative individuals and the percent of seed with physical damage. Unpaired t-tests assuming equal variance were used to compare the number of seed in capitula with eggs to those lacking eggs. Logistic regression was used to compare the number of capitula containing seed with physical damage to capitula without damaged seeds across the weight distribution of capitula. Linear regression was used to determine relationships between the percent of seed with physical damage and the number of eggs per capitulum. Analyses were conducted using SAS/JMP INÒ software (Littell et al., 1996). Growth rate (k) and extinction predictions were made following Morris and Doak (2003) using a count-based population viability analysis in R (R Development Core Team, 2008). To calculate the growth rate (k) the total count of individuals from each plot with eight years of consecutive data was summed. The log of each yearly transition was averaged and a 95% confidence interval on the estimate of k was calculated using a linear regression of the yearly log population growth rate on the amount of time elapsed with the intercept forced through zero (Morris and Doak, 2003). 3. Results 3.1. Population monitoring Plant numbers fluctuated in the plots over time but did not change significantly over the eight years of the study (F(1,4) = 5.90, p = 0.0721) (Fig. 1). The average population size dropped to a low of 9.9 plants per plot in 2002 and increased to a high of 17.4 plants per plot in 2005. Although new individuals, considered seedlings, were documented in all but two years of the study, recruitment of individuals was generally uncommon in the study plot. Vegetative individuals made up over 83% of the individuals observed over the course of the study, reproductive individuals composed 14% and seedlings composed 3% of the individuals (Fig. 1). Most (64%) of the individual plants, tracked by direction and distance from the center marker, lived for two years.
Fig. 1. Average Cirsium ownbeyi S.L. Welsh individuals per plot over the eight years of monitoring. Bars indicate +/ SE.
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The longest life span observed was for two individuals which had persisted for six years and which were both still alive when monitoring stopped. Based on data collected from individuals within the six plots with data for all eight years, the average population growth rate (k) was 1.03 with a 95% confidence interval ranging from k = 0.78 to 1.37. The average number of plants per plot fluctuated by as much as 51% in a single year interval amounting to a change of 7.6 plants per plot. The number of individual plants (171) remaining in 2005 and found in all plots with consecutive data were used to model the population’s viability (count-based) and probability of becoming functionally extirpated (quasi-extinction). If the population is considered functionally extirpated when it falls to two or fewer individuals, and the growth rate remains nearly flat, then the population has approximately a 30% chance of becoming extirpated in 50 years (Fig. 2). 3.2. Capitula, casings and seeds Measurements on capitula are summarized in Table 1. From 1999 through 2001, each reproductive plant produced a mean ± SE of 6 ± 0.99 viable seeds per plant compared to an overall average of 83 seeds produced per reproductive plant per year. Including all seeds (viable, damaged, and undeveloped or aborted) collected in 1999–2004, a mean of 15.4 ± 1.7 seeds were found in the primary terminal capitulum, 12.5 ± 2.1 in the secondary terminal capitulum, and 13.5 ± 2.8 and 15.9 ± 4.9 in the terminal capitula of the successive lateral branches. Overall, 1999 had the highest average number of seeds per capitulum (18.04 ± 2.3). There were significantly fewer seeds (7.3 ± 0.79 per capitulum) over the subsequent four years in which seeds were counted (F(3,25) = 3.391, p = 0.034, r2 = 0.262) (Table 1), with a mean of only 4.0 ± 0.78 viable seed per capitulum during that period. Capitula with no weevil damage produced significantly more seed (16.4 ± 1.4) than capitula with one or more larval entry holes (9.7 ± 1.6), (t = 3.2, p = 0.0017). Of the capitula analyzed in the laboratory, 43% had eggs (Table 1). Heavier capitula were significantly more likely to contain seed damaged by the weevil than lighter capitula (p < 0.001, r2 = 0.17). The number of weevil eggs per capitulum significantly decreased
Fig. 2. Probability of quasi-extinction of Cirsium ownbeyi S.L. Welsh based on the number of individual plants in a plot and a 50 year timeframe. Quasi-extinction is defined as fewer than two individuals.
10.18 (0.41) 49 0.13 (0.01) 51 2.88 (0.51) 51 0.43 (0.07) 51 8.83 (0.43) 37 0.10 (0.01) 47 1.60 (0.54) 47 0.43 (0.07) 47 9.86 (0.45) 43 0.15 (0.01) 43 1.98 (0.56) 43 0.44 (.08) 43 11.6 (1.33) 3 - 1.00 (2.12) 3 0.33 (.29) 3 38.4 39.8 26.9 14.3
(6.21) 23 30.53 (9.66) 15 47.33 (8.47) 9 18.04 (8.26) 13 49.00 (18.70) 4 35.78 (8.47) 9 14.00 (7.02) 18 – – 26.89 (5.99) 18 11.26 (17.2) 3 – – 14.33 (14.67) 3 14.33
(2.30) 49 20.70 (3.37) 27 14.77 (2.74) 22 6.37 (1.31) (2.65) 37 22.00 (4.13) 18 6.42 (2.94) 19 2.13 (1.36) (2.46) 43 13.88 (3.58) 24 7.95 (2.94) 19 3.51 (1.42) (9.30) 3 21.00 (12.38) 2 1 (12.84) 1 0.33 (5.38)
51 47 43 3
4. Discussion
1999 2000 2001 2004
Mean (SE) N Mean (SE) N Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
N
Mean (SE)
Average capitula damaged by weevil Average eggs/ capitulum Average capitulum weight (g) Average Percent of Average # of Average # of seeds/capitulum seeds/capitulum seeds damaged capitulum diameter (cm) by weevil damaged weevil present by weevil Average # of seeds/capitulum no weevil Average # of seeds per capitulum Average # of seeds/plant weevil present Average # of seeds/plant no weevil Average seeds per plant Year
Table 1 Summary seed data per Cirsium ownbeyi plants and per capitula collected in August of 1999–2001 and 2004. All seed, viable and damaged, are included.
as the density of plants found in the plots increased (F(1250) = 6.12, p = 0.014, r2 = 0.024) while the percent of seeds with visible damage was not significantly correlated with plant density. A linear regression of the sum of all seeds damaged by the weevil on the number of eggs per capitulum showed a significant positive relationship (F(1142) = 83.24, p < 0.0001, r2 = 0.37). A repeated measures analysis of variance determined that the number of larval entry holes per capitulum did not vary significantly between years (F(1140) = 1.16, p = 0.3261) (Fig. 3). Overall, 35.5% of potentially viable seed were either physically deformed or a darker color (4 or 5) because of weevil larvae, and a 95% reduction in viable seed was documented in capitula affected by the weevil compared to the potential viable seed produced in capitula that were weevil-free. It has been shown that on Carduus nutans, the number of egg casings tended to decrease on successively lower lateral capitula (Puttler et al., 1978; Denslow and D’Antonio, 2005). However, on C. ownbeyi there was no significant linear relationship between the positions of the capitula and the amount of insect damage (F(6250) = 0.8035, p = 0.568). However, a general pattern was observed of decreasing number of first instar entry holes and lower likelihood to have first instar entry holes on successively lower lateral and later developing capitula.
44.11% 26.89% 31.03% 3.30%
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N
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Despite the limited number of reproductive plants, population fluctuations, low levels of recruitment and moderate to high levels of insect damage to seed (35.5%) and capitula (43%), the Cross Mountain Canyon population of C. ownbeyi appears to be stable in the short-term. The goal of this study was to determine if the introduced biocontrol agent, R. conicus, is a threat to the survival of C. ownbeyi. Collection of plant demographic and insect herbivory data is essential to the decision making processes concerning management of this plant species and management of the biocontrol’s non-target effects. The discovery of R. conicus in the capitula of C. ownbeyi represents a potential threat to the seed production and long-term viability of this population. Since R. conicus has been released for biological control use, it has been documented to migrate as much as 3.2 km from its origin of release in one year and as much as 32 km in six years (Kok and Surles, 1975; Puttler et al., 1978). Movement of R. conicus to nontarget species has been reported as a ‘‘spillover effect” with the largest weevil egg densities on capitula closest to C. nutans populations (Rand and Louda, 2004). Despite the lack of the target species in and around the study plots, R. conicus was able to migrate to these populations of C. ownbeyi. The population size of R. conicus appears to be stable on C. ownbeyi. The weevil had first been observed in Cross Mountain Canyon at least ten years before the start of this study. While R. conicus populations have been shown to exhibit exponential growth rates in the first years of invasion (Louda, 1998), the population at this site has likely reached its carrying capacity. Movement to non-target Cirsium species in low densities has been observed (Laing and Heels, 1978; Louda, 1998). While the presence of the biocontrol agent in other populations of C. ownbeyi was not addressed by this study, unpublished data documented R. conicus in only one of 22 studied C. ownbeyi populations in adjacent Dinosaur National Monument (Gary J. Dodge, 2005). The population of C. ownbeyi at Cross Mountain Canyon fluctuated within the study plots over the course of the study, yet overall experienced a relatively flat growth rate (k = 1.03). Natural surface disturbances (rockslides) continually modify the habitat and often cover known occurrences of C. ownbeyi with talus and scree which tends to destroy existing populations while potentially creating habitat for the establishment of new ones. Large yearly fluctua-
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Fig. 3. Number of first instar boring holes per capitulum per year of inflorescence collection. Whiskers represent the upper fence and points are potential outliers.
tions in population size contribute to the wide confidence interval of k (0.78 to 1.37). This could indicate an episodic or cyclical pattern of seedling recruitment based upon environmental factors not analyzed in this study. These factors indicate that despite the flat growth rate, this population is at great risk of extirpation should multiple years of decline occur. A count-based population viability analysis indicated that the population in 2005 had approximately a 30% chance of extirpation in 50 years. However, if the population size drops as it did by 40% in one year, the threat of local extirpation increases. There is enough variability seen in yearly population size to warrant some concern that this species will become locally extirpated. This count-based viability analysis did not take into account seeds in the soil seed bank which would buffer the threat of local extirpation. A soil seed bank likely exists given the documented increases in population size. The flowerhead weevil, R. conicus, has had a negative effect on C. ownbeyi’s seed development although not on the population size at Cross Mountain Canyon. Rhinocyllus conicus reduces seed set in the capitula it utilizes by 95%; however, it utilizes less than half of the capitula available leaving an average of 83 seed produced per reproductive plant per year. Of these seeds, only an average of six seed per reproductive plant was observed to be potentially viable. The 2005 study by Gary J. Dodge (unpublished data) found no mature seeds in C. ownbeyi capitula infested with the biocontrol agent compared to a mean of 14.98 ± 1.07 seeds per uninfested capitula. Louda (2000) documents that R. conicus has caused an 80% reduction of viable seeds in the common Platte thistle (Cirsium canescens Nutt.) after only four years of exposure to the weevils. Rhinocyllus conicus may not cause a decline in population size but may dampen population increases through reduced reproductive output which could cause local extirpation due to the small population size and environmental stochasticity. The phenology of C. ownbeyi and size of its capitula may restrain the negative effect of R. conicus. Cirsium ownbeyi flowers later in the season than the target host resulting in little overlap between flowering and ovipositing by R. conicus and does not co-occur with other potential host thistles at the Cross Mountain Canyon site that could support a population of R. conicus. Completion of ovipositing has been recorded at different times in different climates (i.e., late June in Nebraska and early August in Utah) (Smith and Kok, 1987;
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Youssef and Evans, 1994; Roduner et al., 2005; Russell and Louda, 2005). In addition, capitula of Carduus ssp. which develop late in the flowering season experience diminished levels of weevil feeding damage (Hodgson and Rees, 1976; Boldt and Jackman, 1993). Later flowering capitula of C. ownbeyi were never observed to contain egg casings. Additional secondary (non-apical) inflorescences may develop after the weevil has finished its ovipositing period thus limiting the time period for phenological synchrony between the plant and weevil (Surles and Kok, 1977; Feldman, 1997; Russell and Louda, 2005). The weak negative correlation of first instar entry holes and the number of developed seeds could be due the small size of C. ownbeyi capitula compared to the target species. Rhinocyllus conicus appears to be limited in the amount of larvae that can survive in one capitulum (Hodgson and Rees, 1976; Goeden and Ricker, 1985). Carduus nutans, however, produces capitula that are nearly 4 times as large as C. ownbeyi (Kok et al., 1986). No more than two larvae were found in one C. ownbeyi capitulum which may afford it the advantage of being a smaller and later flowering species thus limiting the size of the R. conicus population that can be supported by the presence of only one thistle species. The consistent level of impact by R. conicus despite increases in the density of C. ownbeyi could be due to the general absence of the target species, Carduus nutans. When the weevil first established at the Cross Mountain site it is possible there was a decline in the C. ownbeyi population similar to that seen in other Cirsium spp. (Louda, 1998). However, the stable weevil population does not appear to reduce the population size of C. ownbeyi in the short-term but may limit any population growth. Climate change could affect the phenological synchrony of these species and dramatically increase the damage R. conicus exerts on C. ownbeyi. The combination of population size fluctuations and a steady amount of damage by the biocontol puts the long-term viability of C. ownbeyi at risk. If evidence of population extirpation becomes apparent due to the effects of R. conicus, the BLM can take additional action with these data in mind to conserve this species by limiting the spread of the biocontrol agent.
5. Role of the funding source Denver Botanic Gardens provided support for this project along with funding through an Assistance Agreement from the Colorado state office of the Bureau of Land Management (BLM) from 1998– 2005. The BLM conducted surveys to identify the population of Cirsium ownbeyi at Cross Mountain Canyon and developed the study design in 1998. BLM staff collected data with Denver Botanic Gardens in 2000 and were involved in early data analysis. BLM staff also contacted Dr. Svata Louda in April 1999 to use her protocol for seed head collection and analysis. The BLM Colorado State Botanist was consulted on study design and was given annual reports. The BLM did not influence the decision to submit the paper for publication but was given the paper for review before submission.
Acknowledgments We acknowledge Carol Spurrier, a BLM botanist, who provided funding and helped set up the experimental design. We thank Dr. Svata Louda for providing her protocol for seed head analysis and Dr. Jennifer Neale for assistance editing this manuscript. We would also like to acknowledge the many interns and Denver Botanic Gardens’ staff who assisted with summer field work: Bethany Demarco, Dr. Peter Gordon, Chris Story, Nathan Keller, Sierra Smith, Kristin Schou, Jonathan Almond, Cathy Steward, Chris Malone, Aaron Shiels, Michael Denslow, Shannon D. Fehlberg, and R. Powers.
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