Chiefly illustrated by color photographs. / Digitized by Kansas Correctional Industries

Thesis--Virginia Polytechnic Institute and State University. / Vita. Abstract. Includes bibliographical references (leaves 51-70). Also available via the Internet.

Canada thistle is an aggressive perennial weed throughout temperate areas for both the northern and southern hemisphere. As various single tactics have shown limited success in controlling this weed, I believe that a combination of different stress factors is necessary for effective sustainable control of Canada thistle. This study evaluates the competitive abilities of tall fescue grass and crownvetch in conjunction with a thistle feeding insect for Canada thistle control. The goal of the study was to determine what levels of the herbivore and plant competition are needed to suppress Canada thistle. Field studies were conducted to determine the effects of multiple stresses caused by defoliation and plant competition on growth and survival of Canada thistle (<i>Cirsium arvense</i> Scop.). Plant competition was from a cornbination of tall fescue (<i>Festuca arundinaceae</i> Schreb.) and crownvetch (<i>Coronilla varia</i> L). Artificial defoliation was used to determine the influence of level and frequency of defoliation in combination with plant competition on Canada thistle. The effects of various densities of a defoliator, <i>Cassida rubiginosa</i> (Coleop:Chrysonlelidae), in the presence and absence of plant competition were also determined. A three-season study determined the combined effects of <i>C. rubiginosa</i> and plant competitors at various densities on the growth and productivity of Canada thistle plantings of various ages. / Ph. D.

Thesis (Ph. D.)--University of California, Riverside, 2009. / Includes abstract. Includes bibliographical references. Issued in print and online. Available via ProQuest Digital Dissertations.

A biological and integrated control program for Carduus thistles was developed using the biological control agents Rhinocyllus conicus Froelich and Ceuthorhynchidius horridus (Panzer) and the herbicide 2,4-Dichlorophenoxyacetic acid (2,4-D). Field studies on the development of Ceuthorhynchidius horridus (Panzer) on Carduus thistles in Virginia between 1975-1978 showed that the weevil has one generation annually. Oviposition occurred from mid-December until early April, and larvae occurred in rosettes from late December through late May. Teneral adults, which appeared in mid-May through June, underwent an aestival diapause during most of July through September. Adult reappearance in late September coincided with an increase in feeding. Although adult feeding marks, teneral adults and first and third instar larvae were easily found in the field, detection of eggs, second instars or overwintering adults was difficult and time consuming. Acute and chronic effects of spring application of 2,4-D (LVA) on adult C. horridus were examined. LC₅₀ values for males (70.2 kg/ha) and females (61.4 kg/ha) corresponded to 41.7 and 36.6 times, respectively, the recommended application rate of 1.68 kg/ha. Treatment with 1.68 kg/ha did not affect adult survival, but increased dosages (16.8-147.8 kg/ha) caused significantly greater mortality. Adult vitality, measured by number of feeding marks/weevil and weight change/time, was unaffected by the herbicide. Field application of herbicides did not prevent survival, reproduction, or population increase of C. horridus. Herbicidal effect on larval R. conicus was studied by examining the mortality, emergence rates and weights of weevils developing from plants treated with 2,4-D (LVA). Infested heads, obtained by caging ovipositing R. conicus on primary heads of musk thistle (Carduus nutans L.) (resembles C. thoermeri Weinmann), were treated with 2,4-D at 1.68 kg/ha 0-3 weeks after oviposition. Blooms treated immediately following oviposition failed to support larval development beyond the second instar. Developmental times and weights of weevils that emerged from blooms sprayed at 1, 2, and 3 weeks were not significantly different from controls. Plants sprayed up to 2 weeks after oviposition (late-bud to early-bloom) did not produce viable seeds. Treatments at 3 weeks after oviposition (full-bloom) allowed 10% germination of seeds not damaged by R. conicus in primary heads, and plants survived to produce additional heads. Acute and chronic effects of the herbicide 2,4-D on adult R. conicus were also examined. LC₅₀ values for adults prior to over-wintering (males 78.6 kg/ha; females 61.0 kg/ha) were lower than those for overwintered weevils (males 117.1 kg/ha; females 126.6 kg/ha), but were still at least 40 times the recommended application rate of 1.68 kg/ha. Survival was not significantly affected by direct application of 2,4-D at 1.68 kg/ha plus sticker or by exposure to herbicide sprays and residue while on musk thistle rosettes. Mean egg production/ovipositing female/3 day period was not significantly different (range = 5.44 - 7.60), regardless of the 2,4-D dosage applied (range = 0.0 - 147.84 kg/ha); all ovipositing weevils produced viable eggs. Field treatment with up to 2.24 kg/ha of 2,4-D resulted in death of host plants, but did not prevent survival or reproduction of R. conicus populations. / Ph. D.

The compatibility of the rust fungus Puccinia carduorum Jacky with three biological control agents, Trichosirocalus horridus (Panzer), rhinocvllus conicus Froelich, and Cassida rubiginosa Müller and their combined impact on must thistle, Carduus thoermeri Weinmann were studied. / Ph. D.