Browsing by Author "Platt, Thomas"

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    Investigating the above-ground application of EPNs for the control of the vine mealybug Planococcus ficus
    (Stellenbosch : Stellenbosch University, 2017-12) Platt, Thomas; Malan, Antoinette P.; Stokwe, Nomakholwa F.; Stellenbosch University. Faculty of AgriScience. Dept. of Conservation Ecology and Entomology.
    ENGLISH ABSTRACT: The table and wine grape industries in South Africa are of major economic importance, particularly within the Western Cape Province, making the pest control of grapevines a priority. The vine mealybug Planococcus ficus (Signoret) (Hemiptera: Pseudococcidae) is a key pest of South African grapevines, damaging vines by phloem feeding, by disfiguring grapes with waxy residues, by encouraging the growth of sooty moulds, and by serving as a vector for viruses. Chemical insecticides like chlorpyrifos have traditionally been used in their control, though the cryptic habitats on the vine chosen by the most economically significant mealybug life stage complicates pesticide application. Additionally, mealybugs excrete a waxy coating that repels liquids, and their short generation time allows the rapid development of resistance to chemical pesticides. Consequently, alternatives are sought for the control of mealybugs on grapevines. One candidate for their control is entomopathogenic nematodes (EPNs), which are nematode parasites of soil-based insect life stages. Of major interest in this respect are the EPNs of the families Steinernematidae and Heterorhabditidae, the infective juveniles (IJs) of which have been successfully applied to control soil-based insect pests. However, the maladaptation of IJs to non-soil environments (such as foliage) has limited their use as biocontrol agents above ground, due to their susceptibility to extremes of temperature and to prolonged exposure to ultraviolet light (UV), as well as their generally low tolerance for desiccation. The aim of this study was to investigate EPN candidates for the control of P. ficus, and to develop methods for overcoming the weaknesses of EPNs in foliar application. As new species of EPNs are constantly being described, laboratory-based bioassays were performed, screening three newly described EPN species (Steinernema jeffreyense, Heterorhabditis noenieputensis, and Steinernema spp. WS9), as well as Steinernema yirgalemense, for their control of P. ficus. Heterorhabditis noenieputensis was the most effective, causing 90% about 3% mortality, followed by S. yirgalemense (63% about 7%), with both mortalities being significantly greater than was that of the control. The presence of the nematodes within the body cavities of P. ficus cadavers was confirmed. Steinernema yirgalemense was selected as the EPN candidate of choice for experiments going forward, due to the difficulty in mass-producing H. noenieputensis. However, developments in the formulation methods of the Heterorhabditid species will warrant the re-examination of H. noenieputensis in future. On performing a laboratory bioassay to determine the minimum amount of time required for the optimal infectivity of P. ficus by S. yirgalemense, the mortality of P. ficus was found not to improve significantly for individuals exposed to S. yirgalemense for longer than 3h. Subsequently, the effects of varying temperature and relative humidity (%RH) on the ability of S. yirgalemense to cause mortality in P. ficus were tested. The mortality of P. ficus was greatest at 25°C (72% ± 3%), and at 100% RH, during the humidity trial. Each result established targets for the optimal application of S. yirgalemense. The ability of two adjuvants, Zeba® and Nu-Film-P®, to improve the efficacy of S. yirgalemense applications was tested under semi-controlled conditions. The combination of Zeba® and Nu-Film- P® in suspension with S. yirgalemense was shown to deposit significantly more EPNs (30.8 ± 4 IJs / 4 cm2) onto grapevine leaves in the laboratory than did formulations with EPNs and water alone, or with EPNs and Nu-Film-P®, though not significantly more than with EPNs and Zeba® alone. A growth chamber bioassay was conducted to assess the effect of the addition of the adjuvants to S. yirgalemense suspensions on P. ficus mortality. The addition of Zeba® and Nu-Film-P® to S. yirgalemense caused significantly higher mortality (84% ± 5%) in P. ficus in the growth chamber than did any other treatment, including EPNs + Zeba® (47% ± 3%), after 48h. A bioassay carried out in the greenhouse showed similar results, with the S. yirgalemense treatment containing Zeba® and Nu-Film-P® causing 88% ± 3% mortality after 48h, which was significantly higher than was that which was attained with any other EPN treatment. The treatments were then assessed under semi-field conditions that would be capable of inflicting the harshest environmental stress. Application of S. yirgalemense (at a concentration of 4000 IJs/ml) + Zeba® + Nu-Film-P® to P. ficus individuals on grapevine leaf discs hung on grapevines resulted in 66% ± 4% P. ficus mortality after 48h, which was significantly higher (p < 0.01) than was achieved using either S. yirgalemense + Zeba® alone, or EPNs + water alone, though overall less than the control obtained in the glasshouse. A bioassay to assess the impact of reducing EPN concentration was performed, resulting in predictable reductions in P. ficus mortality when progressively lower concentrations of S. yirgalemense (3000, 2000 and 1000 IJs/ml) were applied with Zeba® and Nu- Film-P® to P. ficus on grapevine leaf discs. The control obtained by the formulation containing 3000 IJs/ml was significantly greater than was that which was achieved with each other treatment after 48h (44% ± 4%), though the control overall was lower than was attained with the 4000 IJs/ml concentration used in the previous bioassay. This demonstrates that the EPN concentration remains important to the efficacy of EPN applications. So as to assess the effects of climatic conditions on EPN longevity, a time-of-day application bioassay was performed. Steinernema yirgalemense was formulated with Zeba® and Nu-Film-P® and applied directly to grapevines, the leaves of which were removed and rinsed at timed intervals, whereupon the live nematodes present on them were counted. The experiment was carried out at 8:00 (with conditions being 14.6C and 93.2% RH at application), and repeated at 14:00 (with conditions being 31.0C and 39.9% RH at application). Higher numbers of living nematodes were recorded on the grapevine leaves at all of the time intervals concerned during the 8:00 trial when compared with the same intervals during the 14:00 trial, indicating that the higher percentage RH had a greater effect on IJ survival than did the more optimal temperature (but lower % RH) during the afternoon trial. This study represents an additional step towards the successful utilization of EPNs (in this case, S. yirgalemense) as biocontrol agents of P. ficus on grapevines in South Africa. Steinernema yirgalemense can achieve > 66 % mortality of P. ficus under semi-field conditions, when the humidity (which is the critical factor for IJ survival on foliage) is effectively managed. Future work should examine S. yirgalemense in full-field application, as well as available methods (such as the use of irrigation, or shade netting) for maximizing the relative humidity immediately following IJ application.