Evaluation of a plant-herbivore system in determining potential efficacy of a candidate biological control agent, cornops aquaticum for water hyacinth, eichhornia crassipes
- Authors: Bownes, Angela
- Date: 2009
- Subjects: Water hyacinth -- Control -- South Africa , Eichhornia crassipedes , Pontederiaceae , Grasshoppers , Biological pest control agents -- South Africa , Weeds -- Biological control -- South Africa , Invasive plants -- Biological control -- South Africa
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5687 , http://hdl.handle.net/10962/d1005373
- Description: Water hyacinth, Eichhornia crassipes Mart. Solms-Laubach (Pontederiaceae), a freefloating aquatic macrophyte of Neotropical origin, was introduced into South Africa as an ornamental aquarium plant in the early 1900’s. By the 1970’s it had reached pest proportions in dams and rivers around the country. Due to the sustainability, cost efficiency and low environmental risk associated with biological control, this has been a widely used method in an attempt to reduce infestations to below the threshold where they cause economic and ecological damage. To date, five arthropod and one pathogen biocontrol agents have been introduced for the control of water hyacinth but their impact has been variable. It is believed that their efficacy is hampered by the presence of highly eutrophic systems in South Africa in which plant growth is prolific and the negative effects of herbivory are therefore mitigated. It is for these reasons that new, potentially more damaging biocontrol agents are being considered for release. The water hyacinth grasshopper, Cornops aquaticum Brüner (Orthoptera: Acrididae), which is native to South America and Mexico, was brought into quarantine in Pretoria, South Africa in 1995. Although the grasshopper was identified as one of the most damaging insects associated with water hyacinth in its native range, it has not been considered as a biocontrol agent for water hyacinth anywhere else in the world. After extensive host-range testing which revealed it to be safe for release, a release permit for this candidate agent was issued in 2007. However, host specificity testing is no longer considered to be the only important component of pre-release screening of candidate biocontrol agents. Investigating biological and ecological aspects of the plant-herbivore system that will assist in determination of potential establishment, efficacy and the ability to build up good populations in the recipient environment are some of the important factors. This thesis is a pre-release evaluation of C. aquaticum to determine whether it is sufficiently damaging to water hyacinth to warrant its release. It investigated interactions between the grasshopper and water hyacinth under a range of nutrient conditions found in South African water bodies as well as the impact of the grasshopper on the competitive performance of water hyacinth. Both plant growth rates and the response of water hyacinth to herbivory by the grasshopper were influenced by nutrient availability to the plants. The ability of water hyacinth to compensate for loss of tissue through herbivory was greater under eutrophic nutrient conditions. However, a negative linear relationship was found between grasshopper biomass and water hyacinth performance parameters such as biomass accumulation and leaf production, even under eutrophic conditions. Water hyacinth’s compensatory ability in terms of its potential to mitigate to detrimental effects of insect feeding was dependent on the amount of damage caused by herbivory by the grasshopper. Plant biomass and the competitive ability of water hyacinth in relation to another freefloating aquatic weed species were reduced by C. aquaticum under eutrophic nutrient conditions, in a short space of time. It was also found that grasshopper feeding and characteristics related to their population dynamics such as fecundity and survival were significantly influenced by water nutrient availability and that environmental nutrient availability will influence the control potential of this species should it be released in South Africa. Cornops aquaticum shows promise as a biocontrol agent for water hyacinth but additional factors that were not investigated in this study such as compatibility with the South African climate and the current water hyacinth biocontrol agents need to be combined with these data to make a decision on its release. Possible management options for this species if it is to be introduced into South Africa are discussed.
- Full Text:
The impact on biodiversity, and integrated control, of water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae) on the Lake Nsezi - Nseleni River system
- Authors: Jones, Roy William
- Date: 2009
- Subjects: Water hyacinth -- Control -- South Africa , Eichhornia crassipedes , Pontederiaceae , Aquatic plants -- South Africa -- Nsezi, Lake , Aquatic weeds -- South Africa -- Nsezi, Lake , Invasive plants -- South Africa -- Nsezi, Lake
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5711 , http://hdl.handle.net/10962/d1005397 , Water hyacinth -- Control -- South Africa , Eichhornia crassipedes , Pontederiaceae , Aquatic plants -- South Africa -- Nsezi, Lake , Aquatic weeds -- South Africa -- Nsezi, Lake , Invasive plants -- South Africa -- Nsezi, Lake
- Description: Water hyacinth, Eichhornia crassipes (Martius) Solms-Laubach (Pontederiaceae), a free floating aquatic plant was discovered by C. von Martius in 1823 in Brazil. It is believed to have been introduced into South Africa, as an ornamental plant, in 1908 to the Cape Province and Natal. Since its introduction, water hyacinth has spread throughout South Africa to the detriment of all aquatic systems that it has been introduced to directly or indirectly. The weed was first positively identified on the Nseleni and Mposa rivers on the Nseleni Nature Reserve which is a protected area near Richards Bay in KwaZulu- Natal in 1982 and formed a 100% cover of the river by 1983. An integrated management plan was implemented in 1995 and resulted in a reduction of the weed from a 100% cover to less than 20% cover in 5 years. The keys to success of the water hyacinth integrated management plan, presented here, were finding the source of the weed, mapping the extent of the water hyacinth infestation, identifying sources of nutrient pollution, appointing a champion to drive the programme, dividing the river into management units, consultation with interested and affected parties, judicious use of herbicides and biological control and a commitment to follow-up. This study further showed that water hyacinth on the Nseleni and Mposa river systems had a negative impact on the biodiversity of the protected area and the control of water hyacinth resulted in the recovery of the benthic invertebrate, amphibian, reptile, fish and avian fauna. The implementation of this integrated management plan was very cost-effective and serves as a model approach to the control of water hyacinth in both South Africa and the rest of the world.
- Full Text: