Control Of Yellow Starthistle Research Articles

Effects of Aminopyralid on California Annual Grassland Plant Communities

AbstractAminopyralid is the most commonly used herbicide for the control of yellow starthistle and other invasive thistles in annual grasslands of California. Although the effects of aminopyralid on native plant communities over a 2-yr period have been evaluated in prairies dominated by perennial species in the northern central states, similar evaluations have not been conducted in grassland communities of California, which are generally composed of a high diversity of native and nonnative annual species. In this study we monitored the effects of 53 and 123 g ae aminopyralid ha−1on individual species cover and species richness over three growing seasons in two locations on California annual grassland. Treated plots were compared to untreated plots in randomized complete-block designs. Results were largely consistent between the two trials. In the first season after treatment, both rates of aminopyralid reduced dicot cover significantly, particularly members of the Asteraceae and Fabaceae. Treated plots also showed reduced species richness. However, these differences were less pronounced in the second season after treatment, particularly at the low rate. By the third season after treatment in both sites, there were no longer any significant effects on cover or species richness at the low herbicide rate. On California annual grasslands, winter applications of low rates of aminopyralid have been shown to give excellent control of yellow starthistle, providing long-term benefits to grassland ecosystems. Results of the current study suggest that negative impacts of aminopyralid on the desirable native forb community are transitory.

Integration of Prescribed Burning, Aminopyralid, and Reseeding for Restoration of Yellow Starthistle (Centaurea solstitialis)-Infested Rangeland

AbstractYellow starthistle (Centaurea solstitialis) is among the most invasive plant species in the western United States. The long-term management of yellow starthistle should include an integrated approach that incorporates establishment of competitive vegetation. In this study, conducted in two locations at Fort Hunter Liggett in Monterey County, California, we evaluated the combination of prescribed burning, the herbicide aminopyralid, and reseeding of native broadleaf and grass species on both yellow starthistle control and native plant restoration. Both study sites were burned in late October 2009. Over the following season, aminopyralid was applied at three timings and native plant species were seeded at three timings, using both a drill-seeder and broadcast spreader. Evaluations over the next 3 yr showed that aminopyralid provided complete to nearly complete control of yellow starthistle when applied between January and March, and this level of control was maintained for two seasons. Native plants failed to establish when broadcast seeded, regardless of the timing. December and January drill seeding timings were the most successful in establishing native species. There was a strong herbicide and drill seed timing interaction effect on native grass cover at both study sites. Over the course of the study the native perennial grass Stipa cernua was the most successful seeded species to establish, but establishment was slow and required 3 yr. Our results indicate that a January or March aminopyralid treatment integrated with a native perennial grass drill seeding program in January offers the greatest probability of both successful yellow starthistle control and perennial grass establishment.

Response of Biomass and Seedbanks of Rangeland Functional Groups to Mechanical Control of Yellow Starthistle

Abstract Yellow starthistle ( Centaurea solstitialis L.) is a nonnative pest of rangelands that decreases forage quality and yield. Mowing may control starthistle effectively and complement herbicide use in an integrated pest management strategy, but little research has investigated its effects on nontarget vegetation. We monitored biomass and seedbank size of annual and perennial species, in addition to starthistle, in response to 3 yr of mowing treatments, either mowing alone or in combination with solarization tarps or thatch removal. All mowing treatments were very effective at reducing starthistle biomass and seedbank: mowing alone reduced biomass 92 ± 2%, mowing with thatch removal 91 ± 1%, and mowing with solarization 95 ± 1%. Compared to seedbank sizes in the control plots, yellow starthistle seedbank decreased by 100% (mowing alone), 92% (mowing + thatch removal), and 100% (mowing with solarization) after 3 yr of treatment. Mowing also significantly improved perennial species' biomass. Annual species' biomass varied on a year-to-year basis but was not significantly affected by any treatment. Seedbank sizes of annuals and perennials also did not differ according to mowing treatment. This research indicates that late-season mowing can effectively reduce starthistle biomass without adverse effects on other vegetation and that mowing alone is sufficient to reduce starthistle seedbank size without additional methods of decreasing seed rain.

Control of Yellow Starthistle (Centaurea solstitialis) and Coast Fiddleneck (Amsinckia menziesii) with Aminopyralid

AbstractYellow starthistle is the most widespread broadleaf invasive plant in the western United States, and it is particularly prevalent in California. Prior to the registration of aminopyralid in 2005, the standard for chemical control of yellow starthistle was the herbicide clopyralid. We report on a compilation of several independent trials comparing the efficacy of aminopyralid and clopyralid on yellow starthistle. Treatments were applied at several rates and timings at 11 locations in four states between 2001 and 2007. Treatments were made pre-emergence and postemergence at the seedling and rosette stages of yellow starthistle. Results showed that aminopyralid, even at the low rate of 18 g ae ha−1, provided nearly complete control of yellow starthistle when treatments were made at the seedling stage. However, less consistent control (80 to 100%) resulted with applications made at the pre-emergence and rosette stages. At the seedling stage, aminopyralid is about four times more effective on yellow starthistle compared to clopyralid, based on the rate of acid equivalent. In the Central Valley of California, complete control was obtained at the lowest registered rate (53 g ae ha−1) when applications were made from December through February. At two locations we also evaluated control of the poisonous native plant coast fiddleneck. Although clopyralid does not adequately control coast fiddleneck, aminopyralid provided almost complete control when applied in the winter growing season. Applications of aminopyralid at the rosette stage resulted in a two-fold increase in annual forage grass biomass the following year. These results indicate that aminopyralid is a valuable tool for land managers and can play an important role in integrated management strategies for yellow starthistle and coast fiddleneck.

Synchytrium solstitiale: reclassification based on the function and role of resting spores

Studies were made about resting spores of Synchytrium solstitiale, a chytrid that causes false rust disease of yellow starthistle (YST). During evaluation of this fungus for biological control of YST, a protocol for resting spore germination was developed. Details of resting spore germination and study of long-term survival of the fungus were documented. Resting spores from dried leaves germinated after incubating them on water agar at least 7 d at 10–15 C. Resting spores were viable after storage in air-dried leaves more than 2 y at room temperature, suggesting they have a role in off-season and long-term survival of the fungus. Each resting spore produced a single sorus that contained a single sporangium, which on germination released zoospores through a pore. YST inoculated with germinated resting spores developed symptoms typical of false rust disease. All spore forms of S. solstitiale have been found to be functional, and the life cycle of S. solstitiale has been completed under controlled laboratory and greenhouse conditions. Resting spore galls differ from sporangial galls both morphologically and biologically, and in comparison, each sporangial gall cleaves into several sori and each sorus produces 5–25 sporangia that rupture during release of zoospores. For this reason S. solstitiale should be reclassified as diheterogallic sensu Karling (Am J Bot 42:540–545). Because resting spores function as prosori and produce an external sorus, S. solstitiale is best placed in into the subgenus Exosynchytrium.

Climatic analysis to determine where to collect and release Puccinia jaceae var. solstitialis for biological control of yellow starthistle

Puccinia jaceae var. solstitialis is an autoecious rust fungus that is native to areas of Afro-Eurasia with a Mediterranean climate. An isolate collected near Sivas, Turkey was released for classical biological control of yellow starthistle (YST), which is an invasive alien weed in California, USA. The fungus has been released throughout California, but long-term establishment rates are generally low, apparently because this ecotype is not well adapted to the climate where the weed is most invasive. Using a site with excellent establishment as a target, the Match Climates function in CLIMEX climate modeling software identified similar sites in and around the San Francisco Bay Area, east to the Central Valley and Sierra foothills, and along the coast of Southern California. Similar sites in other states include Walla Walla, Washington, Pendelton, Oregon and Salt Lake City, Utah. A Compare Locations model based primarily on experimentally measured temperature and humidity requirements of the rust produced similar results. Using Sacramento, California, which is in the center of YST distribution, as a target, the Match Climates function predicted that the best locations to search for rust accessions to use in California are near Tunis, Tunisia, Foggia, Italy, Khalkis, Greece, Kayseri, Turkey, and possibly Constantine, Algeria. This generally agrees with the prediction of a Compare Locations model based on the geographic distribution of YST in California. Climatic factors that limit the long-term establishment of the fungus are likely to be summer heat and/or dry stress and short dew periods.

DNA Fingerprinting to Improve Data Collection Efficiency and Yield in an Open-Field Host-Specificity Test of a Weed Biological Control Candidate

AbstractAn open-field test was conducted in southern France to assess the host-specificity of Ceratapion basicorne (Illiger), a candidate for biological control of yellow starthistle. Test plants were infested by naturally occurring populations of C. basicorne but were also exposed to sympatric herbivore species, including other Ceratapion spp. Insects from the test plants were collected directly into tubes of ethanol and were subsequently identified to species according to DNA sequence similarity with morphologically identified reference specimens. This integrated, morphological and molecular identification method was used in an effort to maximize the amount of data gained in the field bioassay and to minimize the number of taxonomist–hours necessary to complete the study. The results obtained showed that the French C. basicorne population only attacked yellow starthistle and cornflower, another known host of C. basicorne. Molecular phylogenetic analysis of the insects collected from all other nonhost plants rejected the possibility that any were C. basicorne.

Late-Season Targeted Grazing of Yellow Starthistle (Centaurea solstitialis) with Goats in Idaho

AbstractYellow starthistle (Centaurea solstitialis) is an exotic winter annual forb that is aggressively invasive and problematic in much of California, Idaho, Oregon, and Washington. Yellow starthistle control is particularly challenging in canyon rangelands where accessibility limits control options. Our objective was to evaluate the effects of late-season targeted goat grazing on yellow starthistle and nontarget grasses and forbs. A 3-yr grazing study was initiated in 2006 on a 380-ha (939 acres) canyon grassland site infested with yellow starthistle near White Bird, ID. Twenty-four paired plots were established, with each pair including a fenced subplot to exclude grazing and a similar-sized adjacent subplot that was grazed. Density of yellow starthistle plants and seedheads was assessed after grazing of each plot in all 3 yr and before grazing in the second and third years. Canopy cover of yellow starthistle, grasses, and forbs also was measured. Grazed subplots had 58% fewer yellow starthistle plants than the ungrazed controls after grazing was applied and 94% fewer seedheads after 3 yr of grazing. Cover of yellow starthistle did not differ between grazed and ungrazed subplots after grazing in 2006, whereas grazing decreased yellow starthistle cover in 2007 and 2008 by about 75%. Goat grazing had little impact on canopy cover of grasses and resident forbs, with the exception of after grazing in 2007 when there was less forb cover in grazed areas compared with ungrazed areas. Late-season (i.e., July to November) targeted goat grazing appears to be an effective way to reduce yellow starthistle plant densities at landscape scales, which creates a large window of opportunity for grazing treatment and flexibility for land and livestock managers.

Rangeland Invasive Plant Management

lands provide a valuable resource for wildlife and recreational activities, as well as livestock grazing. The establishment and spread of invasive plants on rangeland are often symptomatic of long-term cultural practices.3 Invasion of many rangeland sites by non-native plants has been facilitated by multiple factors including overgrazing by domestic livestock, purposeful introduction of non-native plants for agricultural purposes, unintentional introduction of non-native plant seeds as a contaminate in crop seed, changes in fire frequency and intensity, and climatic changes that converge to cause shifts in plant community composition. There are over 300 rangeland weeds in the United States, but perhaps 30 have significant negative impact in the western United States.2 Of these, some of the most problematic include downy brome or cheatgrass {Bromus tectorum), medusahead {Taeniatherum caput-medusae), leafy spurge {Euphorbia esula)> perennial pepperweed {Lepidium latifolium), Dalmatian toadflax {Linaria dalmatica), and several members of the Asteraceae, including yellow starthistle {Centaurea solstitialis), diffuse knapweed (C. diffusa), spotted knapweed {C. stoebe subsp. micranthos), musk thistle {Carduus nutans), Canada thistle {Cirsium arvense), hawkweeds {Hieracium spp.), and Russian knapweed {Acroptilon repens). These and other invasive plants can have adverse effects on rangeland and pastures by disrupting vegetation dynamics and reducing ecosystem capacity to recover after disturbance.3 The financial impacts of invasive plants in rangelands are substantial and have been estimated to cost ranchers US$5 billion annually just for control.4 Other impacts to the livestock industry not included in the estimate are the negative effects of invasive plants on yield and quality of forage, livestock poisoning, interference with grazing, supplemental costs associated with managing and producing livestock, and land values. In addition to the livestock industry, invasive rangeland species can decrease wildlife habitat and forage, deplete soil and water resources, and reduce plant and animal diversity. Mechanical, cultural, biological, and chemical methods have been developed to manage rangeland invasive plants. These methods include mowing, prescribed burning, appropriate grazing management, herbicides, biocontrol agents, and revegetation with desirable plants.1 The merits of each control measure and potential for complementary or synergistic interactions when applied in appropriate sequences and combinations should be considered when developing integrated weed management programs. For example, summer burning resulted in a dramatic increase in yellow starthistle germination the following fall and winter.5 Thus, using an herbicide treatment in the year following a prescribed burn gave far better control of yellow starthistle compared with an herbicide treatment followed by prescribed burning. Successful integrated strategies require careful planning that incorporates a long-term approach consisting of prevention programs, education activities, and management approaches. These are designed to diminish negative impacts of invasive plants by improving the invaded plant community to increase its ecosystem function, niche occupation, and competitiveness. These qualities can increase the communities' resistance to reinvasion by other invasive plants.3

Establishment of the Yellow Starthistle Rust in California: Release, Recovery, and Spread.

The rust fungus Puccinia jaceae var. solstitialis is the first pathogen released for biological control of yellow starthistle (Centaurea solstitialis). From 2004 to 2006, the pathogen was released at 176 sites in 40 counties throughout the state of California. Release sites were evaluated 1 to 3 months and 1, 2, and, in some cases, 3 years after releases to monitor for reemergence. At 1 to 3 months after inoculation, 58 to 93% of sites had rust infection, depending on the year. After 1, 2, and 3 years, the percentages declined to 19 to 21, 9 to 10, and 3% respectively. Spread was detected at 19% of the sites with rust infection, with an average distance of 21 m (±13.3 standard error). The greatest spread occurred at a site in Sonoma County. At this site, the rust spread to over 37 acres 1 year after it was released and has remained in this area for three seasons. Reemergence 1 and 2 years after inoculations was more likely in Northern (above 40°N) compared with Southern California (below 36°N). In general, reemergence was more likely at lower elevations when release sites were within 150 km of the coast. Overall, the rust has not demonstrated a strong record of persistence based on these observations.

Effects of the rust Puccinia jaceae var. solstitialis on Centaurea solstitialis (yellow starthistle) growth and competition

The rust fungus Puccinia jaceae var. solstitialis was introduced to California in 2003 for biological control of yellow starthistle ( Centaurea solstitialis). To test its effectiveness under field conditions, we examined biomass production, chlorophyll levels, and seedhead production of yellow starthistle (YST) infected with P. jaceae. We also evaluated the effect of P. jaceae on the competitive ability of YST grown with wild oat ( Avena fatua), a widespread winter annual grass commonly associated with YST infestations. Chlorophyll levels were reduced by over 50% in severely rust-infested YST leaves. P. jaceae had no effect on growth or reproductive variables in monoculture plots, but caused a modest reduction in YST performance in the competition experiment. In this study, infected plants had fewer leaves than uninfected plants and slightly reduced rosette diameters. P. jaceae also decreased YST biomass from 3.18 kg m −2 in non-inoculated competition plots to 2.62 kg m −2 in inoculated plots over both years of the experiment. Although not significant, there was a trend towards reduced seedhead production in the inoculated plots. These results indicate that P. jaceae may have some negative effects on YST growth, especially under conditions of interspecific competition. However, the effects of P. jaceae appear to be of minor biological significance and are unlikely to cause major declines in YST populations statewide.

Susceptibility of yellow starthistle to Pucciniajaceae var. solstitialis and greenhouse production of inoculum for classical biological control programs

In anticipation of large-scale distribution of a Turkish isolate of Puccinia jaceae var. solstitialis in California for biological control of yellow starthistle ( Centaurea solstitialis, YST), susceptibility of YST within the state was determined and a protocol for bulk inoculum production was developed. Inoculation was made of 62 field accessions of YST representative of the range of habitats in California. These were determined to be equally susceptible to infection by the isolate approved for release in the United States in 2003. To support a program to speed establishment by release at many locations statewide, protocols for artificial increase of inoculum were developed. Over 64 g of urediniospores were produced with a mass-production system under greenhouse conditions from 2003 to 2006. Yield of inoculum varied by season, with peak production occurring from early spring through early summer. A large-scale urediniospore harvest also was made from a field plot at Davis, California. Our results show that susceptibility of YST in California is not likely to limit establishment of P. jaceae for biological control, and that production of this or other obligate pathogenic fungi (biological control agent) is possible for support of statewide release and research programs.

Non-target plant use by a weed biocontrol agent in idaho: host expansion or opportunistic behavior?

Larinus curtus Hochhuth (Coleoptera: Curculionidae) was first introduced into the western United States from Greece for the biological control of yellow starthistle (YST), Centaurea solstitialis L., in 1992. The discovery of L. curtus adults in the open flowerheads of safflower (SF), Carthamus tinctorius L., near Lewiston, Idaho in 2007 suggested this weevil might be expanding its host range to include a non-target crop species closely related to YST. In 2008 field plots near the 2007 observation site, 92 L. curtus adults fed in open SF flowerheads (pollen feeding and minor feeding on corolla tubes). No eggs were found in the ovarioles of 19 pollen-feeding females. No eggs, larvae, or evidence of larval feeding were detected in 39 tagged SF capitula, and no adults emerged from approximately 7,135 post-flowering SF capitula. These collective results are not indicative of an expanding developmental host-range of L. curtus. Also, they are consistent with pre-release host-specificity test results.

Practical Challenges in Private Stewardship of Rangeland Ecosystems: Yellow Starthistle Control in Sierra Nevadan Foothills

Private landowners are often de facto stewards of biodiversity and ecosystem services. In California's Sierra Nevada foothills, ranchers frequently present the only defense against biological invasions in private rangelands. Although ranchers' land management goals (e.g., the desire to control invasive species) can be consistent with ecosystem protection, practical constraints often limit their success. Considerable research on the invasive weed, yellow starthistle (Centaurea solstitialis L.), has produced numerous control strategies. Nevertheless, the range of this noxious weed continues to increase. We used surveys and interviews to document the weed control efforts of 202 ranchers and to identify practical limitations to their efficacy. Overall, 86% of ranchers who had experienced yellow starthistle infestation had attempted control, using one or more of 19 methods. Early response reduced negative effects from yellow starthistle. Control methods learned from agricultural advisors were reported more effective than those learned elsewhere. Limitations to yellow starthistle control in our study population resulted from incomplete information regarding control methods, complexity of weed control in heterogeneous landscapes, inconsistent application of methods, and lack of long-term planning for weed control. Such hindrances make it difficult for landowners to implement control methods promoted by researchers. This gap between science and practice contributes to the continued increase of yellow starthistle within the study region. To shrink this gap, researchers and agricultural advisors can incorporate environmental heterogeneity into applied agricultural research, use land stewards' knowledge and experience, and increase public education.

Puccinia jaceae var. solstitialis Teliospore Priming on Yellow Starthistle

Following the introduction of Puccinia jaceae var. solstitialis to California for biological control of yellow starthistle (Centaurea solstitialis, Asteraceae), teliospores, pycnia, and multiple urediniospore generations have been observed in the field. Because urediniospores have a relatively short life span in the field, functioning teliospores are expected to be necessary for the permanent establishment of P. jaceae var. solstitialis in California. To determine if conditions in California were conducive to this, teliospore emergence and priming were evaluated in the field. A factorial experiment in the laboratory with five incubation times and three incubation temperatures was used to determine teliospore priming requirements. Teliospore production coincided with plant senescence in August and September at two sites in 2 years; fewer teliospores were produced in 2006, suggesting inconsistent teliospore production may limit population growth and contribute to local extinctions in some areas. When teliospores were primed in the field, germination was low through the fall and abruptly peaked in January during both years. In the laboratory, teliospore germination increased as incubation time increased from 2 to 6 weeks and temperatures decreased from 12 to 4 degrees C. A degree-hour model derived from laboratory data accurately predicts when teliospores are primed for germination in the field. Based on the results obtained in this study, it is apparent that teliospore germination can occur over a range of priming conditions. However, lower temperatures and longer incubation periods are superior in breaking teliospore dormancy.

Latent period and viability of Puccinia jaceae var. solstitialis urediniospores: Implications for biological control of yellow starthistle

Since the introduction of Puccinia jaceae var. solstitialis into California in 2003, multiple urediniospore generations have been observed, along with teliospores and pycnia, on yellow starthistle (Centaurea solstitialis). A field and laboratory study of urediniospore viability was conducted to determine the potential of using infected plant material for redistribution of inoculum into new areas. To better understand the life history of P. j. solstitialis, urediniospore viability was measured during the summer and fall when the host plant is usually senescent. Latent period (time from inoculation to pustule eruption) after field and laboratory inoculations was measured in order to determine the potential number of urediniospore generations that occur per year. Viability of urediniospores stored in the laboratory gradually declined over a period of 10 weeks; spore hydration increased germination. Based on the results obtained in this study, introductions of harvested infected plant material would have to be done quickly to offset losses in urediniospore viability. Urediniospore viability in the field decreased exponentially 86% near the coast and 97% in the Central Valley three weeks after production, and was negligible (0–0.2%) thereafter. Latent period ranged from four to five weeks at cool winter temperatures to two to three weeks at warm summer temperatures in both the laboratory and field. Results of this study suggest that urediniospores are not likely to remain viable through the summer and fall dry season. Teliospores, therefore, serve as the inoculum source when YST seedlings germinate in the winter. Approximately six urediniospore generations can be expected to occur each year in central-northern California under suitable conditions for disease.

First Report of Puccinia jaceae var. solstitialis Pycnia on Yellow Starthistle in the United States.

The rust fungus Puccinia jaceae (Otth) var. solstitialis field isolate FDWSRU 84-71 (formerly TR 84-96) was first released in California for the biological control of yellow starthistle (YST; Centaurea solstitialis) in July 2003. This isolate was collected by S. S. Rosenthal in 1984, east of Yarhisar and Hafik (SIVAS), Turkey. It is macrocyclic and autoecious (1), completing its entire life cycle on YST. Observations were made in field plots west of Woodland, CA that had been inoculated on a monthly regimen between January and June of 2005. On February 22 and March 2, 2006, pycnia were observed in and around one of the plots (38°42.767'N, 121°53.732'W at an altitude of 57 m). Pycnia were yellow, flask shaped, and small (less than 100 μm in diameter), occurring in clusters on abaxial leaf surfaces or on petioles. DNA was isolated from pycnial and uredinial samples collected from the site and used for polymerase chain reaction amplification of the ITS2 region with P. jaceae specific primers. Sequences of amplicons from both samples were identical to FDWSRU 84-71 (GenBank Accession No. AF047728). Further amplification of pycnial DNA with PCR primers specific to the released P. jaceae isolate (2) produced the expected 851-bp amplicon. To our knowledge, this is the first report of pycnia from Puccinia jaceae var. solstitialis in the United States, suggesting that the YST rust is fully functional and completes its life cycle in California.

Integrating prescribed burning and clopyralid for the management of yellow starthistle (Centaurea solstitialis)

Prescribed burning and the herbicide clopyralid are very effective tools for the management of yellow starthistle. However, repeated use of either can be impractical or can present other problems. The potential solution is the development of an effective integrated weed management strategy using a combination of the two approaches. In small plot studies (0.2 ha), we tested one of five possible treatments: (1) untreated control, (2) 2 consecutive yr of clopyralid (0.105 kg ha−1), (3) 2 consecutive yr of prescribed summer burning, (4) first-year clopyralid followed by second-year prescribed burning, and (5) first-year prescribed burning followed by second-year clopyralid. Treatments were made in 1999 and 2000 at three study sites in California (San Benito, Yuba, and Siskiyou counties). In 2001, the year following the final treatment, 2 consecutive yr of clopyralid or first-year burning followed by second-year clopyralid consistently reduced yellow starthistle cover in the following year by 92 to 100%. However, at the Yuba site, clopyralid alone increased medusahead and ripgut brome cover. Although 2 consecutive yr of burning was effective in Yuba, very high levels of starthistle infestation in San Benito were not completely burned in the second year because of the lack of available consumable fuel. Clopyralid treatment the first year followed by prescribed burning in the second year stimulated yellow starthistle germination and did not reduce the infestation. In a large-scale study conducted at two sites (13 and 81 ha) in southern Monterey County, we used a first-year burn followed by either 2 yr of clopyralid (0.158 kg ha−1) or a single year of clopyralid (0.210 kg ha−1) and a subsequent burn. Results were in close agreement with those found in the small-scale studies. In the year following the final treatment, control of yellow starthistle was greater than 99% when the burn was followed by 2 yr of clopyralid. In contrast, when a prescribed burn was used in the last year of the program, the level of control was not as good, probably because of the increased germination of the remaining soil seedbank. These results indicate that a first-year prescribed burn followed by a second-year clopyralid treatment can provide consistently good control of yellow starthistle, as well as reduced levels of noxious annual grasses, including medusahead and ripgut brome.

Supplemental risk evaluations of Puccinia jaceae var. solstitialis for biological control of yellow starthistle

Abstract Additional tests of native North American Cirsium species, Saussurea americana , and modern safflower cultivars ( Carthamus tinctorius ) were requested by regulators and specific interest groups during the risk assessment of foreign isolates of Puccinia jaceae var. solstitialis for biological control of yellow starthistle (YST, Centaurea solstitialis ) in the United States. These tests supplement an earlier, extensive host range determination that established P. jaceae from YST as generally host specific and potentially useful for biological control. The additional research was in response to potential hazards identified in an earlier study, changes in safflower cultivars, and concern that P. jaceae might cause a safflower seedling disease similar to hypocotyl infections from infestation by Puccinia carthami teliospores. S. americana , a close relative of yellow starthistle, had not been tested previously. All tests were conducted in a containment greenhouse. Foliage of 19 Cirsium species, 11 safflower cultivars, and S. americana was inoculated with urediniospores and subjected to a 16-h dew period at 18–20 °C. Neither the Cirsium species nor S. americana became infected after foliar inoculations. Compared to foliar infections by P. carthami from safflower in California, only minor infections developed from inoculations with P. jaceae . These were similar to infections observed in earlier studies, and it was not possible to maintain P. jaceae under optimal greenhouse conditions on safflower foliage. Quantitative teliospore inoculations with P. jaceae did not cause infection on safflower hypocotyls, even though large cankers occurred on plants inoculated with P. carthami teliospores. Clear microscopic evidence of infection also was observed in hypocotyls inoculated with P. carthami . These data suggest that native (including rare, threatened, or endangered) Cirsium spp., modern safflower cultivars, and S. americana are not likely to be adversely affected by the use of P. jaceae for biological control of YST. Results from these studies substantiate previous findings and were incorporated in a proposal for permission to use P. jaceae for YST control in California.

Evaluating biological control of yellow starthistle (Centaurea solstitialis) in California: A GIS based supply–demand demographic model

The biological control of yellow starthistle (Centaurea solstitialis) by four capitulum-feeding insects (weevils Bangasternus orientalis and Eustenopus villosus, and flies Urophora sirunaseva and Chaetorellia succinea), as affected by plant competition with annual grasses, is analyzed using a weather-driven, physiologically based, age-structured, simulation model. Seed density in the soil seed bank for yellow starthistle and the number of overwintering insects provide between-season links. Seed germination intensity and pattern is determined by the timing and quantity of autumn rains and temperatures, while season-length is mostly determined by soil–water balance. Output from the yellow starthistle systems model was integrated into a geographic information system to examine regional differences in abundance for all species across several ecological zones of California. A marginal analysis of the simulation results was performed to examine the efficacy of each capitulum-feeding insect, their within-capitulum competitive interactions, and the effect of competition from annual grasses on yellow starthistle populations. The model suggests that lack of complete control of yellow starthistle may be due to plant compensation via increased per-plant seed production at lower plant densities and the incomplete destruction of seed in attacked capitula. The model suggests herbivory that reduces the plant’s ability to compensate and/or kills whole plants before seed maturity would most likely lead to control of yellow starthistle.