Investigations into the ecology of a population of Citrus Red Scale (Aonidiella aurantii Mask.) in the Swaziland lowlands
- Authors: Atkinson, P R
- Date: 1982
- Subjects: Citrus -- Diseases and pests -- Africa, Southern
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5619 , http://hdl.handle.net/10962/d1003733
- Description: In the Swaziland lowlands, biological control of the citrus pest red scale, Aonidiella aurantii (Mask), has been consistently unsuccessful although it is successful in middle altitude regions. However, recent cases of resistance to organophosphate insecticides elsewhere in southern Africa, make it important to reconsider biological control of red scale in the lowlands. Should similar resistance be evolved in this region, the pest could be controlled only by natural means together with oils. This account attempts to answer questions about the natural control of a red scale population in the lowlands, by examining data collected between 1972 and 1975. The population was continuously breeding with overlapped generations. It consisted of three sections, on leaves, twigs and fruit, having different rates of reproduction, development and mortality. These sections are considered separately but that on leaves, being relatively unimportant, is largely ignored. The generation mortality is estimated in the sections on twigs and fruit and related to climatic and biotic indices. The causes of seasonal and between-years variation in the sections on twigs and fruit are elucidated. The effectiveness of each kind of natural enemy is examined and the factors affecting natural enemy numbers are investigated. The question of population regulation is discussed. Differences in mean infestation levels in middle altitude and lowland regions are explained in terms of different rates of reproduction and mortality.
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The factors which influence algal growth in the P.K. le Roux impoundment, Orange River, South Africa
- Authors: Selkirk, Wayne Thomas
- Date: 1982
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5798 , http://hdl.handle.net/10962/d1005900
- Description: Introduction: In South Africa, much of the work on algal limitation in man-made lakes has centered on nutrients rather than the possible effects of physical factors such as turbidity.In order to determine which is the most important factor in algal limitation in the turbid Orange River system, a comprehensive study of spatial and seasonal changes of the following factors was undertaken in the P.K. Ie Roux impoundment: trace elements , silicon, nitrate, phosphate, total phosphate, phytoplankton standing stock, total suspended solids, secthi disc transparency, light attenuation and the spectral quality of the light. None of the chemical factors appeared to be in short supply but the shallowness of the euphotic zone (0.8 m) had two effects. Firstly. the quantity of light in the upper 1.5 metres was extremely small due to the rapid attenuation of the light by silt. Secondly. the quality of the light was poor due to the selective screening of the shorter wavelengths of light. Once light limitation had been implicated as an important factor in the control of phytoplankton stocks a series of column experiments were used to demonstrate that increased circulation rate. and therefore a higher light dosage. would elevate the algal standing stocks without the addition of nutrients. It was demonstrated that light. as a result of the negative effects of turbidity was the principal cause of low algal standing stocks within the impoundment. and not nutrients as has previously been suggested by other workers.
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