Browsing by Author "Erasmus, Talitha"

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    Evaluating the potential biological control of Margarodes prieskaensis using South African entomopathogenic fungi, and/or entomopathogenic nematodes
    (Stellenbosch : Stellenbosch University, 2023-04) Erasmus, Talitha; Stokwe, Nomakholwa F.; Allsopp, Elleunorah; Stellenbosch University. Faculty of AgriSciences. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: Margarodes prieskaensis (Jakubski) (Homoptera: Coccoidea: Margarodidae) occurs naturally in the Northern Cape, Limpopo and Mpumalanga in South Africa, where it is a serious pest on table and raisin grapes (Vitis vinifera L.) The larvae of M. prieskaensis feed on grapevine roots, causing poor growth and reduced vigor which eventually result in the death of the infested plant. Currently, there are no chemicals registered for the control of M. prieskaensis in South Africa, and no resistant rootstocks or natural enemies of M. prieskaensis have been identified to date. The aim of the study was to investigate the potential of local entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPN) to control M. prieskaensis females in table and raisin grapes. Six local EPF isolates, obtained from the Stellenbosch University collection, were screened for their pathogenicity against M. prieskaensis females under laboratory and semi-field conditions: Beauveria bassiana, Metarhizium robertsii, M. pinghaense, M. brunneum, M. majus and M. anisopliae performed the best and achieved high percentages of infection and mortality and were selected for subsequent trials. The other local EPF isolates did not perform adequately, even though infection did occur and thus warrant further investigation. Metarhizium pinghaense outcompeted M. majus in the concentration trials, especially in terms of overall infection success, however both EPF had a high infection success rate. In semi-field trials where M. majus and M. pinghaense were evaluated under optimal conditions, M. majus was outperformed by M. pinghaense, producing higher levels of mycosisi n insect cadavers. The EPF treatments showed high infection rates, while there was no infection in the untreated control. However, the efficacy of the EPF in inducing M. prieskaensis mortality could not be determined accurately due to high levels of insect mortality in the control. Field trials were conducted in the Northern Cape and Limpopo to test the efficacy of M. pinghaense to control females of M. prieskaensis. The infection rate was significantly lower than in the laboratory and semi-field trials. Moisture, temperature ranges and other environmental factors can affect the efficiency of EPF in the soil. Limpopo experienced more optimal temperatures during the trial, with an average infection rate of 28,98%. The trial site in the Northern Cape experienced harsher environmental conditions with extremely cold temperatures during the trial period, resulting in a lower infection rate of 24,61%. Solar radiation also possibly contributed to the overall lower infection rate during the field trials. Suitable formulations of EPF could possibly reduce the dire impact of environmental factors like extreme temperatures, low humidity and solar radiation. The field trials in the Northern Cape also assessed the efficacy of a local EPN species, Steinernema yirgalemense to control females of M. prieskaensis. Overall, little to no infection was achieved. Margarodes prieskaensis presents challenges to field applications of EPN and S. yirgalemense was unable to reproduce within the M. prieskaensis females, indicating that it is possible that M. prieskaensis either inhibit the symbiotic bacteria secreted by the EPN or that the females secrete repellent volatiles that preventinfestation. Before attempts are made to test other EPN species against this pest, the possibility that M. prieskaensis females can inhibit EPN infestation warrants further investigation. A combined application of S. yirgalemense and M. pinghaense was also included in the field trials. The infection rate for both the EPN (3.7%) and EPF (11.1%) was low in the combination application, with indications of antagonism between the EPF and EPN. This might explain why the infection rate for M. pinghaense was lower in the combination treatment than when applied on its own. This should be resolved before further studies with combined applications are done. This study is the first to show that females of M. prieskaensis are susceptible to infection by EPF species and that their use as biocontrol agents warrants further investigation. Male pre-pupae of M. prieskaensis spend between 30 and 50 days just underneath the soil surface before developing into pupae and should also be investigated as targets for biocontrol. It is therefore recommended that the use of EPF as a soil drench application against the pre- pupae should be investigated. This study provided crucial baseline information on the efficacy of local EPF and EPN against M. prieskaensis females. For future studies on EPF to control M. prieskaensis, it would be beneficial to resolve the problems identified in this study, including the method of collecting and handling M. prieskaensis females, and adequate formulation of EPF for protection against environmental factors.