Browsing by Author "Pieterse, Annika"

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    In vitro production and biocontrol potential of nematodes associated with molluscs
    (Stellenbosch : Stellenbosch University, 2020-03) Pieterse, Annika; Malan, Antoinette P.; Ross, Jenna Louise; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Growing concern around the risks associated with the use of some molluscicide chemicals has prompted research into alternative control methods for mollusc pests. One of the most successful biocontrol methods available in Europe is the use of the mollusc-parasitic nematode, Phasmarhabditis hermaphrodita, which cannot be used or sold in South Africa under the terms of the Agricultural Pest Amendment Act, No. 18 of 1989. It is, therefore, necessary to isolate local mollusc-parasitic nematodes and to assess their biocontrol potential. Nematodes associated with slugs from the KwaZulu-Natal province of South Africa was surveyed. Caenorhabditis elegans was the only nematode isolated, and pathogenicity tests concluded that it was not capable of causing mortality to the slug species Deroceras invadens, even when associated with a pathogenic bacterium. During this survey, a number of endemic slug species were collected, including Chlamydephorus gibbonsi. A detailed description of C. gibbonsi’s feeding process is given. Following the quest to isolate other mollusc-nematodes across Africa, a collaborative project led to the isolation and description of Phasmarhabditis sp. (KEN1), a new nematode species from Kenya. Phasmarhabditis sp. (KEN1) can be characterised by the females having a conoid tail shape, the presence of males with a bursa bearing nine bilateral pairs of genital bursal papillae, as well as the infective juvenile (IJ) having the longest body length thus far recorded in the genus. The life cycles of five nematodes associated with molluscs were then studied and described. Based on the ease of mass-culturing, the species were ranked in the following order: C. elegans, P. hermaphrodita and Phasmarhabditis bohemica, with Phasamrhabditis papillosa and Phasmarhabditis sp. (KEN1) being equally difficult to massculture. The in vitro culturing of the nematodes P. papillosa, C. elegans, Phasmarhabditis sp. (KEN1) and P. bohemica was then studied using seven bacterial isolates from slugs and three associated with entomopathogenic nematodes. The results indicated that Kluyvera sp. would be a suitable bacterial candidate to support the growth of C. elegans and Phasmarhabditis sp. (KEN1), while Pseudomonas spp. would make suitable bacterial candidates for P. bohemica and P. papillosa. The nematode P. bohemica was then grown in association with the bacterial isolate Pseudomonas sp. (1), in liquid media cultures to test the effects of bacterial inoculum density and initial IJ inoculum density on the total nematode yield, the IJ yield and the IJ proportion in the cultures. The results showed that 1% bacteria inoculum and a higher initial inoculum concentration of 3000 IJs/ml led to higher total nematode and IJ yields. Lastly, a novel application method of metaldehyde was tested in an apple orchard on the snail Cornu aspersum. Baitchain, a new application method whereby bait pellets can be wrapped around the base of a tree was tested along with Sluggit, a metaldehyde bait pellet applied to the soil, to compare their effectiveness in protecting apple trees from C. aspersum. All treatments caused a significant reduction in snail numbers after 28 days, and all treatments, except Sluggit applied at a concentration of 15 g/kg metaldehyde, caused significant mortality.
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    Investigating the potential of indigenous nematode isolates to control invasive molluscs in canola
    (Stellenbosch : Stellenbosch University, 2016-12) Pieterse, Annika; Malan, Antoinette P.; Ross, Jenna Louise; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Terrestrial molluscs (Mollusca: Gastropoda) are important economic pests worldwide, causing extensive damage to a variety of crop types, and posing a health risk to both humans and wildlife. In South Africa, the climate is favourable for invasive European molluscs, especially in the Western Cape province, where there are mild, damp winters. One crop that is particularly targeted by the pests concerned is canola (Brassica napus), which is a winter arable crop that is commercially produced for its use in cooking, food processing, fertilisers, fuels, pet food, plastics, and animal feed. Molluscs on canola in the Western Cape province are currently controlled using chemical molluscicide pellets. These chemicals have the potential to adversely affect the environment and non-target organisms. The use of mollusc-parasitic nematodes is a possible environmentally-friendly alternative. Current knowledge indicates that there are eight nematode families that associate with molluscs, including Agfidae, Alaninematidae, Alloionematidae, Angiostomatidae, Cosmocercidae, Diplogastridae, Mermithidae, and Rhabditidae. To date, Phasmarhabditis hermaphrodita is the only nematode that has been developed as a biological molluscicide. The nematode, which was commercially released in 1994 by MicroBio Ltd, Littlehampton, UK (formally Becker Underwood, now BASF) under the trade name Nemaslug®, is now sold in fifteen different European countries. Due to current legislation, Nemaslug® cannot be sold or used in South Africa. A survey was therefore conducted in the Western Cape province of South Africa to locate a local nematode isolate capable of causing mortality in invasive mollusc pests. A total of 1944 slugs were collected from 12 different study sites. On the identification of slugs, they were dissected alive, and examined for internal nematodes. Nematodes were identified using morphological and molecular techniques (18S rRNA). Seven of the 12 sites had nematodes present, with 8% of the slugs being found to be infected with nematodes. Six nematode species were identified, including Angiostoma margaretae, Angiostoma sp., Caenorhabditis elegans, a mermitid sp., and Phasmarhabditis spp. (SA3 and SA4). Of the six species mentioned, four were previously undescribed. The isolation of new Phasmarhabditis spp. indicates the importance of conducting further surveys of mollusc-parasitic nematodes in South Africa. Nematodes isolated in the survey were tested for their ability to reproduce on decaying organic matter (consisting of dead frozen slugs), with results demonstrating that one of the nematodes, Phasmarhabditis sp. SA4, could complete its life cycle under such conditions. In addition, pathogenicity tests illustrated that Phasmarhabditis sp. SA4 caused significant mortality of the slug D. panormitanum. Phasmarhabditis sp. SA4 was then fully described and characterised by the shape and length of the female tail, and by the presence of males. Phylogenetic analysis demonstrated that Phasmarhabditis sp. SA4 was placed in a monophyletic clade along with Phasmarhabditis sp. SA2, Phasmarhabditis papillosa, and the mollusc-parasitic nematode, Angiostoma dentiferum. The new species brings the total complement of the genus to seven species. Phasmarhabditis sp. SA4 was then established in monoxenic cultures. Five bacterial isolates were isolated from the intestine of slug hosts, identified using 16S rRNA gene sequences, and their pathogenicity tested by means of injecting directly into the haemocoel of D. reticulatum, and monitoring the mortality over time. Kluyvera sp., which was found to cause the highest mortality rate among the slugs concerned, was chosen for monoxenic culturing. Cultures containing Phasmarhabditis sp. SA4 and Kluyvera sp. were optimised using temperatures ranging from 15°C to 25°C, with results showing that 15°C was the optimum growth temperature.