Biological Control Of Musk Thistle Research Articles

Supplemental risk evaluations and status of Puccinia carduorum for biological control of musk thistle

Abstract Tests of seven rare and endangered native North American Cirsium species and four modern artichoke lines were requested in response to a proposal for introduction of Puccinia carduorum into the United States for biological control of musk thistle ( Carduus nutans ssp. leiophyllus ). These tests were supplemental to an earlier extensive host-range study that established P. carduorum from musk thistle as host specific, useful for biological control, and suitable for limited field tests in Virginia. Test plants in the current study were evaluated in support of a proposal to use the rust in the western United States, and particularly, in California. None of the test plants in this study had been evaluated in previous assessments and all were either rare, endangered or threatened in California. Tests were conducted in both field and greenhouse settings. Field tests were run for two seasons, and test plants were inoculated by natural spread of the pathogen from source plants inside rings of test plants. Greenhouse tests involved direct inoculation under optimal conditions of dew and temperature (18–20 °C, 16 h) for infection. None of the seven Cirsium species or subspecies tested became infected by P. carduorum , either in field or greenhouse tests, compared to infection of 98% of the individual musk thistle plants ( n = 102) from all the studies. Modern artichoke cultivars were tested only by direct inoculation under optimal greenhouse conditions. All artichoke plants ( n = 115) either were immune (no macroscopic symptoms, n = 69) or at most, resistant ( n = 46); pustules on all but two of the resistant plants were very small (⩽0.30 mm diam). Despite infections on artichokes, P. carduorum could not be maintained on artichokes under optimal greenhouse conditions. These results confirm earlier findings from host-range tests and risk assessments of P. carduorum . This information suggests that rare, threatened, or endangered Cirsium spp. and modern artichoke cultivars are not likely to be adversely affected by the use of P. carduorum for biological control of musk thistle. These data have been reviewed by grower groups and regulatory agencies in a proposal for permission to use the rust for musk thistle control throughout the United States.

Indirect effect of early-season infestations of Trichosirocalus horridus on Rhinocyllus conicus (Coleoptera: Curculionidae)

Abstract Field experiments were conducted to determine whether early vegetative feeding by Trichosirocalus horridus alters musk thistle ( Carduus nutans ) as a resource for the later-arriving Rhinocyllus conicus . Results showed that thistles with high larval densities of T. horridus (66 per plant) produced more and shorter flower stems, significantly fewer flower heads, and delayed flowering by 1 week when compared to uninfested thistles. Also, colonization and oviposition on flower heads by adult R. conicus were as much as three and five times lower, respectively, on thistles heavily infested with T. horridus than on uninfested thistles. As a result, 63% fewer R. conicus adults developed from T. horridus -infested thistles. Musk thistles that were infested with lower densities of T. horridus larvae ( R. conicus to T. horridus -infested and uninfested plants. In a greenhouse experiment, the mortality rate of R. conicus was higher on musk thistles that were heavily infested by T. horridus , implying a change in musk thistle quality. However, this effect was apparent only at low R. conicus larval densities. T. horridus can indirectly and negatively affect R. conicus through a variety of mechanisms, but only when densities of T. horridus are high. This finding may have important implications for the continued redistribution of both weevils, which were imported for the classical biological control of musk thistle, a noxious weed in pastures and rangeland.

First Report of Musk Thistle Rust (Puccinia carduorum) in California and Nevada.

Musk thistle, Carduus nutans L., is an introduced weed of pastures, rangelands, and natural areas in much of North America. Puccinia carduorum Jacky, an autoecious rust fungus from Turkey, has been evaluated for biological control of musk thistle since 1978, including a field study near Blacksburg, VA, from 1987 to 1990. After release of the fungus in Virginia, rusted musk thistle was found in eight eastern states by 1992, in Missouri by 1994 (1), and in Oklahoma by 1997 (2). A rust disease was discovered on musk thistle near Mt. Shasta, CA, on 22 September 1998, and near Mogul, NV, on 12 August 1999. The pathogen was identified as P. carduorum on the basis of pathogenicity on musk thistle and urediniospore morphology (ovate spores, 21 μm diameter, three germ pores equatorial in location, and echinulations over the upper two-thirds to three-quarters of urediniospores). Ribosomal RNA internal transcribed spacer DNA sequences (ITS1 and ITS2) were identical to those from the isolate obtained after the field release in Virginia, verifying that the California isolate is P. carduorum. The initial California infestation was observed on a few plants late in the season, and by September 2000, nearly 100% of plants were infected. The occurrence of P. carduorum in California is apparently the result of natural, unaided spread of the fungus on musk thistle from the East Coast of the United States.

Effects ofPuccinia carduorumon Musk Thistle Herbivores

Puccinia carduorum Jacky, a rust fungus recently introduced into the United States for biological control of musk thistle ( Carduus thoermeri Weinmann), may interact with three established thistle herbivores, Trichosirocalus horridus (Panzer) (Coleoptera: Curculionidae), Rhinocyllus conicus Froelich (Coleoptera: Curculionidae), and Cassida rubiginosa Müller (Coleoptera: Chrysomelidae). Tests were conducted to determine the effects of P. carduorum on feeding, oviposition, longevity, egg production, egg hatch, and larval development of the three insects. When offered both rust-infected and healthy musk thistle leaves, adults of C. rubiginosa and T. horridus consumed significantly more healthy than infected foliage. On infected leaves, feeding and oviposition were confined largely to pustule-free areas. The amount of rust-infected and healthy foliage consumed by C. rubiginosa larvae was not significantly different, but feeding on the infected leaves was again confined to rust-free leaf areas. Rust infection did not reduce oviposition by the three insects, and R. conicus oviposited only on healthy portions of the bracts. There were no significant differences in longevity, egg production, percentage of egg hatch, and larval development for any of the three insects fed continuously with both healthy and rust-infected leaves. Thus, effects of P. carduorum on the herbivores were slight, and among the interactions that were found, none is expected to be detrimental to biological control.

Population Dynamics and Spread ofPuccinia carduorumin the Eastern United States

Puccinia carduorum was first introduced into western Virginia in 1987 for biological control of musk thistle (Carduus thoermeri). The pathogen's distribution was surveyed in 1992; it had spread to South Carolina, Georgia, Tennessee, Kentucky, Ohio, southeast Indiana, Maryland, and Delaware, some points more than 500 km from the release site. In 1994, it was found west of the Mississippi River in north central Missouri. Population development of the rust was monitored in several natural musk thistle stands in 1991 and 1992. Average pustule numbers per leaf were 0 to 0.7 in early May during early stem elongation, 0 to 17 (1991) and 8 to 52 (1992) at seed ripening in late June and early July as old plants died, 0.2 to 2.5 on young rosettes in September and October, and declining to near 0 by early December. Germinability of urediniospores from green tissue ranged from 10 to 88% (mean 51%) from May to October, with no significant seasonal trend. Teliospores were present on dead plants in late July and August but did not become prevalent on young rosettes until October and November. Latent periods (days from inoculation to first open pustule) on plants in the field were 13 to 14 days for inoculations in late April and early May, 8 days in June, 17 days in early October, and about 25 days in late October.

Transmission of Puccinia carduorum by the Musk Thistle Herbivores, Cassida rubiginosa (Coleoptera: Chrysomelidae), Trichosirocalus horridus and Rhinocyllus conicus (Coleoptera: Curculionidae)

An autoecious rust fungus, Puccinia carduorum Jacky (Uredinales), imported from Turkey has been introduced for the biological control of musk thistle in Virginia. Studies were conducted to determine if this rust can be transmitted by the three thistle herbivores, Cassida rubiginosa Müller (Coleoptera: Chrysomelidae), Trichosirocalus horridus (Panzer), and Rhinocyllus conicus Froelich (Coleoptera: Curculionidae). Fifty adults of each species were introduced to diseased plants and subsequently transferred to healthy test plants. Fifty adults introduced on and transferred to rust-free plants served as control. Spore dispersal by adults of the three coleopteran species was confirmed. Under greenhouse conditions, pustules developed within two weeks on some test plants where spore-carrying insects were introduced. Most of the urediniospores were observed on setae of legs of all adults.

Field Test of the WeevilHadroplonthus trimaculatusand the FleabeetlePsylliodes chalcomeraAgainst Musk Thistle (Carduus nutans)

This study was conducted to determine if field evaluations could be used to select insects for biological control of musk thistle. Host specificity and larval development of a weevil,Hadroplonthus trimaculatus, and a fleabeetle,Psylliodes chalcomera, were studied in field trials near Rome, Italy, in which insects were allowed free choice of several hosts. Natural populations of these two insects, which do not occur in North America, were exposed to North American species ofCirsium, Carduus, and selected crops. Adult insects and larvae on host plants were identified and counted on test plants from North America and native attraction plants. In addition to infesting musk thistle, weevil adults and larvae were recorded on flodman thistle, wavyleaf thistle, and spinosissimum thistle. Consequently, this insect was not suitable for introduction into North America. The fleabeetle would be satisfactory for biological control since no adults or larvae were recorded onCirsiumspp. or economic plants. These studies show that field trials are a valid method for identifying specific and nonspecific candidate insects for biological control of weeds.

Establishment of Rhinocyllus conicus (Coleoptera: Curculionidae) in Georgia for Control of Musk Thistle

Rhinocyllus conicus Froelich was introduced and successfully established at 21 sites in northern Georgia for biological control of musk thistle, Carduus nutans L. A total of 9,310 adults were released from 1990 through 1992. Releases of reproductively mature adults in the spring were much more successful than summer releases of teneral adults. Progeny were observed in all spring-release sites, and population increases occurred at most sites one and two years after release. Consumption of infested capitula by cattle reduced R. conicus numbers at some sites. After two years, weevils had dispersed up to 0.6 km from the initial release point. Transfer of R. conicus to new sites in Georgia will begin in 1993.

Field Evaluation of Puccinia carduorum for Biological Control of Musk Thistle

A strain of Puccinia carduorum Jacky, an autoecious rust fungus from Turkey, was evaluated for biological control of musk thistle (Carduus thoermeri Weinmann) in a 3-year field trial. Musk thistle plots were inoculated successfully in the fall and spring of each year. To assess the pathogen′s effect on nontarget species that were slightly susceptible in greenhouse trials, artichoke and selected Cirsium spp. were planted between plots of inoculated musk thistle in two of the years. Only one rust pustule was found on an artichoke plant in 1989; all other plants remained rust-free despite severe disease on surrounding musk thistles, indicating that P. carduorum poses no hazard to these nontarget species. Rust overwintered successfully on musk thistle. Its spread was limited during the rosette stage, and disease became severe only during bolting (stem elongation). There was little difference in plant size between inoculated and noninoculated, fungicide-protected plots, but plant senescence was accelerated and seed production was reduced by 20-57% in inoculated plots. The reduction in seed production in rust-inoculated plots was less than that due to attack by three insects. We cannot rule out the possibility that part of the effects on senescence and seed production may have been due to the fungicides used to protect the noninoculated plots from the rust. However, the reduction in seed production, particularly when in combination with insects, suggests that P. carduorum can contribute to the control of musk thistle.

PARASITISM OF RHINOCYLLUS CONICUS FROELICH (COLEOPTERA: CURCULIONIDAE) IN WYOMING

AbstractRhinocyllus conicus Froelich was introduced into Wyoming in 1975 for the biological control of musk thistle, Carduus nutans L. A survey of 15 release sites was conducted in 1986 to determine parasitism of R. conicus. Eight species of parasitic Hymenoptera were identified. Parasitism of R. conicus infecting the flower heads of musk thistle was low (0.6%), but greater for weevils within the peduncles (17%). Parasitism varied among release sites, ranging from 0.0 to 1.3%, but was not significantly correlated with the duration of weevil establishment.

Field Tests With the Musk Thistle Insects Trichosirocalus (Ceuthorhynchidius) horridus and Ceutorhynchus trimaculatus1 to Determine Their Impact on Artichoke

A series of field tests were conducted to clarify the specificity of Trichosirocalus (Ceuthorhynchidius) horridus and Ceutorhynchus trimaculatus (Curculionidae) to the Carduus thistles, one of their natural hosts in Europe. Adult feeding on artichoke, Cynara scolymus L. was recorded, however, the amount was insignificant, and the insects were unable to complete their development. T. horridus has been released in the U.S. for the biological control of musk thistle and C. trimaculatus is under consideration for release.