Browsing by Author "Van Zyl, Carolina"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    The in vivo production of Heterorhabditis zealandica and Heterorhabditis bacteriophora
    (Stellenbosch : Stellenbosch University, 2012-03) Van Zyl, Carolina; Malan, Antoinette P.; Addison, Pia; Addison, M. F.; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The agricultural industry in South Africa is dominated by the use of insecticides. Producers rely heavily on chemicals that cause increased risk to health, the environment and ecology, rapid resistance development in key insect pests and pesticide residues on crops. The increased concern regarding the impact of these pest management practices on the environment and alternative pest management strategies are being investigated. Entomopathogenic nematodes (EPNs) have been identified as being promising biological control agents of key insect pests. The two EPN genera that have shown promise for use as biological control agents within an integrated pest management programme areSteinernema and Heterorhabditis. Commercialisation and the successful use of EPNs to control pests in North America, Australia, Europe and Asia have confirmed the effectiveness of these organisms as biological control agents. Unfortunately, EPNs in large enough numbers for commercial field applications are not yet available on the South African market. Large numbers of EPNs can be produced through either in vivo or in vitro culturing practices. The objective of this study was to streamline the in vivo production process by using two endemic EPN species, Heterorhabditis zealandica (SF41) and H. bacteriophora (SF351). These EPN isolates have been shown to be effective control agents of codling moth Cydia pomonella, false codling moth Thaumatotibia leucotreta, obscure mealybug Pseudococcus viburni, and the banded fruit weevil Phlyctinus callosus. A comparative study was conducted to identify suitable host insects for EPN production of local H. zealandica (SF41) and H. bacteriophora (SF351) strains. Hosts were selected according to their susceptibility to the two EPN species used, their general availability and the ease and cost of rearing. Wax moth larvae Galleria mellonella (WML) and mealworms Tenebrio molitor (MW) were selected as hosts. In order to produce nematodes of consistent quality, a continuous source of host insects reared on a standardised diet was required. WML and MW were each reared on five different diets in the dark at ±26°C. A superior diet for each host was selected according to the diet that produced, on average, the larvae with the highest body mass within a specific timeframe. The heaviest WML, at an average weight of 0.19 g per larva, were produced on a diet consisting of 118 g wheat flour, 206 g wheat bran, 118 g milk powder, 88 g brewer‟s yeast, 24 g wax powder, 175 ml honey and 175 ml glycerol. The heaviest MW larvae weighed, on average, 0.0154 g per larva, and were produced on a diet consisting of 100% wheat bran. To confirm the hypothesis that a linear relationship exists between the weight of a host and the number of nematodes produced from that host, a study was conducted to determine the number of H. zealandica and H. bacteriophora produced per g of host. WML, MW, codling moth larvae and false codling moth larvae were weighed individually and inoculated with the two nematode species respectively. In addition, nematode production in frozen MW and WML was tested. The number of nematodes harvested from each host was counted, and the average number of nematode progeny produced in each host was calculated. A significant linear correlation between the weight of WML and MW and the number of H. zealandica and H. bacteriophora respectively produced confirmed the hypothesis that nematode production within the specified host increases with an increase in host weight. WML produced the highest number of H. zealandica and H. bacteriophora per g of host (1 459 205 ± 113 670 and 1 898 512 ± 94 355), followed by MW larvae (836 690 ± 121 252 and 414 566 ± 67 017). Lower numbers of H. zealandica and H. bacteriophora per g codling moth (57 582 ± 10 026 and 39 653 ± 8 276) and per g false codling moth (192 867 ± 13 488 and 97 652 ± 23 404) were produced. Successful infection of a suitable insect host is one of the key factors in an efficient in vivo nematode production process. Three inoculation techniques were compared using H. zealandica and H. bacteriophora: inoculation with a pipette; shaking of hosts in the nematode inoculum; and immersion of hosts in the nematode suspension. With each inoculation technique, WML and MW were used as host larvae and were inoculated with nematodes at a concentration of 200 infective juveniles (IJs) / larva. The percentage mortality of insect hosts was determined after two days, and EPN infectivity, confirmed by colour change and dissection, after seven days. The highest percentage EPN infection was obtained using pipetting for both nematode isolates and hosts. Nematode infection rates for all nematode-host combinations obtained with pipetting were above 90%, with the exception of MW inoculation with H. bacteriophora, where the percentage of infection obtained was 76%. The current study conclusively demonstrated that variations in infection levels occur, depending on the inoculation technique used. In an additional effort to enhance infectivity during inoculation, H. zealandica, H. bacteriophora and MW were subjected to host-stressor regimes and to nematode- infectivity-enhancing additives. Three treatments, plus a control treatment, were compared. Exposing MW to 70°C tap water prior to inoculation did not increase infection levels. On the contrary, reduced infection levels were observed with host immersion in 70°C tap water followed by inoculation with H. bacteriophora, compared to the control. Only 12% infection was obtained compared to the 48% infection achieved in the control. Infection obtained using H. zealandica was 21%. Treating H. zealandica and H. bacteriophora IJs withMn2+SO4.H20 in a suspension, prior to inoculating MW, did not significantly enhance nematode virulence. Inoculation of MW with treated H. zealandica IJs led to an infection rate of 81%, compared to the control, with which 80% infection rate was obtained. Heterorhabditis bacteriophora caused 47% MW infection, compared to the control, which was subject to 48% infection. A combination of the two above-mentioned treatments did not enhance the infection levels either. Immersing MW into 70°C tap water prior to inoculation with nematodes treated with Mn2+SO4.H20 led to infection levels of 13% and 9% respectively when H. bacteriophora and H. zealandica were used. Future research is required to optimise the protocol used in this study of subjecting MW and local nematode isolates to stressor regimes. The ability of two formulations to maintain biological activity and virulence of H. zealandica was investigated. A quality standard control measure was used to measure the percentage survival and virulence of formulated H. zealandica over a period of 21 days. IJs were formulated into Pesta granules and coconut fibres, while nematodes stored in tap water served as the control. The numbers of live H. zealandica in Pesta granules and coconut fibres decreased drastically after seven days of storage. The survival of nematodes in Pesta granules dropped to 9.79% after 21 days compared to the control, where the survival rate was 79.79%. Nematode survival in coconut fibres was even lower, at 25.84% after seven days and 2.25% after 21 days. After 21 days in storage, 100%+of nematodes survived in the control for coconut fibres. The application of the standard quality control measure, which was used to determine the virulence of formulated H. zealandica, proved to be ineffective. Higher MW mortality rates were obtained in the control where no nematodes were added to larvae, compared to where nematodes were added in varying dosages. However, adjusting certain aspects in the protocol of this quality control measure specifically to accommodate local conditions could possibly make it a more effective tool for measuring endemic nematode virulence.
  • Thumbnail Image
    Item
    Status of the invasive wasp species, Vespula germanica and Polistes dominula in South Africa, and the feasibility of various management strategies
    (Stellenbosch : Stellenbosch University, 2016-12) Van Zyl, Carolina; Veldtman, Ruan; Addison, Pia; Stellenbosch University. Faculty of Agrisciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The main objective of this study was to determine the current status of two invasive wasps, Vespula germanica and Polistes dominula, in South Africa and to explore the feasibility of implementing various management strategies to control and/or eradicate them. Both wasp species pose a potential threat to biodiversity and agriculture in the Cape Floristic Region (CFR), as well as being a nuisance to people. In an effort to identify suitable biocontrol agents, the pathogenicity of three likely indigenous entomopathogenic nematode (EPN) species and one likely indigenous entomopathogenic fungal (EPF) species was tested against both V. germanica and P. dominula larvae in a bioassay trial. The three EPN species were Heterorhabditis bacteriophora,H. noenieputensis and Steinernema yirgalemense. The fungal species tested was Beauveria bassiana. Both V. germanica and P. dominula larvae were highly susceptible to all of the biocontrol agents tested, and died of infection within 4 days after inoculation within EPN and within 7 days after innoculation within the EPF. The EPN Heterorhabditis bacteriophora which performed best in the bioassay trial, as well as the EPF, Beauveria bassiana, were then tested in the field to determine its ability to infect P. dominula larvae by spraying inoculum directly onto nests. Four treatments were applied, namely: an aqueous solution of the EPF, an aqueous solution of the EPN, a mixture of the EPF and EPN species, and a control of distilled water. The combination of EPF and EPN caused the highest mortality in both P. dominula larvae (31.39 %) and pupae (3.42 %) compared to the other treatments, but infection levels were much lower than those obtained under laboratory conditions. An unsuspected discovery was made, when it appeared that 13 % of all nests used in this trial were parasitized by an unclassified fly species, identified to be a species from the Tachinidae family. There was no significant difference between the ability of fly larvae that were treated with the control, to develop into adults over a period of 144 h, compared to those treated with the various biocontrol agents. Landmark-based geometric morphometric analyses were used to identify the potential origin of introduced V. germanica wasps and to determine the possible route of invasion followed in South Africa. Variation in forewing shape among wasp worker samples that were collected from five different countries, including South Africa, Australia, New Zealand, Argentina and France, was compared. An overall direct correlation between the wing shape and the geographic distance between two sites was found. This result suggests that the morphological variation in wasps from South Africa can be explained as isolation-by-distance. Results inferred that wasps had spread from Kirstenbosch to Somerset West, and thence through Stellenbosch to Franschhoek. The wing shape of wasps collected from Kirstenbosch, the area where the first V. germanica specimen was found, mostly resembled the wing shape of samples from France, compared to all the other overseas localities. Therefore, one could conclude V. germanica wasps were most likely transported from Europe to South Africa. The attractiveness of a range of lures and baits to V. germanica and P. dominula females collected in the field were tested using a Y-tube olfactometer. A combination of protein and carbohydrate-based baits were tested. Vespula germanica mostly preferred cooked ham, whereas P. dominula was mostly attracted to the odours emanating from their own nest.