Thesis (M.S.)--University of Wisconsin--Madison, 1968. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis--Wisconsin. / Includes bibliographical references (leaves 132-134).

Thesis (M.S.)--University of Wisconsin--Madison, 1983. / Typescript. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references (leaves 49-54).

No description available.

The aim of this study was to improve our understanding of the biology of two alien, invasive, free-floating species, the Water Fern Azollafiliculoides and the Least Duckweed Lemna minUla, which cause severe weed problems in Britain. Hence, control practices, which had been based upon anecdotal information, could be given a scientific foundation. An appraisal of the known distributions of the two species in Britain was undertaken. From an analysis of spread to date, it can be predicted that L. minUla will be present in 150 and A.filiculoides in 500, 10 x 10 km grid squares by the year 2000. Neither A. filiculoides nor L. minuta produce specialised overwintering structures and both rely heavily on ordinary, vegetative plants to overwinter. It was found that vegetative plants of both species can survive encasement in ice and laboratory studies show that they can withstand short exposure to sub-zero temperatures above approximately -4 °C. However, field evidence suggests that both species can survive much more severe temperature conditions, so both are considerably less cold-sensitive than previously suggested. L. minuta is not known to reproduce sexually in Britain. However, A. filiculoides sporulates regularly over a wide geographical range to produce numerous, viable sporocarps. These sporocarps form a 'seed' bank in the sediment and may ensure population survival because they are more freeze- and desiccation- tolerant than adult plants. Sporocarps require temperatures of not less than 10 °C and a short burst of light to germinate. L. min uta plants vary very little seasonally (although larger summer fronds can be easily confused with the common, native duckweed Lemna minor). In contrast, three forms of A. jilicuJoides can be identified; survival, mat and colonising. Competition experiments suggested the following tentative order of decreasing competitive ability; A. filiculoides > L. minuta > L. minor. This was probably a result of the more erect A filiculoides plants growing over the Lemna fronds. Not all field evidence supports this finding. Floating mats of A.filiculoides and L. minuta cause similar ecological problems because they reduce light, pH and oxygen and increase detrit~:s and probably alter nutrient loading. Laboratory studies showed that morphologically and physiologically plastic species of submerged plants, ego Elodea nuttall;;, could withstand these conditions better than less adaptable species, ego Potamogeton crispus. Four chemical control methods were tested over a range of dosages in the laboratory; diquat (as both spray and sub-surface injection), glyphosate (as spray) and terbutryn (as sub-surface application). These treatments were unsuccessful in controlling L. minuta, in contrast to anecdotal field evidence where terbutryn is considered effective. Glyphosate and diquat sprayed at 1.8 kg ai ha-1 and 1 kg ai ha-1 controlled A. jiliculoides. A. jiliculoides sporocarps were resistant to all chemical treatments.

Thesis (Ph. D.)--University of Wisconsin--Madison, 1972. / Typescript. Vita. eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1970. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (M.S.)--University of Wisconsin--Madison, 1970. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Ph.D, Faculty of Science, University of the Witwatersrand, 2011 / Abstract Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) (water hyacinth), a neotropic noxious weed of South American origin, is counted among the “big five” aquatic weeds in South Africa. The weed causes dramatic ecological and economic losses in infested areas. Its control is facilitated by the release of biocontrol agents, mainly Neochetina eichhorniae (Warner) and Neochetina bruchi Hustache (Coleoptera: Curculionidae). Control efforts via biocontrol are hampered, mainly by the climate incompatibility of the agents, aggravated further by the indiscriminate use of lethal doses of glyphosate based herbicides. The lethal doses interfere with the successful establishment and persistence of the biocontrol agents, thus undermining their impact. Continued use of herbicide kills the water hyacinth mat and as a result, the immature stages of the agents are killed. If biocontrol is to succeed as a control strategy, then low doses of the herbicide need to be advocated. It was hypothesized that a low dose will constrain the vegetative and reproductive capacity of the weed, while maintaining the habitat for the biocontrol agents. Consequently, this study was conducted to identify a retardant dose of glyphosate herbicide and test its effect on the Neochetina weevils. A concentration of 0.8% (0.11g m-² or 2880mg a.i /L) glyphosate based herbicide, sprayed at 150 L ha-1 was proved to retard the vegetative and the reproductive growth of the weed, in terms of leaf and ramet production. Further, the retardant dose did not have any detrimental effects on the adult weevils and its larval stages. Weevil herbivory was also enhanced by the retardant dose. Furthermore, the retardant dose did not have any detrimental effects on ‘plant quality’ as evidenced by % nitrogen level in plant tissues such as crown and leaves. Contrary to expectation however, the combined effects of the retardant dose and Neochetina herbivory (0.8%+Ne) did not result in the production of lower number of ramets or leaves than water hyacinth plants dosed with 0.8% herbicide alone. Water hyacinth biocontrol agents in South Africa are subjected to frosty winters with low temperatures which cause the biocontrol agents to decline to an overwintering larval population that fails to catch up with the weed as it rebounds from the frost in spring. This hypothesis was tested in this study at 12 water hyacinth infested sites, which were grouped as temperate and sub-tropical sites. At both the temperate and subtropical sites, water hyacinth plants produced ramets (daughter plants) through autumn and increased biomass during summer. However, weevil numbers were very low at these sites, as evidenced by adult counts and feeding scars, indicating a marked seasonal asynchrony between the phenologies of the weevils and water hyacinth. Hence, intervention by seasonal applications of the herbicide is crucial to constrain weed growth. Herbicidal applications during autumn and spring inhibited the growth of the weed without adversely affecting the adult weevils or immature, immobile stages. Continued use of herbicides raises concerns of effect on non-target species, such as amphibians. Results from this study indicate that a direct application of a retardant dose of glyphosate did not kill or affect the growth of the Xenopus larvae, as determined by survival and body lengths. However, under laboratory conditions, this study has shown for the first time that an invasive aquatic weed (water hyacinth) was more lethal to an aquatic vertebrate (Xenopus larvae) than a herbicide advocated for its control. This study conclusively shows that retardant dose of glyphosate herbicide can be integrated with biocontrol to provide a sustainable and eco-friendly technique with which to combat water hyacinth infestations in South Africa.

M.Sc. (Botany) / Please refer to full text to view abstract