Investigations into insect-induced plant responses of water hyacinth (Eichhornia crassipes (Mart.) Solms-Laub.) (Pontederiaceae)
- Authors: May, Bronwen
- Date: 2015
- Subjects: Water hyacinth , Water hyacinth -- Biological control , Water hyacinth -- Defenses , Aquatic weeds , Insect-plant relationships , Miridae , Curculionidae
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5940 , http://hdl.handle.net/10962/d1018906
- Description: The water hyacinth (Eichhornia crassipes (Mart.) Solms-Laub (Pontederiaceae)) biological control programme makes use of tight plant-insect interactions to control the weed, by reestablishing the interactions between the plant and its natural enemies. Since the beginning of the water hyacinth biological control initiative, the impact of biological control agent herbivory on water hyacinth’s population growth and fitness have been well documented; however, very few investigations have been conducted to determine whether herbivory elicits insect-induced responses by water hyacinth. Studies were conducted to determine the presence and function of water hyacinth insectinduced responses, using the plant activator, BION®, in attempt to determine the plant hormone-mediated pathways regulating the final expressions of insect-induced defences in response to herbivory by the phloem-feeder, Eccritotarsus catarinensis (Carvalho) (Hemiptera: Miridae) and the leaf chewer, Neochetina bruchi Hustache (Coleoptera: Curculionidae). BION® (Syngenta, acibensolar-S-methyl (benzothiadiazole)) is a dissolvable, granular formulation that contains a chemical analogue of the plant hormone, salicylic acid (SA), which typically regulates defences against pathogens. The application of BION® results in the induction of the SA-mediated defence pathways in plants (activation of defences against pathogens), and consequently the inhibition of the jasmonic acid (JA)- mediated defence pathways (de-activation of defences against insect herbivores). To test for induced defence responses in water hyacinth, plants treated with BION® and then subjected to herbivory, were compared to un-treated plants that were also subjected to herbivory, BION®-only treated plants and control plants. The application of BION® did not confer resistance against the two insect herbivores, as herbivory, reductions in chlorophyll content and plant growth (leaf production and second petiole lengths) significantly increased in comparison to non-BION® treated plants. Furthermore, palatability indices significantly increased (>1.00) in BION® treated plants, reflecting increased weevil preferences for SAinduced water hyacinth plants. This concluded that SA-mediated defences are not effective against insect herbivory in water hyacinth plants, but are in fact palatable to insect herbivores, which reflects ecological and physiological costs of SA-mediated defences (pathogen defences) in water hyacinth. Biochemical analyses of leaves exhibited increases in nitrogen content in BION® treated plants. These elevated levels of nitrogenous compounds account for the increases in mirid and weevil preferences for BION® treated plants. The increases in nitrogenous compounds are probably structural proteins (e.g. peroxidises), because leaves treated with BION® increased in toughness, but only when exposed to herbivory. Regardless, insect herbivory was elevated on these leaves, probably because the nitrogenous compounds were nutritionally viable for the insects.
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- Date Issued: 2015
Studies on the use of essential oils for the control of Sitophilus Zeamais (Motschulsky) (Coleoptera; Curculionidae): a pest of stored maize grains
- Authors: Odeyemi, Oluwakemi Oluwaseyi
- Date: 2008
- Subjects: Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD (Botany)
- Identifier: vital:11301 , http://hdl.handle.net/10353/168 , Curculionidae , Essences and essential oils , Corn -- Diseases and pests -- South Africa , Biological pest control agents -- South Africa
- Description: The common maize weevil, Sitophilus zeamais Motschulsky (Coleoptera: Curculionidae), a well known pest of stored-maize grain in most parts of the world, was identified as one of the major constraints of harvested maize grains in the Eastern Cape Province, South Africa. The use of plants or their products is one of the recent methods being investigated for insect pest control worldwide. Therefore, the main aim of the present study was to identify available plants in the Eastern Cape that could be used to combat the problem of Sitophilus zeamais in stored-maize grains. For the realization of the aims of this research, the following studies were carried out; a preliminary survey was conducted to obtain baseline information on the farmers’ knowledge and experience of indigenous insect pest control methods in the Eastern Cape. Also, studies on the insecticidal potential of the essential oils of some plants were investigated against the maize weevil. The quality parameters of maize grains treated with the essential oils was also studied and, using a rat model, the toxicity of the essential oils was investigated. The outcome from this study revealed that there is awareness amongst the farmers in the Eastern Cape on the use of plants or their products to control insect pests. Unfortunately, such methods are currently being neglected and the knowledge of their application was found to be eroding. Among the various essential oils screened were those from Mentha longifolia L. and Tagetes minuta L. which evoked an appreciable level of contact, fumigant and repellent toxicity on the maize weevil. Further work done to determine the effects of the oils on maize stored over a period of three months revealed that the two oils had no adverse effect on the proximate compositions and some quality parameters of the stored maize. However, the toxicological study conducted on rats showed that the oils at tested concentrations exhibited some level of toxicity. It is, therefore, suggested that the essential oils of M. longifolia and T. minuta should not be used to treat maize grains intended for human consumption.
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- Date Issued: 2008