The effect of shade on the biological control of Salvinia molesta D.S. Mitchell [Salviniaceae] by the weevil, Cyrtobagous salviniae Calder and Sands [Curculionidae]
- Authors: Maseko, Zolile
- Date: 2017
- Subjects: Salvinia molesta , Aquatic weeds -- Biological control , Beetles , Insects as biological pest control agents
- Language: English
- Type: text , Thesis , Masters , MSc
- Identifier: http://hdl.handle.net/10962/4808 , vital:20726
- Description: Salvinia molesta, a floating aquatic weed, is a global menace in many water bodies and waterways. The weed disrupts the ecological balance wherever it invades and also has wide ranging economic and health impacts. Its impact has resulted in the need to control it, and while chemical and mechanical control are often ineffective, biological control by the weevil Cyrtobagous salviniae is largely successful. However, in many parts of the world, including South Africa, biological control of S. molesta has been less effective where the weed grows as an understory species. Shallow and shaded waters characteristically found at the margins of water bodies provide a refuge for S. molesta. Therefore, the aim of the study was to determine the effect of shade on the efficacy of biological control of S. molesta. Investigations into the problems associated with control in the shade were carried out in a greenhouse and in the field. In controlled greenhouse experiments, plants were grown at high and low nutrient levels in individual mesocosms, at three varying levels of shade, where half the mesocosms were inoculated with C. salviniae. Plants in high nutrient conditions were significantly more productive in terms of biomass accumulation, compared to those grown at low nutrient levels, in both the absence and presence of herbivory at all levels of shade tested. Plants grown in the shade and in high nutrient conditions had significantly higher quality compared to the ones exposed to full sun. Higher plant quality in the shade consequently resulted in accelerated fecundity for C. salviniae resulting in significantly higher weevil populations. Furthermore, less damage was recorded on plants in the shade, possibly due to the high nitrogen concentrations which may have deterred grazing by C. salviniae. However, in the absence of shading, plants were of lower quality and consequently sustained more damage from herbivores despite lower weevil populations. In contrast to the greenhouse, there were no differences in biomass recorded in the field at two nutrient levels and in the presence of herbivory for both shade and open sites. Nutrients added to high nutrient treatment quadrats diffused evenly across the water body resulting in uniform nutrient distribution, hence uniform plant biomass and carbon-nitrogen ratio. Despite the lack of statistical differences, more weevils were found in the full sun plots, while modest populations were recorded in the shade. Higher weevil populations consequently led to more damage in the sun, a situation that has been observed in most field sites in South Africa. The results therefore suggest that nutrients were pivotal in plant growth compared to light regimes (amount of PAR). Plant quality significantly influenced weevil populations and plant damage in both studies, demonstrating that plant nitrogen plays a crucial role in the plant- herbivore system. Furthermore, in both the greenhouse and field, despite lack of statistical differences, plants exposed to full sunlight suffered more damage than shade plants, pointing towards better control in the sun compared to the shade, suggesting that there is greater preference of sun-exposed plants for grazing by the weevils. The results from this study add to the growing body of literature that plant quality is a major factor in determining the success of aquatic weed biological control programmes.
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- Date Issued: 2017
The evaluation of Phenrica sp.2 (Coleoptera: Chrysomelidae: Alticinae), as a possible biological control agent for Madeira vine, Anredera cordifolia (Ten.) Steenis in South Africa
- Authors: Van der Westhuizen, Liamé
- Date: 2006
- Subjects: Weeds -- Biological control -- South Africa , Biological pest control agents -- South Africa , Invasive plants -- Biological control -- South Africa , Chrysomelidae , Beetles , Flea beetles , Anredera cordifolia -- Biological control
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5689 , http://hdl.handle.net/10962/d1005375 , Weeds -- Biological control -- South Africa , Biological pest control agents -- South Africa , Invasive plants -- Biological control -- South Africa , Chrysomelidae , Beetles , Flea beetles , Anredera cordifolia -- Biological control
- Description: Anredera cordifolia (Basellaceae), Madeira vine, is a perennial, semi- succulent climber native from Paraguay to southern Brazil and northern Argentina. It has a history of weediness and difficulty of control once established. In South Africa Madeira vine has a wide range and distribution with altitudes ranging from 10-1800m above sea level. Described as a transformer species, its sheer weight is capable of breaking branches off trees, causing the potential collapse of forest canopies. Chemical and mechanical control methods are expensive, labour intensive and may provide only temporary relief. A biological control programme was therefore initiated in 2003. Cf Phenrica sp. 2 (Coleoptera: Chrysomelidae: Alticinae), was field collected from A. cordifolia in Brazil, SSW of Cascavel in the Paraná Province during a survey in November 2003. Eggs are laid in groups of 16 with the average fertility rate being 89%. After going though three larval instars, the larvae pupate in the soil with the adults eclosing after a period of 17 days. The total developmental time for a generation from egg to egg ranges between 7-8 weeks. Biological traits that favour the flea beetle as a possible biological control agent include long-lived adults (up to 5 months) and multiple generations during the summer period. Both adults and larvae feed extensively on leaves and stems and although developmental rates will slow down during the winter period, no indication of a definite diapause was found under the prevailing laboratory conditions. After completing the larval no-choice trials with twenty-six plant species from 14 plant families Phenrica sp. 2 proved to be adequately host specific, as larval development was only supported by 3 Basellaceae species (including the control A. cordifolia) and one Portulacaceae species. All of these are introduced species in South Africa. The only indigenous Basella species could not be tested as it has a very marginal distribution, and because it’s inconspicuous nature, it is seldom seen or collected. Adult multi-choice trials were restricted to species that could sustain larval development to give some indication of the acceptability of these species for adult feeding and oviposition. Although adult feeding was initially concentrated on B. alba, the oviposition preference was clear-cut as females only oviposited on A. cordifolia. In order to quantify the impact of Phenrica sp. 2 on plant biomass and to assess the incidence and intensity of foliar damage, a pair of adults was confined to the host plant, for 2 generations, with different levels of larval densities. The results indicated that the host plant, due to both larval and adult feeding, suffered leaf losses of up to 55%. Anredera cordifolia was however still capable of enlarging the root mass despite suffering huge leaf losses. This would imply that A. cordifolia has an effective re-growth capacity and it will only be vulnerable to attack of the storage organs that enable re-growth, or to repeated attack of other plant parts through which reserves are exhausted. Unfortunately the period of exposure (24 days) was too short to prove that Phenrica sp. 2 impacts on the below ground dry mass, but should the plant be completely defoliated, as was observed in the field, the host plant would be forced to deplete stored resources. Phenrica sp.2 has shown to be very host specific and although A.cordifoia loses its leaves during the winter period in most provinces in South Africa, the adults are long-lived and should be able to survive the leafless periods. Further more the relatively short life cycle, high fecundity and 3 generations per year should theoretically insure a strong population build-up that would improve the chances of establishment in the field. All indications are that Phenrica sp. 2 is an agent well worth considering for the biological control of A. cordifolia.
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- Date Issued: 2006
The role of nutrients in the biological control of water lettuce, Pistia stratiotes lamarck (Araceae) by the leaf-feeding weevil, Neohydronomus affinis Hustache (Coleoptera: Curculionidae) with particular reference to eutrophic conditions
- Authors: Moore, Gareth Ryan
- Date: 2006
- Subjects: Water lettuce -- Biological control , Araceae , Beetles , Curculionidae , Weeds -- Biological control , Pests -- Biological control
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5759 , http://hdl.handle.net/10962/d1005447 , Water lettuce -- Biological control , Araceae , Beetles , Curculionidae , Weeds -- Biological control , Pests -- Biological control
- Description: Water lettuce, Pistia stratiotes Lamarck (Araceae) is a South American plant that has the potential to be a very damaging and important aquatic weed in many tropical countries, including South Africa. It has the potential to rapidly multiply vegetatively and completely cover watercourses in a very short space of time outside of its natural range under ideal conditions and without its natural enemies. In such instances, the weed may cause hindrances to water transport and fishing, increasing chances of malaria, as well as affecting the natural ecology of the system. Water lettuce can also set seed, which may lay dormant for long periods, germinating when conditions are favourable. It is therefore very necessary to adopt control methods against the weed where it is a problem. However, water lettuce has also been effectively and completely controlled in many countries by the leaf-feeding weevil, Neohydronomus affinis Hustache. High nutrient levels in the form of nitrates and phosphates have been shown to have largely negative effects on biological control in several studies, with control being incomplete or taking longer than in similar areas with lower nutrient levels. The effectiveness of N. affinis on the biological control of water lettuce was investigated in a laboratory study, growing P. stratiotes plants with and without insects at different nutrient concentrations. In these studies biological control of water lettuce with N. affinis was found to be complete under eutrophic nutrient conditions, although control took longer when higher nutrient levels were tested. A field site study was conducted at a sewage settlement pond in Cape Recife Nature Reserve near Port Elizabeth, South Africa. This highly eutrophic system was used as a field example for the effectiveness of biocontrol of P. stratiotes by N. affinis under eutrophic conditions. The weevils at Cape Recife caused a massive and rapid crash in the percentage coverage of the weed, from 100% in May 2003, to approximately 0.5 % in September 2003. Plant growth parameters were also found to decrease considerably in size correspondingly with this crash from May 2003 until spring 2003. Plant size only again started to increase gradually but steadily through spring 2003 and into summer. In the laboratory studies, the fecundity of weevils was shown to be much higher on plants grown under higher nutrient concentrations than on plants grown in lower nutrient concentrations. The results from the wing-muscle analysis under different nutrient concentrations were not easy to interpret, and there were few differences in wing muscle state between most of the concentrations. From these findings it is suggested that nutrient concentration, particularly high levels of nitrates and phosphates is not a limiting factor in terms of effective biological control of P. stratiotes with N. affinis, but that under high nutrient conditions biological control might take longer.
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- Date Issued: 2006