Biological studies of insect herbivores associated with some species of Solanum L
- Authors: Olckers, Terence
- Date: 1989
- Subjects: Solanum -- Control -- South Africa Phytophagous insects -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5593 , http://hdl.handle.net/10962/d1002043
- Description: Solanum mauritianum Scop. (bugweed), a serious exotic weed in South Africa, supports a depauperate herbivore fauna relative to indigenous Solanum species. These comprise mainly polyphagous, and some oligophagous, species. The greater diversity of insect herbivores on Solanum hermannii Dun., relative to other indigenous Solanum species in the eastern Cape, suggests that the plant is indigenous and not exotic as has been suggested. Five oligophagous species seasonally caused a high incidence of damage to S.hermannii and other indigenous Solanum species in the eastern Cape, during the study period. Galls of the flowers, stems and leaves of some indigenous Solanum plants are described and biological data on the gall-formers and their natural enemies presented. The tortoise beetle, (Chrysomelidae), defoliates a Conchyloctenia tigrina wide range of indigenous Olivo Solanum species. The exotic S.mauritianum and S.elaeagnifolium Cav. were unfavourable for growth and survival. S.hermannii proved the most favourable of the indigenous hosts tested, for growth and survival, providing further evidence that it is indigenous. Extensive studies on different field populations of the beetle revealed no evidence of host adaptation (host races). All populations tested displayed greater fitness when reared on S.hermannii. South African Solanum faunas are characterized by a scarcity of endophagous and monophagous herbivores, vacant feeding niches and low numbers of species relative to other plant taxa. This may suggest evolutionary immaturity of the insect-plant associations, although comparative studies of solanums from other parts of the world are needed for confirmation. Alternatively, these characteristics may simply be a feature of the genus Solanum in general. The depauperate herbivore fauna of S.mauritianum reflects a lack of local herbivores suitably preadapted to overcome its defences. This appears to be accentuated by absence of any close relatives of bugweed in South Africa. The greater diversity of herbivores in its native South America suggests that biological control may be a potentially useful means of control for bugweed. The potential for biological control is, however, threatened by the susceptability of the cultivated eggplant (Solanum melongena L.) to attack by Solanum-feeding insects. Possible solutions to this problem are discussed
- Full Text:
- Date Issued: 1989
- Authors: Olckers, Terence
- Date: 1989
- Subjects: Solanum -- Control -- South Africa Phytophagous insects -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5593 , http://hdl.handle.net/10962/d1002043
- Description: Solanum mauritianum Scop. (bugweed), a serious exotic weed in South Africa, supports a depauperate herbivore fauna relative to indigenous Solanum species. These comprise mainly polyphagous, and some oligophagous, species. The greater diversity of insect herbivores on Solanum hermannii Dun., relative to other indigenous Solanum species in the eastern Cape, suggests that the plant is indigenous and not exotic as has been suggested. Five oligophagous species seasonally caused a high incidence of damage to S.hermannii and other indigenous Solanum species in the eastern Cape, during the study period. Galls of the flowers, stems and leaves of some indigenous Solanum plants are described and biological data on the gall-formers and their natural enemies presented. The tortoise beetle, (Chrysomelidae), defoliates a Conchyloctenia tigrina wide range of indigenous Olivo Solanum species. The exotic S.mauritianum and S.elaeagnifolium Cav. were unfavourable for growth and survival. S.hermannii proved the most favourable of the indigenous hosts tested, for growth and survival, providing further evidence that it is indigenous. Extensive studies on different field populations of the beetle revealed no evidence of host adaptation (host races). All populations tested displayed greater fitness when reared on S.hermannii. South African Solanum faunas are characterized by a scarcity of endophagous and monophagous herbivores, vacant feeding niches and low numbers of species relative to other plant taxa. This may suggest evolutionary immaturity of the insect-plant associations, although comparative studies of solanums from other parts of the world are needed for confirmation. Alternatively, these characteristics may simply be a feature of the genus Solanum in general. The depauperate herbivore fauna of S.mauritianum reflects a lack of local herbivores suitably preadapted to overcome its defences. This appears to be accentuated by absence of any close relatives of bugweed in South Africa. The greater diversity of herbivores in its native South America suggests that biological control may be a potentially useful means of control for bugweed. The potential for biological control is, however, threatened by the susceptability of the cultivated eggplant (Solanum melongena L.) to attack by Solanum-feeding insects. Possible solutions to this problem are discussed
- Full Text:
- Date Issued: 1989
Interactions between ants, herbivorous insects and bracken (Pteridium aquilinum), a fern with extrafloral nectaries
- Authors: Rashbrook, Vanessa Karin
- Date: 1989
- Subjects: Arthropod pests -- South Africa Ants -- South Africa Ferns -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5592 , http://hdl.handle.net/10962/d1002042
- Description: Bracken (Pteridium aquilinum) is a cosmopolitan species of fern which possesses extrafloral nectaries. A survey of the arthropod fauna associated with the plant throughout its range in South Africa identified the most widespread and damaging bracken herbivores and those species most likely to be influenced by ants visiting the extrafloral nectaries. Seventeen herbivorous arthropods were found to be definitely feeding on bracken and a further thirteen species with a less certain status were recorded. In addition, several species of ants were observed on the fronds and feeding at the extrafloral nectarie. On the basis of their widespread distribution and abundance, four bracken herbivores emerged as being particularly detrimental to the plant in South Africa. They were an eriophyid gall mite, a leafhopper and two moths. The biology of the two lepidopterans, Appana cinisigna and Panotima sp., suggested that they were potentially vulnerable to ants at various stages of their life histories. Laboratory experiments were undertaken to observe ant-lepidopteran interactions under controlled conditions. The ant Crematogaster peringueyi rapidly removed exposed A. cini igna eggs, but did not appear to regard Panotima eggs as food items. Small instar larvae of both species proved vulnerable, but the larger larvae appeared capable of escaping ant predation. Ant exclusion experiments in the field, using natural and artificially augmented ant densities, were carried out at two sites near Grahamstown. The dominant ant species was C. peringueyi. Neither the numbers of lepidopterans nor levels of herbivory were significantly reduced by the presence of ants . Despite the implications of the laboratory tests, the results of these field experiments did not support the hypothesis that ants which visit bracken extrafloral nectaries benefit the plant. Ant-bracken field studies on other continents also reported no significant ant-related effects, though marginal reductions in the abundance of certain herbivores have been noted at one site in the U.K. Since ant protection does not appear to be an inevitable consequence of having extrafloral nectaries, their value to bracken is in some doubt. The most likely situation where effective protection will occur is when high densities of vulnerable herbivores are preyed upon by large numbers of aggressive ants. However, even under these conditions, enhancement of plant fitness is not inevitable. This raises the question of why extrafloral nectaries have been retained in a plant that is as successful and widespread as bracken.
- Full Text:
- Date Issued: 1989
- Authors: Rashbrook, Vanessa Karin
- Date: 1989
- Subjects: Arthropod pests -- South Africa Ants -- South Africa Ferns -- South Africa
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5592 , http://hdl.handle.net/10962/d1002042
- Description: Bracken (Pteridium aquilinum) is a cosmopolitan species of fern which possesses extrafloral nectaries. A survey of the arthropod fauna associated with the plant throughout its range in South Africa identified the most widespread and damaging bracken herbivores and those species most likely to be influenced by ants visiting the extrafloral nectaries. Seventeen herbivorous arthropods were found to be definitely feeding on bracken and a further thirteen species with a less certain status were recorded. In addition, several species of ants were observed on the fronds and feeding at the extrafloral nectarie. On the basis of their widespread distribution and abundance, four bracken herbivores emerged as being particularly detrimental to the plant in South Africa. They were an eriophyid gall mite, a leafhopper and two moths. The biology of the two lepidopterans, Appana cinisigna and Panotima sp., suggested that they were potentially vulnerable to ants at various stages of their life histories. Laboratory experiments were undertaken to observe ant-lepidopteran interactions under controlled conditions. The ant Crematogaster peringueyi rapidly removed exposed A. cini igna eggs, but did not appear to regard Panotima eggs as food items. Small instar larvae of both species proved vulnerable, but the larger larvae appeared capable of escaping ant predation. Ant exclusion experiments in the field, using natural and artificially augmented ant densities, were carried out at two sites near Grahamstown. The dominant ant species was C. peringueyi. Neither the numbers of lepidopterans nor levels of herbivory were significantly reduced by the presence of ants . Despite the implications of the laboratory tests, the results of these field experiments did not support the hypothesis that ants which visit bracken extrafloral nectaries benefit the plant. Ant-bracken field studies on other continents also reported no significant ant-related effects, though marginal reductions in the abundance of certain herbivores have been noted at one site in the U.K. Since ant protection does not appear to be an inevitable consequence of having extrafloral nectaries, their value to bracken is in some doubt. The most likely situation where effective protection will occur is when high densities of vulnerable herbivores are preyed upon by large numbers of aggressive ants. However, even under these conditions, enhancement of plant fitness is not inevitable. This raises the question of why extrafloral nectaries have been retained in a plant that is as successful and widespread as bracken.
- Full Text:
- Date Issued: 1989
Seasonal changes in the physiological and hormonal aspects of reproduction in the male long-fingered bat, Miniopterus schreibersii (Mammalia : Chiroptera)
- Authors: Paton, Joy Carol
- Date: 1989
- Subjects: Bats -- Reproduction
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5590 , http://hdl.handle.net/10962/d1002040
- Description: The reproductive cycle of Miniopterus schreibersii is modified as a consequence of a period of torpor/hibernation over the winter months. This bat is one of three genera which employ the reproductive strategy of delayed implantation in which the blastocyst remains free in the uterine lumen during the winter months. Spermatogenesis is initiated in February, with spermiogenesis occuring in March. Copulation, ovulation and fertilization takes place between March and May after which the testes regress and remain in an inactive condition until the following summer. Activity in the accessory gland complex is initiated in March and continues until early June. Plasma testosterone concentrations reach a peak in May, decline over the winter months and reach a second, unexplainable peak in October. Leydig cells are secretorily active in February/early March after which they undergo vacuolation and final degeneration. The question of the life cyle of Leydig cells is addressed and an increase in a certain cell after Leydig cell activity suggests that this cell type may be the precursor of Leydig cells.
- Full Text:
- Date Issued: 1989
- Authors: Paton, Joy Carol
- Date: 1989
- Subjects: Bats -- Reproduction
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5590 , http://hdl.handle.net/10962/d1002040
- Description: The reproductive cycle of Miniopterus schreibersii is modified as a consequence of a period of torpor/hibernation over the winter months. This bat is one of three genera which employ the reproductive strategy of delayed implantation in which the blastocyst remains free in the uterine lumen during the winter months. Spermatogenesis is initiated in February, with spermiogenesis occuring in March. Copulation, ovulation and fertilization takes place between March and May after which the testes regress and remain in an inactive condition until the following summer. Activity in the accessory gland complex is initiated in March and continues until early June. Plasma testosterone concentrations reach a peak in May, decline over the winter months and reach a second, unexplainable peak in October. Leydig cells are secretorily active in February/early March after which they undergo vacuolation and final degeneration. The question of the life cyle of Leydig cells is addressed and an increase in a certain cell after Leydig cell activity suggests that this cell type may be the precursor of Leydig cells.
- Full Text:
- Date Issued: 1989
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